Core Curriculum, The: Ultrasound
1st Edition

7
Obstetric Ultrasound–Second and Third Trimester
US is widely used in the evaluation of pregnancy with more than 70% of all pregnancies in the United States undergoing sonographic evaluation [1]. Indications for US examination are expansive and include estimation of gestational age (GA), evaluation of fetal growth, determination of fetal position, detection of multiple gestations, evaluation of fetal well-being, and detection of fetal anomalies. The American Institute of Ultrasound in Medicine provides well-accepted guidelines for the performance of obstetric ultrasound examination [2].
Guidelines for Obstetric Ultrasound Examination
Obstetric US examination in the second and third trimester should include the following standards [2], which are endorsed by the American College of Radiology [3]:
  • Documentation of fetal life, number, and presentation.
  • An estimate of the amount of amniotic fluid.
  • The location and appearance of the placenta and its relationship to the internal cervical os.
  • Assessment of GA using a combination of biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL).
  • Evaluation of the uterus and adnexa. The presence, location, and size of myomas and adnexal masses should be reported.
  • The study should encompass evaluation of fetal anatomy including, but not limited to, the cerebral ventricles, four-chamber view of the fetal heart, spine, stomach, urinary bladder, umbilical cord insertion site, and renal region.
Many obstetric US practices expand the evaluation to include the fetal neck, posterior fossa, extremities, ventricular outflow tracts, fetal bowel, and Doppler evaluation of the umbilical artery.
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Fetal Measurements and Growth
Some of the most important aspects of obstetric care are the determination of GA and the assessment of fetal growth. By convention, clinical gestational dating is based on the first day of the last menstrual period (LMP). Conception is assumed to occur on day 14 of the menstrual cycle. A normal, full-term pregnancy is 40 weeks with a range of 37-42 weeks. Clinical dating, based on the mother’s history of LMP, is notoriously inaccurate. Sonographic dating is based on measurement of fetal parameters. Standardized charts correlate GA with measurements of fetal parameters. Serial measurements are used to document fetal growth. In the second and third trimesters, four fetal measurements are routinely used.
Biparietal Diameter
The BPD measurement is greatly affected by shape of the fetal head.
  • The BPD is determined on an axial image of the fetal head at the level of the thalamus (Fig. 7.1). The measurement is taken from the outer edge of the near cranium to the inner edge of the far cranium.
  • The BPD may be low for GA if the head is unusually long and narrow in shape (dolichocephaly) (Fig. 7.2A) [4].
  • The BPD may be high for GA if the head is unusually round (brachycephaly) (Fig. 7.2B) [4].
  • The fetal head may be compressed by excessive transducer pressure or the molding that occurs with oligohydramnios.
Head Circumference
The HC measurement is independent of head shape. The BPD and HC reflect growth of the fetal brain.
  • The HC is measured on the same axial image as the BPD. The HC is a perimeter measurement of the fetal cranium excluding subcutaneous soft tissues (Fig. 7.3).
Figure 7.1 Biparietal Diameter. The biparietal diameter is measured from the outer edge of the near skull to the inner edge of the far skull (between cursors, +) on an image plane through the thalamus (long arrows) and third ventricle (short arrow).
Figure 7.2 Dolichocephaly and Brachycephaly. A. This fetal head is exceptionally elongated in shape (dolichocephaly), disproportionally reducing the biparietal diameter measurement. B. This fetal head is exceptionally round in shape (brachycephaly), disproportionally increasing the biparietal diameter measurement.
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Abdominal Circumference
The AC reflects the growth of intraabdominal organs.
  • The AC is measured in axial plane at the level of the junction of the umbilical vein with the left portal vein (Fig. 7.4). The abdomen should appear round, not oval, when a true axial section is obtained. The fluid-filled stomach is routinely seen on this plane. The AC is measured as the length of the peripheral circumference of the fetal abdomen including subcutaneous soft tissues.
Figure 7.3 Head Circumference. The head circumference is measured on the same image plane as the biparietal diameter. The measurement is the circumference of the fetal cranium with no soft tissues of the scalp included.
Figure 7.4 Abdominal Circumference. The abdominal circumference is measured on a transverse image of the abdomen obtained at the level where the umbilical vein (arrow) is in the substance of the liver. The outer circumference is measured to include all soft tissues.
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Femur Length
The FL serves as a monitor for growth of the long bones.
  • The femoral shaft is seen as a slightly curved, echogenic structure that produces an acoustic shadow. The longest dimension of the femoral shaft is measured for the FL (Fig. 7.5). The femoral epiphysis, seen as a spike on one end of the femoral shaft, is not included in the measurement. The measurement is most accurate when the femur is perpendicular to the US beam.
Figure 7.5 Femur Length. The femur length is the longest dimension of the shaft of the femur (between cursors, +). Note the acoustic shadow cast by the bone.
Figure 7.6 Composite Age–Fetal Biometry Report. Most US units provide a data page that summarizes fetal measurements and calculations. This report (top) compares clinical dating by last menstrual period (LMP) to US dating by fetal measurements. The US estimate of menstrual age (MA) is the composite age based on an average of the four measurements listed. Mean fetal measurements are listed, along with the gestational age (GA) predicted by each measurement. The right hand column indicates the measurement chart used for the computer determination of GA. Measurement ratios and estimated fetal weight (EFW) calculation is provided at the bottom. The LMP% notation provides the EFW percentile for GA based on LMP.
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Estimated Fetal Weight
Estimated fetal weight (EFW) is used to identify fetuses that are small for GA and potentially growth retarded, and fetuses that are large for GA and may be difficult to deliver.
  • EFW may be determined by measurement of BPD and AC [5] or by measurement of AC and FL [6]. Many computer programs include an automatic printout of the EFW with weight percentile compared to GA determined by LMP (Fig. 7.6).
Assignment of GA is an interpretation based on clinical history, physical examination, and sonographic assessment. It is not just a value taken from a chart. GA is routinely determined at the time of the first US examination and is not changed thereafter. Measurements made on subsequent US examinations are compared to the GA determined on the first US examination to determine if interval growth is normal. Sonographic estimates of GA are most accurate in early pregnancy and become progressively less accurate as the pregnancy advances. Sonographic GA in the second and third trimesters is routinely based on composite age, which is the average GA determined by measurement of multiple parameters, usually BPD, HC, AC, and FL (Fig. 7.6). Fetal anomalies may make individual measurements invalid. If so, the affected measurement is excluded and composite age is determined from the remaining measurements. GA based on crown rump length in the first trimester is accurate to approximately 0.5 week. Composite GA based on the four routine measurements is accurate to 1.2 week between 12 and 18 weeks, but is accurate to only 3.1 weeks at 36-42 weeks. Measurement charts are included within the calculation packages in computer software on most US units. The range of error of each measurement is routinely listed. In the United States, most physicians use the Hadlock charts as the standards of reference, although a variety of measurement charts and formulas are available [7,8,9,10,11,12].
Intrauterine Growth Retardation
Intrauterine growth retardation (IUGR) is associated with high perinatal morbidity and mortality and an increased risk of impaired neurodevelopment. Infant mortality rate is 4-8 times greater than non-IUGR infants [13]. The diagnostic challenge is to differentiate fetuses that are pathologically small from those that are normal, but constitutionally small. Causes of IUGR are listed in Box 7.1. The approach to diagnosis of IUGR is as follows:
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  • Estimate the GA. Make the best estimate possible based on early US, clinical history, and physical assessment.
  • Compare the AC measurement to the expected AC value based on GA. An AC below the tenth percentile for GA suggests IUGR.
  • Compare the EFW to the expected EFW for GA. An EFW below the tenth percentile for GA suggests IUGR. If EFW is below the fifth percentile for GA, the risk of IUGR is very high.
  • An FL-to-AC ratio (FL/AC) >23.5 suggests IUGR.
  • Obtain an umbilical artery spectral Doppler tracing (Fig. 7.7) [14]. A systolic-to-diastolic (S/D) velocity ratio >4 suggests IUGR [15]. Absent or reversed flow in diastolic is a highly specific sign of fetal distress, often indicative of imminent fetal death [16]. Normally the umbilical artery shows a low-resistance Doppler spectral pattern [S/D <3, resistance index (RI) <0.70]. A high resistance pattern indicates high vascular resistance within the placenta and impaired blood flow to the placenta.
  • Fetuses that measure small for GA but have normal Doppler studies (Fig. 7.7A) are likely to have a normal outcome [17].
  • Check for oligohydramnios. Low amniotic fluid volume [amniotic fluid index (AFI) <5] is found with severe IUGR [15].
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  • The combination of IUGR with polyhydramnios is also ominous and is associated with a high incidence of chromosome abnormality (38%), major congenital anomalies, and high mortality (59%) [18].
  • Fetuses that are determined by these criteria to be “at risk” for IUGR are routinely followed on a weekly basis with reassessment of the listed parameters. Surveillance of fetal well-being often includes serial biophysical profiles.
Figure 7.7 Umbilical Artery Doppler. A. A normal umbilical artery Doppler spectrum is displayed above the baseline, whereas the umbilical vein spectrum is displayed below the baseline indicating normal blood flow in opposite directions. Distinct, moderately high-velocity blood flow is seen in the umbilical artery throughout diastole, resulting in a resistance index (RI) of 0.58 and a systole/diastole (S/D) ratio of 2.36. B. Doppler spectrum from the umbilical artery of a growth-retarded fetus shows reversal of blood flow direction in diastole (arrow). This is a highly specific finding of severe fetal distress.
Biophysical Profile
The biophysical profile is a commonly performed test used to identify fetuses that are compromised and may require expedited delivery [19]. Four “neurologic” tests are used to assess for acute hypoxia and one test (amniotic fluid) is used to check for chronic hypoxia. A score of 2 is given if the test is normal and a score of 0 is given if the test is abnormal. A total score of 8 or 10 is considered normal. Lower scores correlate with increased risk to the fetus. Abnormal results are reported only after a minimum observation period of 30 minutes.
  • Amniotic fluid. At least one pocket of fluid that measures 2 cm or more in a vertical plane yields a score of 2. No pockets of fluid measuring 2 cm or more in a vertical plane equals a score of 0.
  • Fetal movement. At least three discrete body movements of the limbs or trunk equals a score of 2. Less than three distinct body movements equals a score of 0.
  • Fetal tone. At least one episode of limb extension from a flexed position with return to a flexed position equals a score of 2. No extension or sluggish limb extension with failure of return to full flexion equals a score of 0.
  • Fetal breathing. At least one episode of breathing motion lasting at least 30 seconds equals a score of 2. No breathing motion, or breathing lasting less than 30 seconds, equals a score of 0.
  • Nonstress test. A normal (reactive) stress test is the observation of two or more fetal heart rate accelerations of at least 15 beats per minute (bpm) and of 30 seconds or longer duration equals a score of 2. Anything less constitutes an abnormal (nonreactive) stress test with a score of 0.
Macrosomia
Macrosomia describes babies who are large for GA. For these babies life in utero is usually uncomplicated but they are at high risk for complications during and after delivery. Many large babies are found in mothers who have gestational diabetes. Complications of macrosomia include shoulder dystocia, neurologic damage to the brachial plexus (Erb’s palsy), fractures, perinatal asphyxia, neonatal hypoglycemia, and meconium aspiration.
  • Macrosomia is defined as EFW above the ninetieth percentile for GA or greater than 4,000 grams.
Placenta
Normal Placenta
Normal growth and development of the fetus are critically dependent upon the normal function and integrity of the placenta. The union of chorionic villi, arising from the fertilized ovum, with maternal decidual basalis forms the normal placenta. Spiral arteries carry maternal blood to intervillous spaces between branching chorionic villi. Extensive branching provides a large surface area for exchange of metabolites [20].
  • The placenta is first visualized by US at 8 weeks as a focal thickening along the periphery of the gestational sac at the site of implantation.
  • By 12 weeks GA, the disc shape of the placenta is evident. Its substance appears finely granular and its surface is smooth and sharply defined by the covering chorion (Fig. 7.6).
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  • A retroplacental complex of decidual and myometrial veins creates a network of tubular sonolucent channels along the basal aspect of the placenta (Fig. 7.8). Doppler clearly defines these channels as blood vessels. Excessive transducer pressure may obliterate visualization of these normal vessels.
  • Normal placenta aging is manifest by the appearance of hypoechoic areas, septations, and calcifications (Fig. 7.9). Calcifications occur randomly throughout the substance of
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    the placenta and prominently along placental septations. Acoustic shadowing may or may not be evident. Attempts to correlate the appearance of the placenta with fetal lung maturity have been unsuccessful.
  • Echolucencies within the placenta commonly represent venous lakes (Fig. 7.10). Slow swirling flow may be visualized with real-time US. Flow is often too slow to demonstrate with Doppler. These venous lakes may thrombose and remain as echolucent fibrin deposits [21].
  • Placental thickening normally does not exceed 4 cm. Causes of an abnormally thick placenta are listed in Box 7.2 [22].
  • An abnormally thin placenta (<1 cm) is associated with placental insufficiency %(Box 7.3).
Figure 7.8 Normal Placenta. The normal placenta (P) has a granular appearance with a smooth surface defined by its covering chorionic membrane (C, arrow). The retroplacental complex of blood vessels (black arrows) is an important sonographic landmark in the diagnosis of placental abruption.
Figure 7.9 Placental Aging. This placenta (P) shows normal changes associated with advancing gestational age. The aging placenta develops hypoechoic areas (large arrow), septations (small arrows), and calcifications along the septations and placental surface. FH, fetal head.
Figure 7.10 Normal Placental Venous Lakes. Venous lakes (large arrows) appear as focal echolucent areas just beneath the chorionic membrane (C, small arrow), A, or within the substance of the placenta (P), B. Note the swirling blood flow (small arrow) in B. Venous lakes are incidental finding of no clinical significance.
Placenta Previa
Placenta previa describes low implantation of the placenta that covers all, or a portion of, the internal os of the cervix. Placenta previa is present at term in only 0.3-0.6% of all pregnancies, but may be suggested by US in 45% of pregnancies in the first and second trimesters. Risk factors for placenta previa include previous caesarian section, previous placenta previa, multiparity, and maternal age >35 years. Complications of placenta previa are
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maternal hemorrhage, premature delivery, IUGR, and perinatal death. Patients present with painless vaginal bleeding in the third trimester caused by dilatation of the cervical os, which disrupts placental blood vessels.
  • Marginal previa is present when the edge of the placenta reaches or partially covers the dilating cervical os.
  • Complete previa is present when the os is completely covered by placenta (Fig. 7.11).
  • Taipale reported the risk of placenta previa at term is 5.1% when placenta previa is present on US performed at 12-16 weeks [23]. Follow-up of these patients is controversial. Some suggest that all be re-examined for placenta previa in the third trimester, whereas others recommend follow-up on only patients with risk factors or third trimester bleeding [24]. Rosati suggests following only those patients with a placenta that extends 14 mm or more beyond the internal os [25].
  • US diagnosis of placenta previa should always be made with the bladder empty. A full bladder distorts the appearance of the lower uterine segment and commonly creates a false appearance of previa. The cervix and placenta are easily examined with the bladder empty by a translabial approach (Fig. 7.9) [26]. Transvaginal examination is an alternative. The probe must be inserted cautiously and with direct visualization to stop at the cervix.
  • Vasa previa describes a membranous insertion of the cord that crosses the internal cervical os. The insertion of the cord into the placenta is velamentous. That is, the cord inserts into the peripheral membranes of the placenta rather than into the bulk of the placenta near its center. An accessory lobe of the placenta (a succenturiate lobe) may be connected to the main body of the placenta only by membranous vessels that may cross
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    the os. Disruption of these vessels with cervical dilatation results in rapid exsanguination of the fetus. Doppler shows blood vessels adherent to and crossing the internal os [27].
Figure 7.11 Placenta Previa. A translabial image down the “tube” of the vagina (V) shows the internal (long arrow) and external (short arrow) os of the cervix. The internal os is covered by placenta (P, PL, dual arrows). Also seen are a portion of the near-empty bladder (b), a bit of amniotic fluid (a), and a portion of the fetal head (h). The translabial view allows optimal visualization of the cervix with the bladder empty.
Figure 7.12 Subchorionic Hemorrhage–Marginal Placental Abruption. Subchorionic hemorrhage results from venous bleeding caused by detachment of the margin (curved arrow) of the placenta (p). Low pressure bleeding (b) dissects beneath the chorion (white arrows) separating it from the myometrium (black arrowheads).
Placental Abruption
Abruption is defined as the premature separation of a normally positioned placenta from its myometrial attachment. Hemorrhage occurs from disrupted maternal vessels. Risk factors for placental abruption include maternal hypertension, toxemia, cocaine abuse, smoking, and previous placental abruption. Complications include precipitous delivery, prematurity, coagulopathy, and fetal death.
  • US diagnosis of abruption depends upon visualization of the resulting hematoma [28].
  • Subchorionic hemorrhage represents separation of the placenta at its margin (a “marginal” abruption). Bleeding is primarily venous and extends beneath the chorionic membrane (Fig. 7.12) (see Fig. 6.20). Large hematomas are associated with high risk of early pregnancy loss. Most subchorionic hemorrhages occur before 20 weeks gestation [29,30,31].
  • Retroplacental abruption is much more serious because the associated bleeding is arterial (Fig. 7.13). Extensive placental detachment disrupts placental function, causes placental infarctions, and may result in fetal hypoxia and death. Tears in the amnion allow blood to enter the gestational sac. Amniotic fluid leakage into the maternal bloodstream may cause consumption coagulopathy.
  • The hematoma appears as an anechoic or mixed echogenicity mass beneath the placenta and commonly extending beneath the chorion (Figs. 7.12, 7.13). The appearance of the hematoma varies with its age and physical state. The hematoma is anechoic before clot forms, isoechoic to placenta with clot formation, and becomes hypoechoic to anechoic with hemolysis 1-2 weeks after hemorrhage. When the clot is isoechoic, the placenta may appear only diffusely thickened.
  • Disruption of the retroplacental complex of blood vessels is an important confirmatory finding with abruption (Fig. 7.13A). Myometrial contractions and leiomyomas beneath the placenta simulate the US appearance of abruption but displace rather than disrupt the placental blood vessels.
Figure 7.13 Retroplacental Abruption. A. A large retroplacental hemorrhage (H) displaces the placenta (P) away from the myometrium and disrupts the retroplacental complex of blood vessels. B. A large retroplacental hematoma (H) compresses the umbilical cord (arrow) against the fetal head (h), causing marked fetal distress. This mother was a frequent user of cocaine. p, placenta.
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Placenta Creta
Placenta creta describes abnormal placental invasion of the myometrium with complete or partial absence of the decidua basalis. Severity is graded as accreta with chorionic villi directly contacting the myometrium, increta with chorionic villi invading the myometrium, and percreta with chorionic villi penetrating the myometrium and invading the bladder wall. Risk factors are similar to those for placenta previa and include previous caesarian
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section, increased parity, and previous uterine infection. Definitive US diagnosis may be difficult, but the diagnosis should be suggested when these findings are present [32, 33].
Figure 7.14 Placenta Percreta. A. Gray scale image shows a lumped-up placenta (p) with complete placenta previa. The placenta is in close proximity to the wall of the bladder (B). Note the absence of a normal retroplacental complex of blood vessels and the thin, difficult-to-visualize myometrium. The inner surface of the bladder wall has a lobulated appearance (arrow). B. Color Doppler image shows abnormal placental blood vessels (arrow) penetrating the wall of the bladder and protruding into the bladder lumen. This patient had a previous history of two cesarean sections and previous placenta previa. (See Color Figure 7.14B).
Figure 7.15 Placental Chorioangioma. The tumor (arrows) appears as a well-defined hypoechoic mass within the placenta (P) and bulging from its surface. Spectral Doppler shows blood flow within the mass at fetal heart rate.
  • The placenta is low lying and anterior with placenta previa often present (Fig. 7.14).
  • The retroplacental complex of vessels is partially or completely absent. Care must be taken to avoid compression of these vessels by excessive transducer pressure or bladder overdistention.
  • The myometrium underlying the placenta appears thinned (<1 mm) or absent.
  • The bright reflection of the serosa separating the uterus from the bladder is absent.
  • Color Doppler may show contiguous blood vessels extending from the myometrium into the bladder wall (Fig. 7.14B). The abnormal blood vessels may cause focal elevations of the bladder mucosa.
Placental Chorioangioma
Chorioangioma is a benign tumor of the placenta sometimes classified as a hamartoma. They are found in 1% of placentas pathologically but most are small and not clinically significant [21]. US detects only the larger lesions which are associated with elevation of maternal serum alpha-fetoprotein (MS-AFP).
  • Chorioangiomas appear as well-defined, hypoechoic, or mixed echogenicity masses within the placenta, often near the cord insertion site (Fig. 7.15) [34]. Detected chorioangiomas are usually 1-5 cm in size.
  • Spectral Doppler is diagnostic with demonstration of vessels within the tumor with blood flow pulsating at fetal heart rate.
  • Placental hematomas may have a similar appearance but have no blood flow on Doppler US.
Umbilical Cord
Normal Umbilical Cord
  • The normal umbilical cord contains two arteries and a single vein (Fig. 7.16A, C). The cord is easily visualized in amniotic fluid. Color Doppler shows its spiraling configuration.
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  • Confirmation of one or two umbilical arteries is easily made by examining the fetus and demonstrating the umbilical arteries coursing on both sides of the bladder (Fig. 7.16C, D) [35].
Figure 7.16 Umbilical Cord. A. A normal three-vessel umbilical cord has two smaller arteries carrying blood from the fetus to the placenta and one larger vein carrying oxygenated blood and nutrients from the placenta to the fetus. B. A two-vessel umbilical cord has a single artery and a single vein. Color flow images of the bladder (arrow) confirms the presence of two, C, or one, D, umbilical arteries coursing adjacent to the bladder from the fetal hypogastric arteries to the umbilicus. Imaging the bladder is useful when optimal cross sectional images of the cord cannot be obtained. (See Color Figures 7.16C, D).
Two-Vessel Umbilical Cord
A single umbilical artery is found in up to 1% of pregnancies. A two-vessel cord is associated with chromosome anomalies and a variety of fetal malformations [36].
  • The cord contains one artery and one vein. Only a single umbilical artery is seen adjacent to the bladder in the fetus (Fig. 7.16B, D).
  • A careful and complete anatomic survey of the fetus is indicated to detect developmental anomalies. Most centers do not perform amniocentesis if no anomalies are found [37].
Figure 7.17 Allantoic Cyst. Small cystic mass (arrow) in the umbilical cord is an allantoic cyst.
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Nuchal Cord
The cord encircles the fetal neck in up to 25% of pregnancies. Multiple encircling loops and a tight nuchal cord put the fetus at risk for fetal distress especially during labor [35].
  • Color Doppler provides a high sensitivity for detecting nuchal cord.
Masses in the Umbilical Cord
  • Umbilical cord cysts arise from remnants of the omphalomesenteric duct (vitelline duct) or allantoic duct (Fig. 7.17). Fetal anomalies are commonly associated. Cysts are usually small (4-6 mm).
  • Tumors of the umbilical cord are exceedingly rare and include hemangiomas and teratomas [35].
  • Hematomas of the cord usually occur only with manipulation or puncture of the cord (percutaneous umbilical cord sampling).
Uterus and Cervix
Leiomyomas in Pregnancy
Leiomyomas are the most common pregnancy-associated pelvic tumor present in up to 3% of pregnant women. Myomas are associated with bleeding, premature uterine contractions, malpresentation, and obstruction during labor. Approximately 15% of myomas will increase in size during pregnancy. The remainder remain stable in size or disappear because of progressive stretching of the myometrium [38, 39].
  • Leiomyomas appear as spherical masses that distort the contour of the myometrium, and have heterogeneous, usually decreased, echogenicity compared to myometrium. Calcifications may be present. Color Doppler shows myometrial vessels displaced around the myoma [40].
  • Uterine contractions must be differentiated from myomas. Contractions are transient, although they may persist for 1 hour. Contractions are homogeneous and isoechoic to myometrium. They bulge the inner, but usually not the outer, uterine wall. Color Doppler shows no vessel displacement in the area of a contraction [40].
  • Myomas with a volume >200 cm3 show a higher rate of complications than smaller myomas [39].
Figure 7.18 Normal Translabial View of the Cervix. Positioning the US transducer on the labia directs the US beam down the long axis (long arrow) of the vagina and perpendicular to the cervix (C). The cervix is seen as a muscular cylinder contiguous with the myometrium (tiny arrow). The endocervical canal (between cursors, +) appears as an echogenic line in the middle of the cervix. The internal os (i) is outlined by amniotic fluid. The external os (e) in indicated by the end of the endocervical canal. The bladder (b) contains a small volume of urine on this image. Imaging the cervix with the bladder empty allows an accurate measurement of the length of the cervix. This cervix measured 4.2 cm.
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Normal Cervix
The normal cervix remains closed during pregnancy to physically retain the fetus in utero and to prevent ascending infection of the uterus [24].
  • A translabial approach is optimal to evaluate the cervix with the bladder empty (Fig. 7.18) [41]. A 3.5-4.0-MHz sector transducer is routinely utilized. The transducer is covered by a sterile glove or condom and is placed directly on the patient’s labia. The US beam is directed down the long axis of the vagina and is perpendicular to the cervix.
  • The normal cervix is seen as a hypoechoic cylinder contiguous with the myometrium (Fig. 7.18). The cervical canal is seen as an echogenic line commonly surrounded by a hypoechoic zone. The internal os is defined by the point at which the amniotic sac meets the cervical canal. The external os is interpreted as the point at which echogenic cervical canal is no longer visible [41].
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  • The length of the cervix is measured along the endocervical canal from internal to external os. The normal cervix has a mean length of 3.4-3.7 cm and a minimum length of 2.5 cm [42]. A full bladder compresses the lower uterine segment and falsely elongates the cervix (Fig. 7.19). Cervical length is most accurately measured with the bladder empty (Fig. 7.18).
Figure 7.19 Elongated Cervix. A transabdominal view of the cervix (large arrow) with bladder (B) overdistended falsely elongates the apparent cervix by coapting the myometrium of the lower uterine segment. This cervix measured 5.8 cm. The vagina (tiny arrow) appears as a hypoechoic muscular tube.
Figure 7.20 Incompetent Cervix. A. Translabial view shows a very short cervix (between cursors, +) measuring less than 1 cm in length. B. Transabdominal view in another patient shows a completely dilated cervix distended by amniotic fluid with membranes presenting at the external os.
Incompetent Cervix
A shortened cervix is predictive of cervical incompetence with its associated high risk of premature delivery. Cervical incompetence is responsible for approximately 16% of premature deliveries [24].
  • The cervix is considered abnormally short when the closed endocervical canal is <2.5 cm in length (Fig. 7.20A) [42]. Cervical length >3.0 cm effectively excludes pre-term delivery.
  • Fluid within the cervical canal indicates dilatation of the cervix. Measurement of the distance between the anterior and posterior wall of the cervix indicates the degree of dilatation. The closed portion of the endocervical canal, measured between the dilated portion of the cervix and the external os, is considered the functional cervical length.
  • Membranes may bulge into or through the cervical canal. When membranes bulge into the vagina, delivery is inevitable (Fig. 7.20B).
Amniotic Fluid
Normal Amniotic Fluid
Amniotic fluid protects the fetus from injury, allows growth and fetal movement, and is essential for normal lung maturation. In early pregnancy, fluid in the amnion and chorionic spaces is a filtrate of the membranes. After 16 weeks GA, nearly all of the amniotic fluid originates from fetal urination. Fetuses with bilaterally impaired renal function have profound oligohydramnios by 18 weeks. Amniotic fluid is removed from the amniotic cavity primarily by fetal swallowing.
  • The volume of amniotic fluid rises steadily to a maximum at 22 weeks GA and stays at that level until delivery [43]. Accurate measurement of amniotic fluid volume by US is
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    difficult and most sonographers rely on estimating fluid volume subjectively based on their experience [44].
  • Normal fluid volume allows free but not unlimited movement of fetal limbs. Fluid is seen around the fetus but both the anterior and posterior uterine walls are in contact with the fetus.
  • The amniotic fluid index (AFI) is widely utilized in an attempt to be more objective. The index is determined by measuring the vertical height of the deepest fluid pocket in each quadrant of the uterus and summing the 4 measurements. Umbilical cord and fetal parts are excluded from any measurement. The normal range of the AFI is 5-20 cm.
  • Fine particulate matter suspended in the amniotic fluid is usually a normal finding. It usually represents vernix in the third trimester, but may also result from blood or meconium [45].
Oligohydramnios
Oligohydramnios indicates abnormally low volume of amniotic fluid. Oligohydramnios is associated with increased perinatal morbidity. Causes include reduced urine output (renal agenesis, bilateral renal dysplasia, and urinary tract obstruction), IUGR, premature rupture of membranes, and post-term pregnancy.
  • AFI below 5 cm is indicative of oligohydramnios (Fig. 7.21A).
Polyhydramnios
Polyhydramnios is an excessive volume of amniotic fluid. Polyhydramnios is associated with maternal diabetes under poor control, gastrointestinal and central nervous system anomalies, lethal skeletal dysplasias, and chromosome anomalies. Severe polyhydramnios may cause abdominal pain, breathing difficulty, premature rupture of membranes and premature
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delivery. Polyhydramnios is commonly idiopathic and many fetuses with mild polyhydramnios will have a normal outcome.
Figure 7.21 Oligohydramnios and Polyhydramnios. A. Oligohydramnios. The abdominal circumference is measured on a fetus with renal agenesis. No amniotic fluid was seen in the uterine cavity. Visualization of fetal anatomy is very difficult when severe oligohydramnios is present. B. Polyhydramnios. A huge volume of fluid surrounds the fetus. At least 8 cm of amniotic fluid separates the abdomen from the anterior wall of the uterus. This fetus had esophageal atresia.
  • AFI above 20 cm is indicative of polyhydramnios.
  • A single fluid pocket >8 cm in vertical height indicates polyhydramnios.
  • The fetus is observed to float in the excessive fluid (Fig. 7.21B).
  • Fluid is seen anteriorly between the fetus and the anterior uterine wall.
Membranes
Chorioamniotic Separation
The amnion is seen separately from the chorion until 16 weeks GA when the two membranes normally fuse (see Chapter 6). Persistent separation of chorion and amnion is a normal variant but may also result from amniocentesis. No morbidity is associated with persistent separation [46].
  • The amnion appears as a thin, undulating membrane suspended in fluid (Fig. 7.22). While the chorion is tightly adherent to the surface of the placenta, the amnion is commonly seen separately over the placenta.
Amniotic Sheets
Amniotic sheets develop over uterine synechiae that cross the uterine cavity. Synechiae result from previous uterine surgery or infection. Membranes drape over the synechiae as the pregnancy develops and the sac enlarges [47, 48].
  • Visualized membranes are thick because they consist of two layers of chorion and two layers of amnion. The membrane forms a shelf-like structure about which the fetus moves freely (Fig. 7.23).
  • Visualization of a free edge is diagnostic. The edge is usually rounded and is thicker than the membrane because it includes the synechiae.
  • A Y-shaped splitting of the membrane is seen at the attachment to the uterine wall as the double layers of chorion and amnion separate [47, 48].
  • No fetal anomalies are associated with amniotic sheets. The risk of malpresentation or poor pregnancy outcome is not increased [49].
Figure 7.22 Normal Chorioamniotic Separation. The amnion (white arrows) is a delicate membrane, uniform in thickness that floats in fluid. The amnion may separate from the surface of the placenta, whereas the chorion is fixed to the placenta. The chorion (black arrows) defines the limit of the fluid-filled gestation sac. The chorionic cavity is between the amnion and chorion. f, fluid in the chorionic cavity.
Figure 7.23 Amniotic Sheet. Amniotic sheets are caused by layers of amnion and chorion folding over a uterine synechiae to form a thick membranous shelf (arrow). The folded membrane always has a thickened free edge. The fetus moves freely on both sides of the shelf.
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Amniotic Band Syndrome
Amniotic band syndrome is a common cause of fetal malformations present in 1 in 1200 births [50]. Disruption of the amnion allows the fetus to enter the chorionic cavity where it becomes entangled by sticky fibrous septa. Entrapment of random fetal parts results in amputations and slash defects that are nonembryological in distribution.
  • Amniotic bands appear as septa of varying thickness that may produce a spider web appearance (Fig. 7.24). Extension of bands to entangled limbs may be visualized. Absence of US visualization of amniotic bands does not exclude the diagnosis of amniotic band syndrome.
  • Extremities are most frequently involved with asymmetric amputations that involve only one digit or the entire limb. Focal constriction may result in marked lymphedema of the peripheral limb.
  • Head defects include asymmetric anencephaly, encephaloceles away from the midline, and facial clefts that extend beyond normal boundaries.
  • Truncal deformities may include the chest and abdomen and resemble gastroschisis with associated angulation deformities of the spine. The distal spine may be amputated.
Figure 7.24 Amniotic Bands. Amniotic bands (arrows) entangle the arm and leg of this fetus, restricting both movement and limb growth.
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Chromosome Anomalies and Biochemical Screening
Approximately 4 infants per 1000 live births have a significant chromosome abnormality [51]. The incidence of chromosome abnormalities is much higher in spontaneous abortions and stillbirths. Aneuploidy is the state of having an abnormal number of chromosomes (more or less than 46). The risk of aneuploidy increases with increasing age. Triploidy refers to the presence of a complete extra set of chromosomes (69 chromosomes). Most triploidies result in spontaneous abortions. Triploidy is associated with partial hydatidiform mole. Trisomy is the state of having three, instead of the usual pair, of any one chromosome. Trisomy implies aneuploidy with 47 chromosomes. Common trisomies are trisomy 21 (Down’s syndrome), trisomy 18 [52], and trisomy 13 [53].
Initial screening for chromosome abnormalities was based on maternal age alone [54]. The risk of Down’s syndrome is 1 in 910 at age 30 and progressively increases to 1 in 110 at age 40. At age 37 the risk is 1 in 240, which is approximately equal to the risk of fetal loss associated with amniocentesis, approximately 1 in 200. If amniocentesis is offered to all pregnant women age 35 and older, approximately 40-60% of Down’s syndrome fetuses will be detected [54]. Additional efforts at detection of chromosome anomalies are based on biochemical screening of maternal serum and on US examination.
Maternal Serum Alpha-Fetoprotein
Biochemical screening for fetal abnormalities initially focused on alpha-fetoprotein (AFP) as a marker of neural tube and other fetal anatomic defects [55]. AFP is a glycoprotein made initially in the yolk sac and later by the fetal liver. MS-AFP levels vary with GA and peak at 28-32 weeks. Fetal tissue not covered by skin leaks AFP into the amniotic fluid where it is absorbed into the maternal bloodstream. Levels of AFP in maternal blood that exceed 2.5 multiples of the median (MOM) for GA are considered abnormal for singleton pregnancies. Levels >4.5 MOM are abnormal for multiple fetus pregnancies. MS-AFP screening detects 98% of open spina bifida defects and anencephaly. See Box 7.4 for a summary of abnormalities associated with elevated MS-AFP.
Low levels of MS-AFP (0.63-1.0 MOM) are associated with increased risk of Down’s syndrome. However the sensitivity of low MS-AFP for Down’s is only approximately 21%. This has led to the use of “triple marker” maternal serum screening.
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Triple Marker Screening
Expanded AFP, or triple marker, screening measures the levels of AFP, human chorionic gonadotropin, and unconjugated estriol in maternal serum. Race, ethnicity, maternal age, and accurate determination of GA are used to determine the risk level for chromosome abnormality.
Sonographic Screening for Down’s Syndrome
US findings associated with Down’s syndrome are summarized in Box 7.5. The presence of an abnormal nuchal fold, a “hard” marker, or two soft markers is considered an indication for parental counseling and consideration of amniocentesis for chromosome analysis [56, 57]. Use of sonographic markers in a high-risk population can identify up to 75% of fetuses with Down’s syndrome [58]. A number of centers recommend against counseling if only one “soft” marker is present because of the high price of parental anxiety for what is most likely to be an insignificant problem [54, 59].
  • Nuchal thickening is one of the strongest predictive signs of chromosome abnormality. Nuchal thickness is measured on the transcerebellar view. Nuchal thickness >3 mm in the first trimester [60] or >6 mm in the second trimester is associated with the presence of chromosome anomalies (Fig. 7.25) [61].
Multiple Pregnancy
Mortality of twins is 15% higher than mortality of singleton pregnancies. The incidence of congenital anomalies is increased 4-fold in twin pregnancies. In addition, twins may experience a variety of disorders that are unique to twinning.
Approximately 1 in 80-90 deliveries involves twins. Triplets are much less common, approximately 1 in 6400 births. Quadruplets occur in 1 in 512,000 births.
Figure 7.25 Nuchal Thickening. A. On this 9-week fetus, the nuchal lucency (arrow) measures 4 mm. This fetus was proven to have Down’s syndrome on amniocentesis. B. On this 20-week fetus, the nuchal thickness (between cursors, +) measures 9 mm. This fetus has Turner’s syndrome (XO).
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Placentation
The risks of twin pregnancies are related to the type of twinning [62]. Dizygotic twins are always dichorionic, diamniotic. The placentation of monozygotic twins depends upon the timing of the division of the fertilized ovum that results in twinning. When division is early (first 3-4 days), the twins are dichorionic, diamniotic (1/3). When division occurs at 4-7 days, the twins are monochorionic, diamniotic (2/3). When division occurs after 8 days, the twins are monoamniotic (<1%) and at highest risk for adverse outcome. Division later than 13 days results in conjoined twins.
  • In the first trimester, the presence of a thick membrane separating the two sacs indicates dichorionic twins. If the membrane is thin or not seen, the pregnancy is monochorionic. Because the amnion membrane separating the twins may not be evident, amnionicity cannot be reliably determined until the second trimester. The presence of two yolk sacs indicates diamniotic twins. If only one yolk sac is present, the pregnancy is monoamniotic [63].
  • In the second trimester the pregnancy is dichorionic if the twins are of different gender or if two separate placentas are clearly visualized.
  • A single placental mass may consist of two abutting placentas or a single placenta that supplies both twins. A thick membrane, consisting of two chorions and two amnions, separating the twins indicates dichorionic twinning. A thin membrane, consisting of two layers of amnion, separating the twins indicates monochorionic, diamniotic twins. An easy-to-see membrane of 1-2 mm is considered “thick.” A difficult-to-see membrane is “thin.” Differentiation of thin and thick is obviously subjective and not very accurate (Fig. 7.26).
  • The “twin-peak sign” is more definitive [64]. A beak-like tongue of placenta protrudes between the two double-membranes of dichorionic diamniotic twins (Fig. 7.26A). The single chorion of a monochorionic pregnancy prevents this protrusion of placenta between membranes.
  • Visualization of an amnion in the second trimester is definitive for diamniotic pregnancy. However, when the amnion is not visualized it still may be present. Diagnosis of monoamniotic twinning requires additional findings such as entangled umbilical cords or fetal parts (Fig. 7.26C).
Figure 7.26 Membranes–Twin Peak Sign. A. Image where the membrane (small arrow) that separates twins joins the placenta shows the “peak” of placental tissue (large arrow) that protrudes between the membrane layers. This is, therefore, a “thick” membrane of two layers of chorion and two layers of amnion in a dichorionic diamniotic twin pregnancy. Compare to the “thin” membrane (arrow) in B and note the minimal difference in apparent thickness. The apparent thickness of the membrane is determined more by US physics and angle of the US beam than by the physical thickness of the membrane. B. This membrane (arrow) of a monochorionic diamniotic twin pregnancy varies little in apparent thickness compared to A. Note that the membrane is fairly closely applied to the head of one twin, simulating a cystic hygroma; however, the characteristic midline septum is not present. Membranes may simulate nuchal lucencies and cystic hygroma. C. Monoamniotic twins have no separating membrane and demonstrate intermingling of fetal parts and umbilical cords. Monoamniotic twins have the highest rate of complications.
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Growth of Twins
Twins closely parallel the growth of singletons in the first and second trimesters, so the same growth charts can be used [65, 66]. In the third trimester, the weight gain of normal twins slows and charts specific to twins may be used for more accurate assessment. IUGR affects approximately 25% of twin pregnancies.
  • GA is determined at the first US examination by averaging the composite GA of the twins, provided they are not grossly discordant. Subsequent US examinations are compared to the first examination for growth. Once established, the GA of the twins is not changed [62]. If the first US examination is performed late in pregnancy, FL measurements appear to be most reliable for determining GA.
  • A 15% difference in EFW or AC between the twins is considered evidence of significant growth discordance.
Figure 7.27 Twin-Twin Transfusion Syndrome–Stuck Twin. The donor twin suffering from twin-twin transfusion syndrome is trapped in a small sac with almost no amniotic fluid by a tightly adherent membrane (arrow). The recipient twin floated in a huge volume of amniotic fluid. H, head; T, trunk of the donor twin.
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Twin-Twin Transfusion Syndrome
In monoamniotic twin pregnancies, vascular communications exist in the placenta between the two twins. If a large arterial to venous shunt exists, blood and nutrients are progressively transfused between a donor twin and a recipient twin. The donor twin becomes anemic and growth restricted. The recipient twin becomes hypervolemic and hydropic [67]. This condition affects 15-30% of monochorionic pregnancies and is commonly fatal to both twins (40-87%) [62].
  • Significant (>20%) difference in EFW between the twins is caused by growth restriction of the donor twin.
  • The size and amniotic fluid volume of the two sacs are greatly different. In some cases the donor twin appears “stuck” to the wall of the uterus (Fig. 7.27). This twin experiences severe oligohydramnios caused by diminished urine output and its movement is greatly restricted by adherent amnion. The stuck twin is commonly fixed in position in a nondependent portion of the uterus. The recipient twin floats freely in polyhydramnios.
  • The recipient twin commonly shows evidence of congestive heart failure or hydrops.
  • Doppler shows a marked difference in the umbilical artery spectrum of the twins with the growth-restricted donor twin showing high resistance [68].
Intrauterine Demise of One Twin
Death of one twin may occur at any time during gestation. Death late in gestation carries increased risk to the surviving fetus.
  • Loss of one twin early in pregnancy is very common, affecting as many as 50% of early twins. Often one twin will “vanish” with no residual evidence of its presence. A blighted twin is evidenced by the presence of a recognizable second fetus without a heartbeat. The non-viable twin may persist in the uterus throughout gestation as a flattened fetal remnant (fetus papyraceous).
  • The live twin may lose blood to the dead twin resulting in hypovolemic shock and death.
  • The dead twin may embolize necrotic tissue to the live twin resulting in disseminated intravascular coagulation, tissue ischemia, and infarction. This is called twin-twin embolization syndrome. This syndrome occurs only in monochorionic twins with significant vascular intercommunications in the placenta.
Twin Reversed Arterial Perfusion Sequence
Intraplacental shunts result in pairing of arteries of one twin to the arteries of the other twin and vein-to-vein pairing between the twins. Preferential perfusion to the lower body of one twin results in absence of development of the upper body and absence of the heart (acardiac twin).
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Blood flow is reversed in the acardiac twin while the “pump” twin experiences high output cardiac failure. If the acardiac twin is large (>70% EFW of the co-twin), the co-twin will usually die.
Figure 7.28 Acardiac Twin. The acardiac twin (larger arrow) is grossly deformed with no identifiable head or heart. The normal twin (smaller arrow) shows no evidence of hydrops or cardiac decompensation at this stage of pregnancy.
  • No beating heart is present in the severely deformed acardiac twin (Fig. 7.28).
  • Doppler shows umbilical artery flow toward and umbilical vein flow away from the acardiac twin. The blood flow to the acardiac twin is deoxygenated by flow through the pump twin. In most cases only a single umbilical artery perfuses the acardiac twin.
  • The head and upper body of the acardiac twin is either absent or shows major anomalies of development.
  • The pump twin is usually anatomically normal and will survive if the strain on its heart is not excessive.
Normal Fetal Anatomy
Brain
Three image planes provide effective US screening of the brain for anomalies [69].
  • The transthalamic plane is used to measure the BPD and HC (Figs. 7.1, 7.3). Anomalies of head shape and head size are evident on this image plane. The third ventricle is routinely identified on this image plane [70].
  • The transventricular plane is used to assess the size and appearance of the lateral ventricles (Fig. 7.29). It is an axial plane through the fetal cranium at the level of the atria of the lateral ventricles. The diameter of each atrium is measured on this plane and remains unchanged throughout the second and third trimesters. Normal atria measure 7-8 mm in diameter and do not exceed 10 mm throughout pregnancy. The echogenic choroid plexus nearly completely fills the atria. More than 3 mm of separation of the choroid plexus from the medial wall of the lateral ventricle is a sign of ventriculomegaly.
  • The transcerebellar plane provides a standardized image for assessment of the posterior fossa (Fig. 7.30). This plane is an axial plane obtained by angling the transducer 10-15 degrees posteriorly and inferiorly from the transthalamic plane. The anatomic landmarks of this plane are the posterior aspect of the third ventricle and the prominent cerebellar hemispheres and vermis outlined by fluid in the cisterna magna. Demonstration of normal size and appearance of the cisterna magna virtually excludes the presence of lumbosacral meningomyelocele. The width of the cisterna magna is measured in the
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    midline from the inner table of the occiput to the posterior aspect of the vermis. The normal cisterna magna measures 2-11 mm in diameter. A small cisterna magna (<2 mm) suggests a Chiari II malformation. A large cisterna magna (>11 mm) suggests Dandy-Walker malformation, cerebellar hypoplasia, arachnoid cyst, or maybe a normal variant “mega-cisterna magna.” Echogenic lines that cross the cisterna magna have been shown to represent bridging arachnoid septations. A cyst-like configuration of these septa is a normal variant of no significance [71].
Figure 7.29 Normal Transventricular Plane. A. The echogenic choroid plexus nearly completely fills the lateral ventricle at the atrium (between cursors, +). Note that only the downside ventricle can be visualized. The upside ventricle is obscured by shadowing and reverberation artifact from the near skull. B. The upside ventricle can be imaged by angling the transducer slightly from a direct axial plane.
Face and Neck
  • By 13-14 weeks GA, a sagittal profile view (Fig. 7.31A) will show normal features of the fetal face including nose, maxilla, mandible, and orbits [72].
  • A coronal view shows the lips and mouth (Fig. 7.31B). The normal depression, called the fulcrum, in the upper lip beneath the nose should not be mistaken for a facial cleft. Tooth buds are seen as echogenic structures within the maxilla and mandible. The tongue is seen to move within the open mouth.
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  • Axial views show the orbits (Fig. 7.31C). The distance between the medial and lateral walls of the orbits can be measured and compared to charts related to GA to diagnose hypo- and hypertelorism.
  • The neck is well seen on axial (Fig. 7.32) and longitudinal views [73]. Landmarks are the larynx, thyroid gland, and pulsating carotid arteries.
Figure 7.30 Normal Transcerebellar Plane. By angling the transducer posteriorly from the transthalamic plane, the posterior fossa, cerebellum (between cursors, +), and cisterna magna (between cursors, x) can be visualized and measured. This same plane is used to measure nuchal thickness from the outer aspect of the cranium to the surface of the skin (arrow). The width of the cerebellum in millimeters is approximately equal to gestational age in weeks.
Figure 7.31 Normal Fetal Face. A. A midline sagittal view shows the profile of the fetal face, including the forehead, nose, lips, and mandible. B. A coronal view shows the chin, lips, and nose. Swallowing and tongue motion can be observed on this view. C. An axial view of the orbits is used to diagnose hypo- and hypertelorism by measuring the distance (between cursors, +) between the orbits. The shape, size, and symmetry of the orbits (arrows) and eyes are also assessed. This infant has hypertelorism of unknown cause.
Spine
  • Ossification of the fetal spine occurs in three prominent ossification centers; paired dorsal centers that will become the lateral masses and posterior arch and a single central ventral center that will become the vertebral body (Fig. 7.33) [74].
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  • Transverse views allow simultaneous imaging of all three ossification centers, whereas sagittal and coronal views provide images of only two centers at a time but allow visualization of many vertebral segments (Fig. 7.33). Transverse images are best for detection of subtle spina bifida anomalies whereas sagittal and coronal images are best for demonstration of scoliosis, hemivertebrae, and disorganized vertebral development. Normal dorsal ossification centers converge toward each other on transverse views. Divergence is evidence of spina bifida. Longitudinal views are inspected for normal thoracic kyphosis and lumbar lordosis [74].
Figure 7.32 Normal Neck. Axial view of the fetal neck shows the centrally positioned spine (S), the fluid-filled trachea (large arrow), and the carotid artery (small arrow). Vascular pulsations in the carotid arteries are prominent on real-time imaging.
Figure 7.33 Normal Spine. A. Transverse view of the spine at the level of L5 vertebra shows the three ossification centers, two (large arrows) for the posterior elements and one anteriorly for the vertebral body. With advancing GA, the two posterior ossification centers elongate and converge on each other. When spina bifida is present, the posterior ossification centers diverge. Note that the skin (arrowhead) is intact over the spine. The iliac wings (skinny arrows) are prominent landmarks of the bony pelvis. B. Transverse view of L4 vertebra shows converging posterior ossification centers and intact skin (arrowhead). The hypoechoic paraspinal muscles (arrows) are well seen. C. Longitudinal view of the spine in coronal plane shows normal tapering of the distance between posterior elements toward the sacrum.
Chest
The chest cavity is defined by the shadowing ribs and the dome-shaped, thin, hypoechoic muscle of the diaphragm.
  • On axial view the heart occupies approximately one-third of the cross-sectional area of the thorax (Fig. 7.34A).
  • Normal fetal lungs surround the heart and have homogeneous echogenicity approximately equal to the liver. Color Doppler will show the vascularity of the lungs.
  • The normal esophagus may be seen in the lower neck and posterior thorax as a multilayered tubular structure [75].
Figure 7.34 Normal Chest and Heart. A. Axial 4-chamber view of the heart clearly shows both cardiac atria and ventricles, interventricular and interatrial septa, and the atrioventricular valves. See Box 7.6 for a description of normal features of this view. R, fetal right side. L, fetal left side. B. The left ventricular outflow tract view shows the left ventricle (white arrow) and root of the aorta (black arrow). C. The right ventricular outflow tract view shows the right ventricle (white arrow) and the main pulmonary artery (black arrow).
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Heart
Routine screening of the fetal heart for anomalies of the cardiovascular system and thorax includes the 4-chamber view and views of the right and left ventricular outflow tracts (RVOT and LVOT) [76, 77]. These three views will identify the majority (83%) of cardiac anomalies [78].
  • The 4-chamber view is an axial section through the lower thorax and heart (Fig. 7.34A) [77, 79]. The cross-sectional area of the normal heart occupies approximately one-third of the cross-sectional area of the thorax on this view. The cardiac axis is evaluated by drawing one line to connect the spine and sternum and a second line through the interventricular septum. The normal cardiac axis is 45 degrees to the left (normal range = 22-75 degrees). Situs is determined by evaluating fetal position and confirming that both the cardiac apex and the stomach are on the left. The right and left ventricles are equal in size in the fetus and are separated by a muscular septum of uniform thickness. The ventricular septum has a normal thin membranous portion near the atrio-ventricular valves that should not be mistaken for a ventricular septal defect. The moderator band of muscle near the apex of the right ventricle is commonly prominent. The papillary muscles are occasionally brightly echogenic. Echogenic foci in the ventricular chambers may be associated with chromosome anomalies as previously discussed. The atrioventricular valves are inspected for symmetry of size and motion. Normal features of the 4-chamber view are listed in Box 7.6 [77, 79, 80].
  • The LVOT view (Fig. 7.34B) is obtained by rotating the transducer approximately 90 degrees from the 4-chamber view to visualize the aorta exiting from the center of the heart. The left ventricle, aortic valve, and ascending aorta are well visualized on this view [77].
  • The RVOT view (Fig. 7.34C) is obtained by rotating the transducer approximately 90 degrees from the 4-chamber view in the opposite direction from that used to obtain the LVOT view. At the same time that the transducer is rotated, it is also angled cephalad. The pulmonary artery exits the heart anterior to the aorta. The RVOT view shows the right ventricle, pulmonic valve, and main pulmonary artery [77].
  • The size of the pulmonary artery should be approximately equal to the size of the aorta. Distinct differences in size of the great vessels suggests atresia or cardiac shunts with asymmetric volumes of blood passing out of each side of the heart.
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  • If the pulmonary artery is not clearly anterior to the aorta, transposition of the great vessels must be suspected.
  • M-mode US is used to document fetal heart rate and rhythm (Fig. 7.35). If the fetal heart rate is below 100 bpm, above 180 bpm, or is irregular, M-mode tracings should be obtained and compared to both the atrial rate and the ventricular rate. Doppler is not recommended for this indication because of the high-energy settings required for Doppler US.
Figure 7.35 M-Mode Documentation of Fetal Heart Rate. The direction of the M-mode US beam can be steered by the operator and is indicated by the dotted line (arrow). In this case, the beam is directed through both the right ventricle and the right atrium of this fetal heart seen in 4-chamber view. The M-mode tracing shows an identical heart rate for the atrium and the ventricle at 137 beats per minute.
Abdominal Wall
  • The umbilical cord enters the fetus at the umbilicus and diverges immediately into the two umbilical arteries, which extend caudally, and the umbilical vein, which courses superiorly and dorsally.
  • The umbilical arteries course from their origin on the internal iliac arteries around both sides of the bladder and along the anterior abdominal wall to the umbilicus. A single umbilical artery (two-vessel cord) is easily confirmed by observing that only one umbilical artery is seen adjacent to the bladder.
  • The umbilical vein crosses the peritoneal cavity in the free edge of the falciform ligament to enter the liver and join the left portal vein. Oxygenated blood is divided equally between liver via the portal circulation and the IVC via the ductus venosus.
  • Midgut herniation into the base of the umbilical cord is normal between 8 and 12 weeks GA. US shows a round or oval echogenic mass 4-10 mm in size in the base of the umbilical cord.
  • The cord insertion site should be inspected on every second and third trimester examination (Fig. 7.36).
  • The abdominal wall musculature may be mistaken for ascites [81]. Abdominal wall muscles create a sonolucent band of uniform thickness that merges with the rib ends. No fluid is seen within the peritoneal recesses.
Abdomen–Gastrointestinal
  • The fetal stomach is visualized as a fluid-filled structure (Fig. 7.37) in the left upper quadrant as early as 11 weeks and in 98% of normal fetuses after 12 weeks.
  • The liver occupies most of the upper abdomen. In the fetus the left lobe is larger than the right lobe and commonly extends to the left flank.
  • The gallbladder is seen as an ovoid cystic structure to the right of the intrahepatic umbilical vein.
  • The spleen is seen as a solid organ posterior and to the left of the stomach.
  • In the first and second trimester, bowel is ill defined, somewhat heterogeneous, and moderately echogenic in mid- to lower abdomen.
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  • After 20 weeks, large bowel is seen as a continuous hypoechoic tubular structure in the periphery of the abdomen. Normal size is 3-5 mm at 20 weeks, increasing to a maximum of 23 mm at term [82]. Liquid meconium in the colon may be mistaken for a cystic mass near term.
  • Small bowel is seen more centrally in the abdomen. Meconium becomes increasingly echogenic with advancing GA. Normal loops are <6 mm diameter [82]. Active peristalsis is evident in the third trimester and helps to differentiate small from large bowel.
  • Compared to adults, the fetal abdomen is relatively large and the fetal pelvis is relatively small resulting in the bladder, uterus, and ovaries lying mainly in the abdomen.
Figure 7.36 Normal Umbilical Cord Insertion. Axial view through the fetal abdomen at the level of the umbilicus shows the normal appearance of the umbilical cord insertion (arrow).
Figure 7.37 Normal Fetal Abdomen. Axial view of the abdomen in a 30-week fetus shows the umbilical vein (short arrow) passing through the liver (l) to enter the left portal vein (tiny arrow). A normal, prominent fetal adrenal gland (open arrow) is apparent. The fetal stomach (curved arrow) is fluid filled. The spleen (S) occupies the left upper quadrant posterior and lateral to the stomach.
Figure 7.38 Normal Kidneys. Long axis, A, and transverse, B, views of the fetal kidneys (arrows) show prominent corticomedullary differentiation and mild pelviectasis. Mild dilatation of the fetal renal pelvis is a common normal finding. The dilatation will commonly disappear when the fetus empties its bladder. Note the prominent shadow of the spine on the transverse view. Abnormal structures that touch the fetal spine are likely renal in origin.
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Abdomen–Genitourinary Tract
  • The fetal adrenal glands are 20 times their relative size in the adult and form prominent masses above the kidneys. The inner medulla is echogenic and the thick outer cortex is hypoechoic (Fig. 7.37).
  • Kidneys are seen as distinct structures at 14 weeks and show characteristic internal morphology by 18-20 weeks. The kidneys are identified by their characteristic shape and their position adjacent to the spine (Fig. 7.38). Fetal lobulation becomes more prominent with advancing GA. The pyramids are lucent and may be mistaken for hydronephrosis. The kidneys grow throughout gestation with normal fetal renal length in mm being approximately equal to GA in weeks.
  • Ureters are not seen unless they are dilated. Visualization of a fetal ureter indicates the ureter is dilated.
  • The urinary bladder may be observed to fill and empty every 30-45 minutes. When distended, its wall is very thin.
  • Amniotic fluid volume beyond 16 weeks reflects the volume of urine production and excretion.
Gender Determination
Although fetal gender is probably the most common question asked by prospective parents, answering the question correctly may be difficult [83].
  • In approximately 30% of cases, the perineum is not adequately visualized.
  • Seeing the testes in the scrotum is reliable in confirming a male fetus; however, the testes do not descend to the scrotum until 28-34 weeks (Fig. 7.39).
  • The labia may be enlarged by circulating maternal hormones and may closely resemble the scrotum (Fig. 7.39). A hypertrophied clitoris may mimic the penis.
Figure 7.39 Boy or Girl? A. The penis (larger arrow) and scrotum (smaller arrow) are obvious as this boy cooperatively spreads his legs. The umbilical cord (UC) may extend into the perineum and simulate a penis. Doppler makes differentiation easy. B. The labia (arrow) of a girl are commonly prominent because of stimulation by maternal hormones. The labia may be mistaken for a scrotum. UC, umbilical cord. C. Testes (arrow) are seen in the scrotum providing clear evidence of gender in this 30-week fetus. Gender may be impossible to determine if the fetus keeps its legs tightly together.
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Skeletal System
The high reflectivity of bone makes the skeletal system easy to visualize with US. However, complete examination is difficult because of wide variations in fetal position and orientation of the limbs. The sonographer is completely at the mercy of the fetus. However, with experience the skeletal system can usually be comprehensively evaluated.
  • Identify individual bones by anatomic continuity, not just by position. A limb adjacent to the fetal head may be either an arm or a leg.
  • Image the short axis of a limb to confirm whether one or two bones are present. The presence of two bones differentiates the forearm and leg from the upper arm and thigh. Bones that begin and end at the same level are the tibia and fibula. The ulna is longer than the radius and extends more proximally than the radius at the elbow.
  • Make sure that both upper and lower extremities are examined and that the same extremity is not examined twice because the fetus has moved.
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  • Measure the greatest length of the echogenic surface of each long bone and refer to appropriate charts that relate bone length to GA to determine if measurements are within normal range.
  • Documentation of normal opening and closing of the fetal hand is more important than counting the number of phalanges (Fig. 7.40).
Figure 7.40 Normal Hand. Bones, even tiny ones like the phalanges, are highly echogenic and easy to visualize. This baby is celebrating its peaceful life in utero.
Fetal Anomalies
Hydrops
Fetal Hydrops
Fetal hydrops is defined as the abnormal accumulation of serous fluid in at least two body cavities or tissues. Causes of hydrops are usually classified as immune and nonimmune. Immune hydrops is caused by maternal antibodies that cross the placenta to attack and hemolyze fetal red blood cells, resulting in severe fetal anemia, tissue hypoxia, and eventually fetal death (erythroblastosis fetalis). The majority of cases are associated with maternal sensitization to Rh factor from previous pregnancy. Nonimmune hydrops is the terminal stage of many severe fetal anomalies. Most cases in the United States are caused by severe congenital heart disease, congenital infections, and chromosome abnormalities.
  • US diagnosis is based on the presence of two or more of the following abnormalities: ascites, pleural effusion, pericardial effusion, and subcutaneous edema.
  • Placental edema may be present late in the course of hydrops. The placenta is thickened (>4-5 cm) and diffusely decreased in echogenicity.
  • Additional signs of fetal distress include abnormal biophysical profile and abnormal umbilical artery Doppler.
Brain
Anencephaly
Anencephaly is the most common of the neural tube defects reported in 1 in 1000 births. Anencephaly is uniformly fatal, although some infants may survive several months [84].
  • Absence of the cranium and cerebral hemispheres above the orbits is the characteristic finding of anencephaly (Fig. 7.41).
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  • The brainstem, midbrain (mesencephalon), and base of the cranium are typically present.
  • A variable amount of disorganized soft tissue commonly protrudes from the cranial defect. This vascular (angiomatous) stroma is called area cerebrovasculosa.
  • Polyhydramnios is present in most cases after 25 weeks GA.
Figure 7.41 Anencephaly. A. The cranium and cerebral cortex (large arrow) are obviously absent on this sagittal view of the fetus. Small arrow indicates the orbit. B. Coronal view of the same fetus shows the spine ending abruptly at the flattened cranium. Disorganized brain tissue (arrow) floats freely not covered by bone.
Cephalocele
The term cephalocele describes herniation of brain, meninges, or cerebrospinal fluid (CSF) through defects in the cranium. Prognosis depends on the size of the defect and the presence of brain in the herniated tissue [85].
  • Encephalocele describes herniation of brain and meninges through a cranial defect (Fig. 7.42). The herniated mass is solid and shows a gyral pattern contiguous with intracranial brain.
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  • Meningoencephaloceles involve herniation of brain tissue, CSF, and meninges. The mass has cystic and solid elements.
  • Meningocele is less common (10%) and describes herniation of meninges and CSF without brain tissue. The herniated mass is purely cystic and is defined by its covering of meninges.
  • The cranial defect is characteristically midline and may be occipital (75%), frontoethmoid (13%), or parietal (12%).
  • Hydrocephalus and microcephaly are usually present.
  • Cephaloceles in atypical location, asymmetric and not in the midline, are often caused by amniotic band syndrome.
  • Cephaloceles are found in a variety of multiple anomaly syndromes including Meckel-Gruber syndrome.
Figure 7.42 Encephalocele. A. Occipital encephalocele. Axial image shows a large posterior protrusion of brain tissue (arrows) through a defect in the occiput. The biparietal diameter (between cursors, +) is smaller than normal for gestational age. B. Frontoethmoid encephalocele. Polypoid masses of brain tissue (arrows) protrude through a frontoethmoid cranial defect seen on this axial image.
Ventriculomegaly
The term ventriculomegalyrefers to abnormal enlargement of the ventricles. The term hydrocephalus should be reserved for cases of ventriculomegaly that are caused by obstruction to the flow of CSF. Non-obstructive causes of ventriculomegaly include brain maldevelopment and diffuse brain atrophy %(Box 7.7).
  • The lateral ventricles are considered to be enlarged when the width of the ventricular atrium exceeds 10 mm on the standard transventricular plane image (Fig. 7.43A) [86]. Ventriculomegaly is differentiated from hydranencephaly and holoprosencephaly by the presence of the falx and a distinct rim of peripheral cortex (Fig. 7.43B).
  • Separation of the choroid plexus from the medial wall of the lateral ventricle ≥3 mm is another widely accepted sign of ventriculomegaly (Fig. 7.43A) [87]. This measurement is also made on the transventricular plane image.
  • A choroid angle of >29 degrees is indicative of ventriculomegaly. The choroid angle is measured between the midline and the long axis of the choroid plexus on the transventricular view. The normal choroid angle is between 16 degrees and 22 degrees.
  • Aqueduct stenosis is a common cause of hydrocephalus with enlargement of the lateral and third ventricles. Aqueduct stenosis may be inherited (X-linked recessive) or caused by tumor, infection, or hemorrhage. Many cases are not evident until late second or third trimester.
  • Ventriculomegaly is commonly associated with additional anomalies. The diagnosis of ventriculomegaly is an indication for careful and detailed fetal survey.
Chiari II Malformation
Chiari II malformation accompanies nearly all cases of lumbosacral myelomeningocele. The posterior fossa is small and the cerebellum is squeezed upward against the tentorium, downward through the foramen magnum, and anteriorly around the brainstem.
Figure 7.43 Ventriculomegaly. A. Axial view of an 18-week fetal head shows dilatation of the ventricular atrium (between cursors, +) to 11 mm. Separation (large arrow) of the choroid plexus from the medial wall of the ventricle exceeds 4 mm. Flattening of the frontal bones (small arrows) creates a lemon-shaped head. This is an example of hydrocephalus associated with Chiari II defect. A myelomeningocele was also present. B. Another fetus has more severe ventriculomegaly caused by idiopathic aqueductal stenosis. Note the presence of a thinned rim of peripheral cortex (arrow) and the falx (arrowhead). These findings differentiate severe hydrocephalus from hydranencephaly and holoprosencephaly.
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  • The lemon sign is seen as a concave deformity of the frontal bones that results in a lemon-shaped outline of the fetal head on axial section (Figs. 7.43A, 7.44A). The lemon sign usually disappears by 34 weeks gestation.
  • The banana sign describes the appearance of the cerebellar hemispheres as they are compressed downward toward the foramen magnum and anteriorly around the brainstem (Fig. 7.44A). Sonographically recognizing obliteration of the cisterna magna is far more important than identifying the banana. Compression of the posterior fossa with absence of fluid in the cisterna magna is a highly specific finding of Chiari II malformation.
  • Hydrocephaly is seen in 80-90% of cases of Chiari II (Fig. 7.43A). The degree of ventricular dilatation is mild in the second trimester and increases in the third trimester.
  • The corpus callosum is partially or completely absent.
Holoprosencephaly
Holoprosencephaly is a spectrum of disorders resulting from failure of cleavage of the forebrain during early development.
  • Alobar holoprosencephaly is the most severe and presents with a large central monoventricle, small brain, fused thalami, and absence of the corpus callosum, cavum septum pellucidum, and falx (Fig. 7.45A).
  • Semilobar holoprosencephaly has partial fusion of the hemispheres and formation of a portion of the posterior interhemispheric fissure and falx.
  • Lobar holoprosencephaly has partial absence of the anterior interhemispheric fissure and absent cavum septum pellucidum.
  • Facial anomalies are commonly associated with alobar and semilobar holoprosencephaly. Abnormalities include midline clefts, cyclopia, cebocephaly, and proboscis (Fig. 7.45B) [88].
Hydranencephaly
Hydranencephaly is believed to result from total cerebral infarction in utero caused by bilateral thrombosis of the internal carotid arteries.
  • Cerebral hemispheres are completely absent (Fig. 7.46).
  • The cranium, falx, skin, meninges, midbrain, and brainstem are present.
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  • No facial anomalies are associated with hydranencephaly.
Figure 7.44 Spina Bifida Defect. A. Axial transthalamic plane image of the fetal head demonstrates the frontal concavities (closed arrows) that cause the lemon sign, and the compression of the cerebral hemispheres that produce the banana sign (open arrow). Absence of a visible cisterna magna (curved arrow) is indicative of Chiari II defect. B. Transverse image of the lumbar spine shows an open spina bifida defect. Note the divergence of the posterior ossification centers (closed arrows) and the absence of skin over the defect (open arrow). C. Longitudinal image of the spine in another fetus shows a myelomeningocele (arrows). D. Clubfoot deformity is commonly present, reflecting the neurologic deficit caused by the spine defect. A clubfoot deformity is diagnosed by US when the long axis of the foot (short arrow) is visible in the same coronal plane that shows the tibia and fibula (long arrow).
Dandy-Walker Malformation
In distinction to the Chiari II malformation, Dandy-Walker syndrome is characterized by a large posterior fossa. Dandy-Walker malformation is a midline posterior fossa cyst associated with absence of the vermis. Dandy-Walker variants are similar but less severe anomalies.
  • The posterior fossa is expanded by cystic dilatation of the fourth ventricle, which exerts mass effect on adjacent structures (Fig. 7.47).
  • The cerebellar hemispheres are hypoplastic and the vermis is hypoplastic or absent.
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  • The size of the posterior fossa is usually normal.
  • Hydrocephalus and agenesis of the corpus callosum are commonly present.
  • Dandy-Walker variant has a normal-sized posterior fossa, lesser dilatation of the fourth ventricle, and lesser degree of cerebellar hypoplasia with no mass effect on adjacent structures.
  • Arachnoid cysts are benign developmental cysts of the arachnoid membrane. The cyst does not communicate with the ventricular system. The vermis and remainder of the cerebellum are normally developed but may be compressed.
  • Mega cisterna magna has a large (>11 mm) cisterna magna with normal size of the posterior fossa and cerebellum. No mass effect is present.
Figure 7.45 Alobar Holoprosencephaly. A. Coronal image through the fetal brain shows a large intracranial fluid space representing a monoventricle. The fused thalami (white arrow) protrude upward into the fused ventricle. Note the presence of a thin rim of cortex (black arrow) peripherally that aids in the differentiation of holoprosencephaly from hydranencephaly. B. Midline sagittal image of the face of another fetus with alobar holoprosencephaly shows absence of the nose, a midline fused eye (straight arrow) representing cyclopia, and a proboscis (open arrow) protruding from above the fused eye. The curved arrow indicates the mouth. Severe facial anomalies are common with holoprosencephaly.
Figure 7.46 Hydranencephaly. Axial image of the fetal head shows fluid replacing all visible brain tissue. No rim of cerebral cortex is present (large arrow). This finding is best evaluated adjacent to the far cranium because reverberation artifact (small arrow) obscures visualization deep to the near cranium. The falx (F) is clearly present.
Figure 7.47 Dandy-Walker Malformation. A. A large cystic cavity (large arrow) fills the posterior fossa. The tentorium is seen as a thin echogenic line (small arrow). B. In another fetus, the posterior fossa (large arrow) is characteristically expanded by a fluid-filled cavity. Hydrocephaly (small arrow) is also present.
Vein of Galen Aneurysm
The primary defect is an arteriovenous malformation (AVM) in cerebral tissue. Marked increased blood flow from the AVM causes striking dilatation of the midline vein of Galen that is the major draining vein of both cerebral hemispheres.
  • The dilated vein of Galen produces a cystic mass just above the tentorium and just posterior to the corpus callosum.
  • Doppler shows turbulent blood flow within the cyst.
  • The AVM is occasionally identified as a region of abnormal vascularity in the cerebral hemispheres.
Choroid Plexus Cysts
Choroid plexus cysts are associated with chromosome abnormalities including trisomy 18, trisomy 21, Turner’s syndrome (XO), and Klinefelter’s syndrome (XXY). Discovery of a choroid plexus cyst is an indication for detailed examination of the fetus to look for additional abnormalities. In the absence of additional findings, trisomy 18 is very unlikely. Genetic counseling and amniocentesis are often offered.
  • Choroid plexus cysts appear as round or oval anechoic cystic masses within the echogenic choroid plexus (Fig. 7.48). Most are <10 mm in size. They may be single or multiple, bilateral or unilateral, multiseptated or unilocular. Nearly all disappear before birth regardless of whether they are found in a normal or abnormal fetus.
Figure 7.48 Choroid Plexus Cyst. Transventricular image shows a 9-mm (between cursors, +) cyst (arrow) in the echogenic choroid plexus.
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Spine
Spina Bifida
Spina bifida refers to failure of normal fusion of the posterior vertebral elements to complete the bony ring of the spinal canal. Defects may occur at any level but are most common in the lumbosacral region. The defect may involve one or several consecutive vertebrae [74].
  • Open spina bifida defects refer to defects that are either uncovered or are covered by a very thin translucent membrane (Fig. 7.44B).
  • Closed spina bifida defects are covered by either skin or a thick opaque membrane.
  • Spina bifida occulta is a minor internal defect of spine closure seen in 2-3% of the population and not detected prenatally. This condition is nearly always asymptomatic.
  • The posterior ossification centers are divergent and often abnormally widely separated (Fig. 7.44).
  • The skin is absent over open spina bifida defects.
  • A myelomeningocele sac protrudes through the bone and skin defect. A thin membrane defines the sac.
  • A myelomeningocele contains both neural elements and CSF.
  • A meningocele contains only CSF.
  • Chiari II malformations are found in association with nearly all (99%) cases of myelomeningocele.
  • Clubfoot deformities and absence of leg movements are commonly associated with large lumbosacral defects (Fig. 7.44D).
  • Parallel posterior ossification centers are commonly seen early in the second trimester caused by incomplete ossification. The soft tissues overlying the spine should be carefully inspected for a mass or skin defect. If no defect or mass is present and the cisterna magna is normal, no spina bifida defect is present.
Face and Neck
Facial Clefts
Facial clefts are the most common anomaly of the fetal face [72]. Many cases are inherited and up to 30% are associated with chromosome anomalies. Many centers offer amniocentesis [73].
  • Lateral facial clefts include isolated cleft lip (25%), isolated cleft palate (25%), and combined cleft lip and cleft palate (50%). The condition may be bilateral (20%).
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  • Cleft lip appears as a linear defect extending from either naris through the lip, just lateral of midline (Fig. 7.49). Nearly all of these defects are detected if coronal views of the lips and nose are obtained.
  • Cleft palate is much more difficult to detect. Cleft palate without cleft lip is almost never detected prenatally. The major finding is a linear defect through the maxilla with abnormal separation of the tooth buds.
  • Median facial clefts are rare and are usually associated with holoprosencephaly. Central facial development is markedly abnormal.
  • Amniotic band syndrome causes irregular facial clefting. An amniotic band may be visualized extending through the lip and into the mouth.
Figure 7.49 Facial Clefts. A. Lateral cleft lip is seen as a linear lucent defect (arrow) in the fetal lip extending into the left nares. The arm overlies part of the face on this coronal image. B. This infant has bilateral cleft lip (larger arrow) and palate that has resulted in premaxillary mass (smaller arrow) of soft tissue beneath the nose and between the clefts.
Cystic Hygroma
Cystic hygroma results from abnormal connections of the lymphatic system [72]. Most cases are associated with chromosome abnormalities, most commonly Turner’s syndrome (XO) and Down’s syndrome (trisomy 21). Amniocentesis is indicated [73].
  • A complex cystic mass symmetrically surrounds the posterior neck and may extend over the head and upper thorax.
  • A midline septum representing the nuchal ligament is always present and helps to differentiate cystic hygroma from occipital meningocele. Additional randomly placed septations are common (Fig. 7.50).
  • Fetal hydrops and diffuse cystic lymphangiectasis are commonly present and are usually associated with fetal demise in utero.
  • Cystic hygromas will commonly be observed to involute with advancing gestation. The residual skin thickening is responsible for the web neck characteristic of Turner’s syndrome.
Chest
Displacement of the fetal heart is a primary US sign of an intrathoracic mass (Boxes 7.8, 7.9) [89].
Figure 7.50 Cystic Hygroma. A. A cystic mass (large arrows) surrounds the posterior neck and occiput. Several septations are evident, one of which (small arrow) is the characteristic midline septum that represents the nuchal ligament. B. In another fetus, a larger cystic hygroma also has the midline nuchal ligament (arrow) and additional septa.
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Pleural Effusion
Most pleural effusions in the fetus are associated with hydrops, are bilateral, and consist of serous fluid. Isolated unilateral pleural effusions are most likely chylothorax associated with malformations of the thoracic duct. These are found as isolated abnormalities, or in association with chromosome anomalies, especially trisomy 21 and Turner’s syndrome (XO).
  • Pleural effusions appear as well-defined anechoic fluid surrounding sharply marginated, triangular-shaped, echogenic lung (Fig. 7.51).
  • Isolated unilateral pleural effusions occur most commonly on the right side and usually are chylothorax.
Congenital Diaphragmatic Hernia
Congenital diaphragmatic hernias occur most often at the posterolateral foramen of Bochdalek (90%), predominantly on the left side (85-90%). The remainder are herniations through the anteromedial foramen of Morgagni. Approximately 50% of affected fetuses have chromosome abnormalities or additional structural defects [90].
  • Hernias appear as a complex cystic mass of displaced bowel in the chest. Peristalsis in the herniated bowel is often seen (Fig. 7.52).
  • The heart is displaced to the right by left-sided hernias.
  • No fluid-filled stomach is seen in the abdomen.
  • Polyhydramnios is often present.
  • The AC is often small for dates. AC below the fifth percentile for GA is associated with large hernias and poor prognosis [91].
  • Large hernias compress the ipsilateral lung directly and compress the opposite lung by displacement of the mediastinum. This commonly results in severe pulmonary hypoplasia.
Figure 7.51 Pleural Effusions. Bilateral pleural effusions (small arrows) are seen as crescentic-shaped fluid spaces compressing the lungs (L). The heart (large arrow) is also seen on this axial plane image.
Figure 7.52 Diaphragmatic Hernia. A. Transverse image through the chest shows the stomach (S) and a large volume of small bowel (straight arrows) occupying the left chest and displacing the heart (curved arrow) into the right thorax. No lung is visible, indicating that severe pulmonary hypoplasia is very likely. B. In another fetus, a moderate volume of lung (L) is visible in the right thorax even though the heart (long arrow) is displaced rightward by the herniation of bowel (short arrows) and stomach (S) into the left thorax.
Figure 7.53 Cystic Adenomatoid Malformation. A. Type I. Markedly echogenic bilateral cystic adenomatoid malformations replace all visible lung. The heart (large arrow) appears compressed by the lung mass. The presence of several large cysts (small arrow) indicates a type I malformation. B. Type III. The left lower lobe (small arrows) is replaced by markedly echogenic cystic adenomatoid malformation. Normal appearing lung (L) is present bilaterally. The heart (arrowhead) is seen anteriorly.
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Cystic Adenomatoid Malformation
Cystic adenomatoid malformation (CAM) is a hamartoma of the lung. Three pathologic types are described. CAM usually involves only one lobe or a portion of one lobe [92].
  • CAM type I consists of one or more large cysts, 2-10 cm in diameter, surrounded by numerous smaller cysts (Fig. 7.53A).
  • CAM type II consists of multiple small cysts (<2 cm) of uniform size.
  • CAM type III consists of innumerable tiny cysts (<5 mm size) that produce the appearance of a solid echogenic mass (Fig. 7.53B).
  • CAM may shrink or disappear in utero. However, CT of the neonatal lung will usually reveal a residual pulmonary abnormality.
  • Pulmonary hypoplasia is a risk when the CAM is large.
  • Hydrops and polyhydramnios may be present.
Pulmonary Sequestration
Pulmonary sequestration is a lung mass separate from the normal bronchial system and supplied by a systemic artery.
  • Identification of the systemic artery supply is diagnostic of pulmonary sequestration. Doppler is used to identify the systemic artery that usually arises from the thoracic or upper abdominal aorta.
  • Intralobar sequestrations (75%) share the visceral pleura with the normal lung and drain via pulmonary veins into the left atrium [93].
  • Extralobar sequestrations (25%) are separated from normal lung by a separate covering of visceral pleura and drain via systemic veins into the azygous or hemiazygous system to the right atrium [94]. Although less common overall, extralobar sequestrations are more likely to present in utero.
  • Approximately 5% of pulmonary sequestrations are below the diaphragm.
  • Sequestrations appear as homogeneous well-defined, usually solid, echogenic masses in the posterior basal thorax, most commonly on the left. Occasionally cystic areas are seen within the mass. The mass is usually triangular in shape.
  • Polyhydramnios and hydrops may occur.
  • Additional developmental defects occur in 15-60% of fetuses with extralobar sequestrations. Foci of CAM type II are found in 15-25% of extralobar sequestrations [92].
Figure 7.54 Bronchogenic Cyst. Transverse scan of the chest shows a well-defined unilocular cystic mass (arrow) in the mediastinum. This is a characteristic appearance and location for a bronchogenic cyst.
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Foregut Duplication Cysts
Foregut malformations cause cystic masses in the thorax.
  • Bronchogenic cysts are unilocular cysts found in the anterior mediastinum or in the lung parenchyma (Fig. 7.54) [95]. They are isolated defects usually not associated with other congenital anomalies. Cysts are sharply marginated and unilocular or multilocular. Echogenic debris may layer within the fluid.
  • Enteric cysts in the thorax are duplications of the esophagus and appear as unilocular cysts in the posterior mediastinum.
  • Neurenteric cysts are attached to the spine and are associated with spinal dysraphism, absent vertebrae, hemivertebrae, and meningomyelocele.
Pulmonary Hypoplasia
Pulmonary hypoplasia is an absolute decrease in lung volume or lung weight for GA. Any process that interferes with distension of the lung with lung fluid or fetal respiratory movements may result in pulmonary hypoplasia. Etiologic factors include severe or prolonged oligohydramnios (bilateral renal disease), chest mass or fluid that compresses the lung, skeletal dysplasia with small thorax, neurological conditions resulting in decreased breathing movements, and chromosomal abnormalities.
  • Determination of lung volume and development is difficult to assess directly.
  • Small thoracic circumference (TC) is associated with pulmonary hypoplasia. TC is measured on the axial plane of the chest that shows the 4-chambered heart. Measurements of TC below the fifth percentile for GA predict pulmonary hypoplasia [96].
  • The ratio of TC/AC normally exceeds 0.80 beyond 20 weeks GA. Smaller ratios suggest pulmonary hypoplasia [97].
  • Shift in cardiac position in the absence of an intrathoracic mass is evidence of unilateral lung hypoplasia (Fig. 7.52) [98].
Heart
The fetal heart is probably the most challenging organ to evaluate with US. Cardiac anomalies are common but quite diverse and complicated. Emphasis should be placed on screening
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for cardiac anomalies with precise diagnosis often left to tertiary centers with highly experienced pediatric cardiologists. A dedicated fetal cardiac examination can easily take as long to perform as a complete fetal survey. Because the examinations may be long and tedious for the parents, as well as the examining physician, each study should be scheduled on a different day.
Figure 7.55 Ectopia Cordis and Cardiomegaly. Four-chamber view of the chest shows a disproportionately large heart, part of which protrudes out of the chest (arrows). A pericardial effusion is also evident (arrowhead). This infant had complex congenital heart disease in addition to multiple anomalies caused by trisomy 18.
Abnormal 4-Chamber View
Approximately 63% of cardiac defects can be detected on the 4-chamber view alone [78].
  • When position of the heart is abnormal, consider an extracardiac thoracic mass, unilateral lung hypoplasia, and situs abnormalities (Fig. 7.52).
  • When heart size is disproportional to size of the thorax, consider cardiomegaly and skeletal dysplasias (Fig. 7.55).
  • When the left ventricle appears small compared to the right ventricle, consider hypoplastic left heart, coarctation of the aorta, and hypoplastic aortic arch [79, 99].
  • When the right ventricle appears small compared to the left ventricle, consider hypoplastic right heart (pulmonary atresia, tricuspid atresia) or aortic stenosis or insufficiency (large left ventricle) [79, 99].
  • Abnormal position of the atrioventricular valves is most commonly caused by Ebstein anomaly with downward displacement of the tricuspid valve, a huge right atrium, and a small right ventricle.
  • An echogenic focus in the ventricular chamber (Fig. 7.56) is caused by mineralization of the papillary muscle but may be associated with chromosome abnormalities [100, 101]. If an echogenic mass is seen within the ventricle consider a rhabdomyoma, which is often associated with tuberous sclerosis (Fig. 7.57) [79].
  • If the heart wall is grossly thickened, consider a cardiomyopathy or endocardial fibroelastosis [79].
  • A ventricular septal defect appears as an opening in the ventricular septum (Fig. 7.58). The normal, thin, membranous septum near the mitral valve may be mistaken for a septal defect.
Abnormal Views of the Great Vessels
Including images of the RVOT and LVOT adds detection of an additional 25% of congenital heart anomalies compared to obtaining only the 4-chamber view [102].
  • When the pulmonary artery is small compared to the aorta, consider tetralogy of Fallot and hypoplastic right heart [76, 103].
  • When the aorta is small compared to the pulmonary artery, consider hypoplastic left heart or coarctation of the aorta [76, 103].
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  • When the position of the great vessels appears abnormal, consider transposition of the great vessels and double outlet right ventricle [76, 103].
  • When only a single, large great vessel is seen, consider truncus arteriosus and tetralogy of Fallot (Fig. 7.59) [76, 103].
Figure 7.56 Echogenic Focus in Left Ventricle. A focus of high echogenicity (arrow) corresponds to the location of the papillary muscles in the left ventricle. This finding is not indicative of a congenital heart defect but may be associated with chromosome anomalies.
Pericardial Effusion
Pericardial effusions are seen with hydrops and with cardiac structural anomalies. Isolated pericardial effusions 2-7 mm in thickness are not associated with adverse outcomes [104].
  • Pericardial fluid is seen between the thin rim of echogenic pericardium and the thin rim of hypoechoic myocardium (Fig. 7.60). M-mode is useful to confirm the presence of pericardial effusion. Normal pericardial fluid is <2 mm in thickness at its point of maximum width. Pericardial fluid >2 mm is considered a pericardial effusion [104].
Figure 7.57 Cardiac Rhabdomyomas. Axial view of the heart reveals multiple echogenic masses (arrows) in the heart. These were proven to be rhabdomyomas in a fetus with tuberous sclerosis.
Figure 7.58 Ventricular Septal Defect. A large defect is evident in the ventricular septum. LV, left ventricle; RV, right ventricle.
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Cardiac Arrhythmias
Arrhythmias are defined as heart rates that are irregular, or too fast, or too slow. Fetuses with arrhythmias deserve complete echocardiography to detect morphological abnormalities. M-mode US is used to determine the atrial and ventricular rates separately [105]. The M-mode cursor must be placed to demonstrate atrial and ventricular motion simultaneously. Ventricular rates are commonly slower than atrial rates because some degree of atrioventricular block is usually present.
  • Irregular rhythms are usually caused by premature atrial or ventricular contractions. Isolated premature ventricular contractions are the most common fetal arrhythmia [105]. Diagnosis is confirmed by an M-mode tracing that shows an early beat followed by a prolonged compensatory pause. Isolated premature contractions are usually innocent. Frequent premature contraction may be induced by maternal smoking, or caffeine or alcohol use. These activities should be discontinued and the fetus rechecked. Premature contractions resulting in tachycardia are treated with digoxin.
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  • Tachycardia is usually defined as fetal heart rate >180 bpm [105]. Causes include supraventricular tachycardia, atrial fibrillation, and atrial flutter. Tachycardia may lead to congestive heart failure, hydrops, and fetal death. Digoxin, administered intravenously through the maternal circulation, is the usual treatment of choice.
  • Bradycardia is defined as fetal heart rate <100 bpm [105]. Transient bradycardia is common and is usually caused by vasovagal reaction, sometimes induced by excessive transducer pressure. Scanning should be temporarily stopped. Heart rate usually returns to normal in 15-30 seconds. Complete heart block is the most common cause of sustained bradycardia. Atrial rates are usually 120-140 bpm, while ventricular rates are 40-60 bpm. Sustained bradycardia may result in hydrops. Prognosis is poor if cardiac malformations are also present.
Figure 7.59 Truncus Arteriosus Anomaly. Only one great vessel (the truncus arteriosus) was shown to arise from the heart primarily from the left ventricle (LV). This solitary vessel continued as the aorta (AO). Both pulmonary arteries (arrows) arose from the truncus arteriosus. RV, right ventricle.
Figure 7.60 Pericardial Effusion. A 4-chamber view of the heart shows a small anechoic pericardial effusion (arrow). The shape and width of the pericardial effusion change with cardiac motion viewed with real-time imaging.
Abdominal Wall
Gastroschisis
Gastroschisis results from a defect in the anterior abdominal wall on the right side of the umbilicus [106]. Bowel and abdominal contents herniate through the defects to float freely in the amniotic cavity [107]. Gastroschisis is nearly always surgically repairable in the postnatal period. Prognosis depends upon presence and severity of other anomalies [108].
  • Free-floating bowel loops are seen outside of the abdominal cavity (Fig. 7.61).
  • No covering membrane is present.
  • The cord insertion is normal. The cord is most easily identified and the insertion site visualized by use of color flow US (Fig. 7.61C). Herniation occurs to the right of the cord insertion site.
  • Large defects may contain liver, urinary bladder, uterus, and adnexa.
  • Gastroschisis is usually (75%) an isolated defect without associated chromosome abnormality or risk of recurrence. Amniocentesis is usually not indicated.
  • Approximately 25% of affected fetuses have other anomalies including cardiac defects, cleft palate, scoliosis, or diaphragmatic hernia.
  • Complications include non-rotated bowel (always present), ischemia caused by rotation and kinking of the mesenteric vessels, perforation, obstruction, and bowel atresia.
  • US demonstration of bowel wall thickening or intestinal dilatation is not a useful predictor of the presence of complications [109].
Figure 7.61 Gastroschisis. A. Bowel loops (small arrow) float freely in amniotic fluid. No covering membrane is present. These are diagnostic findings of gastroschisis. The umbilical vein (large arrow) in the cord courses adjacent to the protruding bowel loops. B. The abdominal wall defect (between cursors, +) is shown on this transverse image. Bowel loops (arrow) extend from the abdomen to the amniotic cavity through this defect. C. Color Doppler, shown here as a gray scale image, shows a normal insertion (small arrow) of the umbilical cord on the abdominal wall. The gastroschisis defect (large arrow) is adjacent to the cord insertion site on the right side.
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Omphalocele
Omphalocele is a midline defect in the anterior abdominal wall through which abdominal contents herniate into the base of the umbilical cord [106,107,108].
  • US shows a midline mass protruding through the abdominal wall (Fig. 7.62). The mass commonly contains liver, stomach, and bowel.
  • A covering membrane produces a smooth, well-defined surface to the mass.
  • The umbilical cord inserts into, rather than next to, the mass (Fig. 7.62B, C).
  • Ascites is common and helps to define the covering membrane.
  • Polyhydramnios is present in one-third of cases.
  • Most cases (67-88%) have associated anomalies including cardiac, central nervous system, urinary tract, and gastrointestinal malformations.
  • Chromosome anomalies, most commonly trisomy 13 or 18, are found in 30-45% of cases. Amniocentesis is usually performed.
  • Pentalogy of Cantrell is the association of an ectopic heart (ectopia cordis) with omphalocele. The anterior wall defect includes the thorax as well as the abdomen. The fetal heart is seen outside of the thoracic cavity (Fig. 7.55) [110].
Figure 7.62 Omphalocele. A. An omphalocele is seen as a rounded mass protruding from the anterior abdominal wall. A covering membrane (short arrow) is clearly visible. Note the characteristic obtuse angle (long arrow) that the omphalocele forms with the abdominal wall. This omphalocele contains liver and bowel. B. In a 14-week fetus, an omphalocele forms a 20-mm mass (arrow) protruding from the anterior abdominal wall. C. Color Doppler image (shown in gray scale) of the same fetus as in B shows the umbilical cord (long arrow) inserting into the mass (short arrow) confirming an omphalocele.
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Amniotic Band Syndrome
Disruption of the amnion allows the fetus to enter the chorionic cavity where fibrous bands form and entangle the fetus. Entrapment of fetal parts results in amputation deformities that range from partial finger amputations to being incompatible with life [50].
  • Amniotic bands trapping the fetus are commonly visualized.
  • Extension of bands across the abdomen results in gastroschisis and truncal defects.
  • Bands across the cranium may result in anencephaly, encephaloceles, and facial clefts.
  • Spine deformities, marked scoliosis, and extremity amputations are common.
Abdomen–Gastrointestinal
Esophageal Atresia
Almost two-thirds of cases of esophageal atresia are associated with additional anomalies including chromosome anomalies (20%) [111].
  • Association of polyhydramnios with non-visualization of a fluid-filled stomach is strong but not perfect evidence of esophageal atresia (Fig. 7.21B).
  • Visualization of a fluid-filled stomach does not exclude esophageal atresia because the stomach may fill with amniotic fluid through a tracheoesophageal fistula.
  • Rarely, a blind-ended pouch of proximal esophagus may be visualized [111].
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Non-Visualization of Fluid-Filled Stomach
The normal fetal stomach is visualized as a fluid-filled structure in the left upper quadrant of the abdomen in most fetuses by 14-15 weeks GA [112, 113].
  • Non-visualization of the stomach is indication for follow-up examination. Look for associated abnormalities. Causes are listed in Box 7.10 [114, 115].
Duodenal Obstruction–The Double Bubble
The duodenum is not fluid-filled in normal fetuses %(Box 7.11) [116]. Approximately one-half of the cases have additional anomalies. Amniocentesis is indicated because of a 30% incidence of Down’s syndrome.
  • “Double bubble” describes the appearance of a fluid-distended duodenal bulb associated with an overdistended stomach (Fig. 7.63). Careful scanning will usually reveal the pylorus connecting the two structures. This finding confirms a true double bubble and is highly indicative of duodenal obstruction.
  • Care must be taken to confirm a true double bubble. Other cystic masses in the upper abdomen include choledochal cyst, renal cyst, bowel duplication, omental or mesenteric cyst, hepatic cyst, and gallbladder.
Echogenic Bowel
Echogenic bowel may be a normal variant or may be associated with significant fetal abnormalities %(Box 7.12) [114, 115].
  • Normal fetal bowel varies from being isoechoic with liver to being moderately echogenic compared to liver.
  • Bowel is abnormally echogenic when its echogenicity is equal to or greater than that of bone. Comparison is often made to the echogenicity of the iliac crest (Fig. 7.64).
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  • When abnormally echogenic bowel is present, parents are routinely offered genetic counseling and opportunity for amniocentesis.
  • High-frequency transducers (8 MHz) increase the apparent echogenicity of bowel compared to lower frequency transducers (5 MHz) [117].
Figure 7.63 Double Bubble–Duodenal Atresia. Image of the fetal abdomen shows a distended stomach (S) in contiguity with a dilated duodenal bulb (large arrow) making the “double bubble” sign of duodenal obstruction. Care must be taken to not mistake the gallbladder (small arrow) for a second “bubble.” This fetus had duodenal atresia and Down’s syndrome.
Small Bowel Obstruction
Dilatation of small bowel is indicative of obstruction %(Box 7.13).
  • Diameter of small bowel >6 mm is considered dilated. Multiple, interconnecting, dilated, small bowel loops indicate small bowel obstruction (Fig. 7.65).
  • Small bowel dilatation is generally not seen until after 16-20 weeks because insufficient meconium is present to distend the small bowel.
  • Polyhydramnios is commonly present.
  • A dilated and tortuous ureter may simulate dilated small bowel loops. Differentiation is made by careful examination of the kidney for hydronephrosis.
  • Normal colon is differentiated from dilated small bowel by its normal more peripheral location.
Meconium Ileus
Meconium ileus is small bowel obstruction caused by abnormally thick meconium in the distal ileum. Most cases (90%) are associated with cystic fibrosis. Approximately 10-15% of fetuses with cystic fibrosis present with meconium ileus [118].
Meconium Peritonitis
Perforation of the fetal small bowel in utero spills meconium into the peritoneal cavity and incites a chemical peritonitis. Approximately 50% of cases are idiopathic, 15-20% are
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caused by small bowel atresia, and 15-40% of cases are associated with cystic fibrosis. Isolated small bowel perforation has a good prognosis [118].
Figure 7.64 Echogenic Bowel. This bowel (large arrow) is considered to be abnormally echogenic because it is equal in echogenicity to the nearby iliac crest (small arrow).
  • Calcifications occur on peritoneal surfaces and appear as linear or punctate bright echoes with or without acoustic shadowing (Fig. 7.66).
  • Ascites is commonly present (Fig. 7.66).
  • Loculations of fluid may form in the peritoneal cavity from the chemical peritonitis. These thick-walled cystic masses are called meconium pseudocysts and may have a calcified wall (Fig. 7.66).
  • Fetal bowel is commonly dilated.
Large Bowel Obstruction
  • Normal colon can be seen as a prominent structure in the periphery of the abdomen as early as 22 weeks. Normal colon can be especially prominent and have echolucent contents in the third trimester [113].
  • Colonic dilatation >23 mm is reliable evidence of colon obstruction %(Box 7.14).
  • Normal meconium-filled rectum may be mistaken for a presacral mass, especially in the third trimester %(Box 7.15) [119].
  • Anal atresia results in dilatation of the colon apparent only late in the third trimester. VATER syndrome refers to the association of vertebral anomalies, anal atresia, tracheoesophageal fistula, radial dysplasia, and renal dysplasia.
  • Meconium plug syndrome describes transient colonic obstruction caused by inspissated meconium. Approximately 25% of patients have cystic fibrosis.
Ascites
Ascites is always abnormal in the fetus. Most ascites is associated with hydrops. Additional causes include bowel perforation and urine ascites resulting from rupture of an obstructed bladder [120].
  • Ascites appears as anechoic fluid surrounding abdominal organs and distending peritoneal recesses. Fluid outlines the falciform ligament and umbilical vein (Fig. 7.67A).
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  • Fluid may accumulate between the leaves of the unfused greater omentum resulting in a cystic intraperitoneal mass.
  • The thin lucent rim of abdominal musculature must not be mistaken for ascites (Fig. 7.67B).
Figure 7.65 Small Bowel Obstruction. Transverse image of the fetal abdomen demonstrates multiple dilated loops of small bowel measuring up to 10-11 mm in diameter. This infant had ileal atresia.
Figure 7.66 Meconium Peritonitis. A. View of the abdomen shows the umbilical vein (large arrow) crossing ascites in the peritoneal cavity. Several calcifications (small arrows) are present that provide diagnostic evidence of meconium peritonitis. B. Image of another fetus shows a large, cystic, abdominal mass with calcifications in its wall. Calcifications seen elsewhere in the abdominal cavity confirm meconium peritonitis and a diagnosis of meconium pseudocyst.
Abdominal Calcifications
Isolated abdominal calcifications without associated abnormality are usually incidental findings associated with a normal fetal outcome (Fig. 7.68) %(Box 7.16).
  • Calcifications appear as echogenic foci with or without acoustic shadowing.
  • Localize the calcifications as being associated with a mass, within an organ, within the bowel lumen, or within the peritoneal cavity %(Box 7.16).
Cystic Abdominal Masses
Cystic masses in the fetal abdomen are virtually never malignant %(Box 7.17).
  • If the cyst touches the spine, renal origin is likely.
  • Ovarian cysts may be huge and appear simple, complex, or septated. Simple ovarian cysts result from excessive stimulation of fetal ovaries by maternal and placental hormones. Most disappear spontaneously by 6 months of age.
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  • Enteric duplication cysts may occur anywhere in the GI tract. They may appear entirely cystic to solid, commonly have thick walls and contain internal debris [111].
  • Mesenteric and omental cysts are solitary, unilocular or multilocular and are generally found as isolated defects.
  • Choledochal cysts are found near the porta hepatis. Intrahepatic bile ducts may be dilated [111].
  • Meconium pseudocysts have calcification in the walls and usually elsewhere in the abdomen (Fig. 7.66B).
Figure 7.67 Ascites and Pseudoascites. A. Ascites. A recipient twin victim of twin-twin transfusion syndrome shows large volume ascites surrounding the liver (L) and outlining the umbilical vein (arrow). B. Pseudoascites. The abdominal wall muscles (small arrows) appear hypoechoic, especially when well seen late in pregnancy. This finding has been mistaken for ascites. Note the thin uniform width of the musculature and the fact that the muscles merge with the ribs (large arrow). Use these findings to differentiate musculature from true ascites, which should be seen in numerous peritoneal recesses.
Figure 7.68 Isolated Abdominal Calcification. Image of the abdomen shows a solitary coarse calcification (large arrow) with acoustic shadowing (small arrow) within the liver. As an isolated finding, this is of no consequence.
Sacrococcygeal Teratoma
The sacrococcygeal region is the most common location for teratomas in children. Additional anomalies are present in approximately 18% of patients [121]. Tumors may be mature benign (60%), immature benign (18%), or malignant (22%). Differential diagnosis of a sacral region mass is presented in Box 7.18.
  • The mass occupies the presacral region and usually extends caudally as an obvious exophytic mass (Fig. 7.69A).
  • The most common appearance is a complex mass of approximately equal cystic and solid components. Echogenic areas represent fat. Calcification or ossification is present in 50%.
  • Some tumors are predominantly cystic, unilocular, or multilocular, with a small solid nodule often present within the cyst.
  • Entirely solid masses are uncommon.
  • The tumor may obstruct the urinary tract.
  • US does not reliably differentiate benign from malignant teratomas.
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  • Polyhydramnios is common. The presence of hydrops caused by arteriovenous shunting within the mass indicates a poor prognosis.
  • A normal, fluid-filled rectum may simulate a pelvic teratoma late in pregnancy (Fig. 7.69B).
Abdomen–Urinary Tract
Because nearly all amniotic fluid after 16 weeks GA is fetal urine, evaluation of the urinary tract starts with assessment of amniotic fluid volume as a measure of fetal urine production.
Renal Agenesis
Bilateral renal agenesis (Potter’s syndrome) is incompatible with life because of the severe pulmonary hypoplasia that results from the lack of amniotic fluid. Unilateral agenesis is 4-20 times more common than bilateral renal agenesis. It has a good prognosis provided the opposite kidney is normal.
  • Severe oligohydramnios is always present with bilateral renal agenesis (Fig. 7.21A).
  • The fetal kidneys are not visualized.
  • The normally large fetal adrenal gland may “lie down” and mimic the appearance of the missing fetal kidney.
  • The bladder is not visualized or is small.
  • If one renal fossa is empty, search for an ectopic kidney.
Hydronephrosis
Hydronephrosis is the most common cause of an abdominal mass in a neonate and is readily detected by prenatal US %(Box 7.19).
  • Significant fetal hydronephrosis is unequivocally present when the anteroposterior (AP) diameter of the renal pelvis is ≥ 10 mm, or when the ratio of AP diameter of the renal pelvis to AP diameter of the kidney is >0.5 (Fig. 7.70A) [122].
  • Calyceal dilatation is a sign of significant hydronephrosis independent of the size of the renal pelvis (7.70B, C).
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  • Patients with significant hydronephrosis are usually followed with periodic US in utero and undergo postnatal evaluation of the urinary tract with renal US, voiding cystourethrography, and radionuclide renography with diuretic challenge. Postnatal renal US should not be performed until after 48 hours of life to allow for sufficient urine production to dilate the obstructed system.
  • Mild pyelectasis in common in normal fetuses. The upper limit of “normal” pyelectasis in a fetus of <20 weeks GA has been defined as 4 mm or 5 mm depending on the publication. This small degree of dilatation is not associated with significant pathology and does not require follow-up [123]. The upper limit of normal pelvis AP diameter is increased to 7 mm after 33 weeks GA [124].
  • Dilatation of the renal pelvis between 5 and 10 mm without dilatation of the calyces remains a source of controversy as to proper management. The incidence of significant urinary pathology at birth is low in this group. Some authors recommend aggressive evaluation and others challenge that approach as not being necessary or cost effective [125, 126].
  • Ureteropelvic junction obstruction is the most common cause of fetal hydronephrosis (Fig. 7.70B, C). The renal pelvis and calyces are dilated but the ureter is not. The degree of pelvicalyectasis is dependent upon the degree of obstruction. Polyhydramnios may occur because of increased production of dilute urine by the affected kidney. Oligohydramnios is uncommon unless the opposite kidney is also abnormal. Rarely, obstruction may be severe enough to rupture the collecting system and result in a perinephric urinoma. These severely affected kidneys usually have minimal renal function.
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  • Ureterovesical junction (UVJ) obstruction is the cause of approximately 23% of fetal hydronephrosis.
  • Megaureter may be caused by distal obstruction (ureterovesical junction), vesicoureteral reflux, or congenital megaureter. These three entities cannot be differentiated by prenatal US. Postnatal testing is required.
  • Complete ureteral duplication shows characteristic findings. The upper pole ureter is commonly obstructed because of its ectopic insertion inferior and medial to the normal bladder insertion of the lower pole ureter. Severe obstruction may result in dysplasia of the upper pole. Ureteroceles appear as a thin-walled cystic mass in or below the bladder.
Figure 7.69 Sacrococcygeal Teratoma. A. Longitudinal image shows a large cystic mass (big arrow) protruding inferiorly from the pelvis. Careful inspection reveals another cystic component of the mass within the sacrum (small arrow) and a solid component of the mass filling the pelvis (arrowheads). B. A fluid-filled rectum (between cursors, +, x) may create a pelvic mass and simulate a sacrococcygeal teratoma. Note the characteristic location anterior to the sacrum and elongated normal shape of the rectum.
Figure 7.70 Hydronephrosis. A. Transverse image shows dilatation of both renal pelvises (arrows). The right pelvis (RT) measures 18 mm in anteroposterior diameter, and the left pelvis (LT) measures 8 mm in anteroposterior diameter. B. Longitudinal image of the kidney shows pelviectasis (long arrow) with mild calyectasis (short arrow) caused by mild ureteropelvic junction obstruction. C. Longitudinal image of the kidney in another fetus shows a greater degree of hydronephrosis. Note the prominently dilated calyces (arrows).
Posterior Urethral Valves
A flap-like structure or web blocks the prostatic urethra causing bladder outlet obstruction. This condition is seen only in males. The condition is easily corrected with surgery but irreversible damage may occur in utero before the diagnosis is made. Catheter placement to drain the obstructed bladder may be performed in utero using US guidance. Females may have similar findings when urethral obstruction is caused by urethral atresia or a cloacal anomaly.
  • The bladder is dilated, frequently massively. The wall undergoes smooth muscle hypertrophy and is thickened and trabeculated.
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  • Ureters are dilated and sometimes thick-walled.
  • The kidneys are hydronephrotic and commonly dysplastic with abnormally echogenic parenchyma and cyst formation.
  • Oligohydramnios may be profound, depending upon the degree of urethral obstruction.
  • Dilatation of the prostatic urethra produces a characteristic “keyhole” appearance to the bladder (Fig. 7.71).
Figure 7.71 Posterior Urethral Valve. The fetal bladder (B) is massively distended. The posterior urethra (arrow) is dilated to the level of the obstructing valve.
Renal Dysplasia
Early (before 20 weeks) severe obstruction results in renal dysplasia. Dysplastic kidneys have disorganized parenchymal development and a marked increase in fibrous tissue [127].
  • Renal cysts in the setting of renal obstruction indicate renal dysplasia (Fig. 7.72) [128].
  • Increased echogenicity of the renal parenchyma indicates dysplasia when the kidney is hydronephrotic.
  • Some normal appearing kidneys may be severely dysplastic.
Figure 7.72 Renal Dysplasia. A. A fetal kidney (between arrowheads) is abnormally small with poor corticomedullary differentiation. Several discrete renal cysts (arrows) are evident, providing strong evidence of renal dysplasia. B. Similar findings are present in a newborn. The kidney is abnormally echogenic with renal cysts (arrows).
Figure 7.73 Multicystic Dysplastic Kidney. Both fetal kidneys (arrows) are completely replaced by cysts of varying size. No amniotic fluid was detected. Bilateral multicystic dysplastic kidneys are a fatal condition. Without amniotic fluid, the fetal lungs will not develop properly. S, spine.
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Multicystic Dysplastic Kidney
  • The kidney is replaced by numerous cysts of varying size that do not communicate (Fig. 7.73).
  • Approximately 40% of cases are associated with anomalous development of the opposite kidney. Anomalies include ureteropelvic junction obstruction, agenesis, and bilateral multicystic dysplastic kidney.
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  • Bilateral multicystic dysplastic kidneys produce no urine, have severe oligohydramnios, and are always fatal.
Figure 7.74 Polycystic Kidney Disease. A. Bilateral large, diffusely echogenic kidneys (arrows) are indicative of autosomal recessive polycystic disease in the fetus. Image is in transverse plane. B. Multiple bilateral discrete renal cysts in enlarged kidneys (arrows) are indicative of autosomal dominant polycystic disease in the fetus. Image is in coronal plane. HT, heart.
Figure 7.75 Mesoblastic Nephroma. Transverse image of the abdomen demonstrates a large solid mass (straight arrow) replacing the kidney. Note that the mass touches the spine (curved arrow), providing excellent evidence that the mass is renal in origin.
Polycystic Kidney Disease
  • Autosomal recessive polycystic disease produces large, very echogenic kidneys (Fig. 7.74A). The numerous very tiny cysts are usually too small to see with US. Amniotic fluid is minimal or absent.
  • Autosomal dominant polycystic disease may produce large echogenic kidneys with multiple cysts (Fig. 7.74B), but amniotic fluid volume is usually normal.
Solid Renal Mass
Mesoblastic nephroma is the most common solid renal mass seen in the fetus. The tumor is a benign hamartoma.
  • A solid heterogeneous infiltrating mass replaces some or the entire affected kidney (Fig. 7.75). When only a portion of the kidney is replaced by tumor, the margin between tumor and normal parenchyma is ill defined.
  • Polyhydramnios is usually present.
  • Wilms tumor is exceedingly rare in the fetus. The tumor is seen as a solid mass with a discreet capsule separating tumor from normal parenchyma.
Skeletal System
Fetal limb anomalies are associated with skeletal dysplasias, exposure to teratogens, and certain metabolic disorders [129]. More than 200 skeletal dysplasias have been described
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and differentiating between them is a daunting task. Fortunately, skeletal dysplasias are rare, affecting only 0.03% of births. Unfortunately, approximately half of all skeletal dysplasias are lethal. Keep several features in mind. If a family has a history of skeletal anomalies and a skeletal anomaly is found in the fetus, the anomaly is very likely to be the same one. Extremely short bones (more than 4 standard deviations from the mean for GA) are indicative of a lethal bone dysplasia %(Box 7.20) [130, 131]. Demonstrating a small fetal thorax is confirmatory of a lethal dysplasia. Precise identification of the lethal syndrome is not necessary for management.
Figure 7.76 Osteogenesis Imperfecta. The femur (between cursors, +) is abnormally thin and obviously fractured in its mid-portion (arrow). Multiple bones were affected in this fetus with a family history of osteogenesis imperfecta.
Approach to Skeletal Dysplasias
Spirt and Mahony provide excellent algorithmic approaches to the diagnosis of skeletal dysplasias [130, 132].
  • A short femur (below fifth percentile for GA) is a highly sensitive but not specific sign of a skeletal dysplasia [133]. If FL is more than 2 standard deviations below the mean length for GA, measure all the long bones and refer to appropriate tables for normal values. IUGR is another common cause of a short FL.
  • If FL is normal and a skeletal anomaly is suspected on the basis of family history, reexamine the fetus at a later date. Limb shortening may be evident only late in pregnancy (after 25 weeks) [129].
  • Evaluate bones for fractures, bowing, and mineralization (Fig. 7.76). Signs of decreased mineralization are decreased echogenicity of bone, decreased or absent acoustic shadowing, poor visualization of the fetal spine, and increased prominence of the falx [130].
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    The brain is seen “too well” (Fig. 7.76). Bones appear abnormally thickened because the US beam is only weakly attenuated.
  • Evaluate for evidence of small thorax. Measure TC and compare to tables for GA. Measure cardiothoracic ratio. Cardiac anomalies are associated with increased cardiothoracic ratio with normal TC. Bone dysplasias have increased cardiothoracic ratio and decreased TC.
  • Evaluate hands and feet for polydactyly [134].
  • Micromelia refers to proportional shortening of all long bones (Fig. 7.77).
  • Rhizomelia refers to shortening of the humerus and/or the femur.
  • Mesomelia refers to shortening of the radius and ulna and/or tibia and fibula. Mesomelic dysplasias are rare and non-lethal.
  • Acromelia refers to shortening of the bones of the hands and feet.
Figure 7.77 Micromelic Dwarfism. Image of the femur (between cursors, +) shows a markedly decreased length for gestational age. Bowing is present. Note how short the femur appears compared to the length of the soft tissues of the thigh.
Figure 7.78 Thanatophoric Dysplasia. A. Coronal view of the fetal brain reveals remarkable detail. The skull is so thin that the US beams penetrate it easily. Excellent visualization through bone is a sign of poor bone mineralization. B. A view of the arm (arrow) shows severe deformity. This is a lethal dysplasia.
Lethal Skeletal Dysplasias
Diagnosis of a lethal, or possibly lethal, skeletal dysplasia is essential to appropriate family counseling and delivery planning. Postnatal evaluation is usually required to confirm a specific diagnosis.
  • Severe bone shortening is a hallmark of a lethal skeletal dysplasia (Figs. 7.77, 7.78) [131].
  • Small thorax results in pulmonary hypoplasia and respiratory insufficiency.
  • Cloverleaf skull deformity is seen with thanatophoric dysplasia.
  • Polydactyly is seen with Jeune syndrome, Ellis-van Creveld syndrome, and short rib-polydactyly syndrome.
  • Fractures, bowing, and severely deficient mineralization are seen with osteogenesis imperfecta, type 2.
References
1. American College of Obstetrics and Gynecology. Ultrasonography in pregnancy. Technical Bulletin 1993;187:1-9.
2. American Institute of Ultrasound in Medicine. Guidelines for performance of the antepartum obstetrical ultrasound examination. Rockville: American Institute of Ultrasound in Medicine, 1991.
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3. American College of Radiology. ACR standards for the performance of antepartum obstetrical ultrasound. 1990, revised 1995.
4. Hadlock FP, Deter RL, Carpenter RJ, et al. Estimating fetal age: effect of head shape on BPD. AJR Am J Roentgenol 1981;137:83-85.
5. Shepard MJ, Richards VA, Berkowitz RL, et al. An evaluation of two equations for predicting fetal weight by ultrasound. Am J Obstet Gynecol 1982;142:47-52.
6. Hadlock FP, Harrist RB, Carpenter RJ, et al. Sonographic estimation of fetal weight: the value of the femur in addition to head and abdomen measurements. Radiology 1984;150:535-540.
7. Hadlock FP, Deter RL, Harrist RB, et al. Fetal biparietal diameter: a critical re-evaluation of the relation to menstrual age by means of real-time ultrasound. J Ultrasound Med 1982;1:97-100.
8. Hadlock FP, Deter RL, Harrist RB, et al. Fetal head circumference: relation to menstrual age. AJR Am J Roentgenol 1982;138:649-653.
9. Hadlock FP, Deter RL, Harrist RB, et al. Fetal abdominal circumference as a predictor of menstrual age. AJR Am J Roentgenol 1982;139:367-370.
10. Hadlock FP, Harrist RB, Deter RL, et al. Fetal femur length as a predictor of menstrual age: sonographically measured. AJR Am J Roentgenol 1982;138:875.
11. Hadlock FP, Deter RL, Harrist RB, et al. Estimating fetal age: computer-assisted analysis of multiple growth parameters. Radiology 1984;152:497-501.
12. Lessoway VA, Schlzer M, Wittman BK, et al. Ultrasound fetal biometry charts for a North American Caucasian population. J Clin Ultrasound 1998;26:433-453.
13. Doubilet PM, Benson CB. Sonographic evaluation of intrauterine growth retardation. AJR Am J Roentgenol 1995;164:709-717.
14. Fong DW, Ohlsson A, Hannah ME, et al. Prediction of perinatal outcome in fetuses suspected to have intrauterine growth restriction: Doppler US study of fetal cerebral, renal, and umbilical arteries. Radiology 1999;213:681-689.
15. Dubinsky T, Lau M, Powell F, et al. Predicting poor neonatal outcome: a comparative study of noninvasive antenatal testing methods. AJR Am J Roentgenol 1997;168:827-831.
16. Woo JSK, Liang ST, Lo RLS. Significance of an absent or reversed end diastolic flow in Doppler umbilical artery waveforms. J Ultrasound Med 1987;6:291-297.
17. Ott WJ. Intrauterine growth restriction and Doppler ultrasonography. J Ultrasound Med 2000;19:661-665.
18. Sickler GK, Nyberg DA, Sohaey R, et al. Polyhydramnios and fetal intrauterine growth restriction: ominous combination. J Ultrasound Med 1997;16:609-614.
19. Finberg HJ, Kurtz AB, Johnson RL, et al. The biophysical profile: a literature review and reassessment of its usefulness in the evaluation of fetal well-being. J Ultrasound Med 1990;9:583-591.
20. Brant WE. Ultrasonography of the placenta. Perspectives in Radiology 1989;2:157-170.
21. Harris RD, Cho C, Wells WA. Sonography of the placenta with emphasis on pathological correlation. Semin Ultrasound CT MRI 1996;17:66-89.
22. Spirt BA, Gordon LP. Practical aspects of placental evaluation. Sem Roentgen 1991;26:32-49.
23. Taipale P, Hilesma V, Ylostalo P. Diagnosis of placenta previa by transvaginal sonographic screening at 12-16 weeks in a nonselected population. Obstet Gynecol 1997;89:364-368.
24. Wong G, Levine D. Sonographic assessment of the cervix in pregnancy. Semin Ultrasound CT MRI 1998;19:370-380.
25. Rosati P, Guariglia L. Clinical significance of placenta previa detected at early routine transvaginal scan. J Ultrasound Med 2000;19:581-585.
26. Hertzberg BS, Bowie JD, Carroll BA, et al. Diagnosis of placenta previa during the third trimester: role of transperineal sonography. AJR Am J Roentgenol 1992;159:83-87.
27. Hertzberg BS, Kliewer MA. Vasa previa: prenatal diagnosis by transperineal sonography with Doppler evaluation. J Clin Ultrasound 1998;26:405-408.
28. Kaakaji Y, Nghiem HV, Nodell C, et al. Sonography of obstetric and gynecologic emergencies: Part I, obstetric emergencies. AJR Am J Roentgenol 2000;174:641-649.
29. Pederson JF, Mantoni M. Prevalence and significance of subchorionic hemorrhage in threatened abortion: a sonographic study. AJR Am J Roentgenol 1990;154:535-537.
30. Sauerbrei EE, Pham DH. Placental abruption and subchorionic hemorrhage in the first half of pregnancy: US appearance and clinical outcome. Radiology 1986;160:109-112.
31. Bennett GL, Bromley B, Lieberman E, et al. Subchorionic hemorrhage in first-trimester pregnancies: prediction of pregnancy outcome with sonography. Radiology 1996;200:803-806.
32. Finberg HJ, Williams JW. Placenta accreta: prospective sonographic diagnosis in patients with placenta previa and prior Cesarean section. J Ultrasound Med 1992;11:333-343.
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33. Kim H, Hill MC, Winick AB, et al. Prenatal diagnosis of placenta accreta with pathologic correlation. Radiographics 1998;18:237-242.
34. Bromley B, Benacerraf BR. Solid masses on the fetal surface of the placenta: differential diagnosis and clinical outcome. J Ultrasound Med 1994;13:883-886.
35. Dudiak CM, Salomon CG, Posniak HV, et al. Sonography of the umbilical cord. Radiographics 1995;15:1035-1050.
36. Nyberg DA, Mahony BS, Luthy D, et al. Single umbilical artery–prenatal detection of concurrent abnormalities. J Ultrasound Med 1991;10:247-253.
37. Wu M-H, Chang F-M, Shen M-R, et al. Prenatal sonographic diagnosis of single umbilical artery. J Clin Ultrasound 1997;25:425-430.
38. Strobelt N, Ghidini A, Cavallone M, et al. Natural history of uterine leiomyomas in pregnancy. J Ultrasound Medicine 1994;13:399-401.
39. Rosati P, Exacoustos C, Mancuso S. Longitudinal evaluation of uterine myoma growth during pregnancy–a sonographic study. J Ultrasound Med 1992;11:511-515.
40. Kessler A, Mitchell DG, Kuhlman K, et al. Myoma vs. contraction in pregnancy: differentiation with color Doppler imaging. J Clin Ultrasound 1993;21:241-244.
41. Mahony BS, Nyberg DA, Luthy DA, et al. Translabial ultrasound of the third-trimester uterine cervix–correlation with digital examination. J Ultrasound Med 1990;9:717-723.
42. Iams JD, Goldenberg RL, Meis PJ, et al. The length of the cervix and the risk of spontaneous premature delivery. N Engl J Med 1996;334:567-572.
43. Brace RA, Wolf EJ. Normal amniotic fluid volume changes throughout pregnancy. Am J Obstet Gynecol 1989;161:382-388.
44. Magann EF, Perry KG, Jr., Chauhan SP, et al. The accuracy of ultrasound evaluation of amniotic fluid volume in singleton pregnancies: the effect of operator experience and ultrasound interpretative technique. J Clin Ultrasound 1997;25:249-253.
45. Petrikovsky B, Schneider EP, Gross B. Clinical significance of echogenic amniotic fluid. J Clin Ultrasound 1997;26:191-193.
46. Levine D, Callen PW, Pender SG, et al. Chorioamniotic separation after second-trimester genetic amniocentesis: importance and frequency. Radiology 1998;209:175-181.
47. Randel SB, Filly RA, Callen PW, et al. Amniotic sheets. Radiology 1988;166:633-636.
48. Finberg HJ. Uterine synechiae in pregnancy: expanded criteria for recognition and clinical significance in 28 cases. J Ultrasound Med 1991;10:547-555.
49. Ball RH, Buchmeier SE, Longnecker M. Clinical significance of sonographically detected uterine synechiae in pregnant patients. J Ultrasound Med 1997;16:465-469.
50. Burton DJ, Filly RA. Sonographic diagnosis of the amniotic band syndrome. AJR Am J Roentgenol 1991;156:555-558.
51. Evans HJ. Chromosome anomalies among live births. J Med Genet 1977;14:309-312.
52. Nyberg DA, Kramer D, Resta RG, et al. Prenatal sonographic findings of trisomy 18: review of 47 cases. J Ultrasound Med 1993;2:103-113.
53. Lehman CD, Nyberg DA, Winter TC, III, et al. Trisomy 13 syndrome: prenatal US findings in a review of 33 cases. Radiology 1995;194:217-222.
54. Dubbins PA. Screening for chromosome abnormality. Semin Ultrasound CT MRI 1998;19:310-317.
55. Filly RA, Callen PW, Goldstein RB. Alpha-fetoprotein screening programs: what every obstetric sonologist should know. Radiology 1993;188:1-9.
56. Benacerraf BR, Nadel A, Bromley B. Identification of second-trimester fetuses with autosomal trisomy by use of a sonographic scoring index. Radiology 1994;193:135-140.
57. Benacerraf BR. Use of sonographic markers to determine the risk of Down syndrome in second-trimester fetuses. Radiology 1996;201:619-620.
58. Bromley B, Shipp T, Benacerraf BR. Genetic sonogram scoring index: accuracy and clinical utility. J Ultrasound Med 1999;18:523-528.
59. Filly RA. Obstetrical sonography: the best way to terrify a pregnant woman. J Ultrasound Med 2000;19:1-5.
60. Nicolaides KH, Brizot ML, Snijders RJM. Fetal nuchal translucency: ultrasound screening for fetal trisomy in the first trimester of pregnancy. Br J Obstet Gynecol 1994;101:782-786.
61. Watson WJ, Miller RC, Menard MK, et al. Ultrasonographic measurement of fetal nuchal skin to screen for chromosomal abnormalities. Am J Obstet Gynecol 1994;170:583-586.
62. Benson CB, Doubilet PM. Sonography in multiple gestations. Radiologist 1994;1:147-154.
63. Bromley B, Benacerraf BR. Using the number of yolk sacs to determine amnionicity in early first trimester monochorionic twins. J Ultrasound Med 1995;14:415-419.
P.327

64. Finberg HJ. The “twin peak” sign: reliable evidence of dichorionic twinning. J Ultrasound Med 1992;11:571-577.
65. Reece EA, Yarkoni S, Abdalla M, et al. A prospective longitudinal study of growth in twin gestations compared to growth in singleton pregnancies: I. The fetal head. J Ultrasound Med 1991;10:439-443.
66. Reece EA, Yarkoni S, Abdalla M, et al. A prospective longitudinal study of growth in twin gestations compared to growth in singleton pregnancies: II. The fetal limbs. J Ultrasound Med 1991;10:445-450.
67. Brown DL, Benson CB, Driscoll SG, et al. Twin-twin transfusion syndrome: sonographic findings. Radiology 1989;170:61-63.
68. Hecher K, Ville Y, Nicolaides KH. Fetal arterial Doppler studies in twin-twin transfusion syndrome. J Ultrasound Med 1995;14:101-108.
69. Filly RA, Cardoza JD, Goldstein RB, et al. Detection of fetal central nervous system anomalies: a practical level of effort for a routine sonogram. Radiology 1989;172:403-408.
70. Hertzberg BS, Kliewer MA, Freed KS, et al. Third ventricle: size and appearance in normal fetuses through gestation. Radiology 1997;203:641-644.
71. Knutson RK, McGahan JP, Salamat MS, et al. Fetal cisterna magna septa: a normal anatomic finding. Radiology 1991;180:799-801.
72. Brant WE. Ultrasound of the fetal face and neck. Radiologist 1994;1:235-244.
73. Mernagh JR, Mohide PT, Lappalainen RE, et al. US assessment of the fetal head and neck: a state-of-the-art pictorial review. Radiographics 1999;19:S229-S241.
74. Budorick NE, Pretorius DH, Nelson TR. Sonography of the fetal spine: technique, imaging findings, and clinical implications. AJR Am J Roentgenol 1995;164:421-428.
75. Avni EF, Rypens R, Milaire J. Fetal esophagus: normal appearance. J Ultrasound Med 1994;13: 175-180.
76. Benacerraf BR. Sonographic detection of fetal anomalies of the aortic and pulmonary arteries: value of the four-chamber view vs direct images. AJR Am J Roentgenol 1994;163:1483-1489.
77. Frates MC. Sonography of the normal fetal heart: a practical approach. AJR Am J Roentgenol 1999;173:1363-1370.
78. Bromley B, Estroff JA, Sanders SP. Fetal echocardiography: accuracy and limitations in a population at high and low risk for heart defects. Am J Obstet Gynecol 1992;166:1473-1481.
79. McGahan JP. Sonography of the fetal heart: findings on the four chamber view. AJR Am J Roentgenol 1991;156:547-553.
80. Brown DL, DiSalvo DN, Frates MC, et al. Sonography of the fetal heart: normal variants and pitfalls. AJR Am J Roentgenol 1993;160:1251-1255.
81. Hashimoto BE, Filly RA, Callen PW. Fetal pseudoascites: further anatomic observations. J Ultrasound Med 1986;5:151-152.
82. Parulekar SG. Sonography of normal fetal bowel. J Ultrasound Med 1991;10:211-220.
83. Elejalde B, Elejalde M, Heitman T. Visualization of the fetal genitalia by ultrasonography: a review of the literature and analysis of its accuracy and ethical implications. J Ultrasound Med 1985;4:633-636.
84. Goldstein RB, Filly RA. Prenatal diagnosis of anencephaly: spectrum of sonographic appearances and distinction from the amniotic band syndrome. AJR Am J Roentgenol 1988;151:547-550.
85. Goldstein RB, LaPidus AS, Filly RA. Fetal cephaloceles: diagnosis with US. Radiology 1991; 180:803-808.
86. Cardoza JD, Goldstein RB, Filly RA. Exclusion of fetal ventriculomegaly with a single measurement: the width of the lateral ventricular atrium. Radiology 1988;169:711-714.
87. Hertzberg BS, Lile R, Foosaner DE, et al. Choroid plexus-ventricular wall separation in fetuses with normal-sized cerebral ventricles at sonography: postnatal outcome. AJR Am J Roentgenol 1994;163:405-410.
88. McGahan JP, Nyberg DA, Mack LA. Sonography of facial features of alobar and semilobar holoprosencephaly. AJR Am J Roentgenol 1990;154:143-148.
89. Sohaey R, Zwiebel WJ. The fetal thorax: noncardiac chest anomalies. Semin Ultrasound CT MR 1996;17:34-50.
90. Guibaud L, Filiatrault D, Garel L, et al. Fetal congenital diaphragmatic hernia: accuracy of sonography in the diagnosis and prediction of outcome after birth. AJR Am J Roentgenol 1996;166:1195-1202.
91. Teixeira J, Sepulveda W, Hassan J, et al. Abdominal circumference in fetuses with congenital diaphragmatic hernia: correlation with hernia content and pregnancy outcome. J Ultrasound Med 1997;16:407-410.
P.328

92. Rosado-de-Christenson ML, Stocker JT. Congenital cystic adenomatoid malformation. Radiographics 1991;11:865-886.
93. Frazier AA, Rosado-de-Christenson ML, Stocker JT, et al. Intralobar sequestration: radiologic-pathologic correlation. Radiographics 1997;17:725-745.
94. Rosado-de-Christenson ML, Frazier AA, Stocker JT, et al. Extralobar sequestration: radiologic-pathologic correlation. Radiographics 1993;13:425-441.
95. McAdams HP, Kirejczyk WM, Rosado-de-Christenson ML, et al. Bronchogenic cyst: imaging features with clinical and histopathologic correlation. Radiology 2000;217:441-446.
96. Ohlsson A, Fong K, Rose T, et al. Prenatal ultrasonic prediction of autopsy proven pulmonary hypoplasia. Am J Perinatol 1992;9:334-337.
97. D’Alton M, Mercer B, Riddick E, et al. Serial thoracic versus abdominal circumference ratios for the prediction of pulmonary hypoplasia in premature rupture of the membranes remote from term. Am J Obstet Gynecol 1992;166:658-662.
98. Abdullah MM, Lacro RV, Smallhorn J, et al. Fetal cardiac dextroposition in the absence of an intrathoracic mass: sign of significant right lung hypoplasia. J Ultrasound Med 2000;19:669-676.
99. McGahan JP, Choy M, Parrish MD, et al. Sonographic spectrum of fetal cardiac hypoplasia. J Ultrasound Med 1991;10:539-546.
100. Brown DL, Roberts DJ, Miller WA. Left ventricular echogenic focus in the fetal heart: pathologic correlation. J Ultrasound Med 1994;13:613-616.
101. Bromley B, Lieberman E, Shipp TD, et al. Significance of an echogenic intracardiac focus in fetuses at high and low risk for aneuploidy. J Ultrasound Med 1998;17:127-131.
102. Wigton TR, Sabbagha RE, Tamura RK, et al. Sonographic diagnosis of congenital heart disease: comparison between the four-chamber view and multiple cardiac views. Obstet Gynecol 1993;82:219-224.
103. Yoo S-J, Lee YH, Cho KS. Abnormal three-vessel view on sonography: a clue to the diagnosis of congenital heart disease in the fetus. AJR Am J Roentgenol 1999;172:825-830.
104. Di Salvo DN, Brown DL, Doubilet PM, et al. Clinical significance of isolated fetal pericardial effusion. J Ultrasound Med 1994;13:291-293.
105. Brown DL. Sonographic assessment of fetal arrhythmias. AJR Am J Roentgenol 1997;169: 1029-1033.
106. Emanuel PG, Garcia GI, Angtuaco TL. Prenatal detection of anterior abdominal wall defects with US. Radiographics 1995;15:517-530.
107. Brant WE. Sonographic evaluation of the fetal abdominal wall. Radiologist 1995;2:149-161.
108. Calzolari E, Volpato S, Bianchi F, et al. Omphalocele and gastroschisis: a collaborative study of five Italian congenital malformation registries. Teratology 1993;47:47-55.
109. Babcook CJ, Hedrick MH, Goldstein RB, et al. Gastroschisis: can sonography of the fetal bowel accurately predict postnatal outcome? J Ultrasound Med 1994;13:701-706.
110. Tongsong T, Wanapirak C, Sirivatanapa P, et al. Prenatal sonographic diagnosis of ectopia cordis. J Clin Ultrasound 1999;27:440-445.
111. Robertson FM, Crombleholme TM, Paidas M, et al. Prenatal diagnosis and management of gastrointestinal anomalies. Semin Perinat 1994;18:182-195.
112. McKenna KM, Goldstein RB, Stringer MD. Small or absent fetal stomach: prognostic significance. Radiology 1995;197:729-733.
113. Hertzberg BS. Sonography of the fetal gastrointestinal tract: anatomic variants, diagnostic pitfalls, and abnormalities. AJR Am J Roentgenol 1994;162:1175-1182.
114. Nyberg DA, Dubinsky T, Resta RG, et al. Echogenic fetal bowel during the second trimester: clinical importance. Radiology 1993;188:527-531.
115. Perez CG, Goldstein RB. Sonographic borderlands in the fetal abdomen. Semin Ultrasound CT MR 1998;19:336-346.
116. Levine D, Goldstein RB, Cadrin C. Distention of the fetal duodenum: abnormal finding? J Ultrasound Med 1998;17:213-215.
117. Vincoff NS, Callen PW, Smith-Bindman R, et al. Effect of ultrasound transducer frequency on the appearance of the fetal bowel. J Ultrasound Med 1999;18:799-803.
118. Rypens FF, Avni EF, Abehsera MM, et al. Areas of increased echogenicity in the fetal abdomen: diagnosis and significance. Radiographics 1995;15:1329-1344.
119. Karcnik TJ, Rubenstein JB, Swayne LC. The fetal presacral pseudomass: a normal sonographic variant. J Ultrasound Med 1991;10:579-581.
120. Zelop C, Benacerraf BR. The causes and natural history of fetal ascites. Prenat Diag 1994;14:941-946.
P.329

121. Keslar PJ, Buck JL, Suarez ES. Germ cell tumors of the sacrococcygeal region: radiologic-pathologic correlation. Radiographics 1994;14:607-620.
122. Arger PH, Coleman BG, Mintz MC, et al. Routine fetal genitourinary tract screening. Radiology 1985;156:485-489.
123. Bronshtein M, Bar-Hava I, Lightman A. The significance of early second-trimester sonographic detection of minor fetal renal anomalies. Prenat Diagn 1995;15:627-632.
124. Anderson N, Clautice-Engle T, Allan R, et al. Detection of obstructive uropathy in the fetus: predictive value of sonographic measurements of renal pelvic diameter at various gestational ages. AJR Am J Roentgenol 1995;164:719-723.
125. Corteville JE, Gray DL, Crane JP. Congenital hydronephrosis: correlation of fetal ultrasonographic findings with infant outcome. Am J Obstet Gynecol 1991;165:384-388.
126. Filly RA. Fetal hydronephrosis. Annual meeting of American Roentgen Ray Society, 1998.
127. Risdon RA. Renal dysplasia. J Clin Pathol 1971;24:57-71.
128. Zhou Q, Cardoza JD, Barth R. Prenatal sonography of congenital renal malformations. AJR Am J Roentgenol 1999;173:1371-1376.
129. Machado LE, Bonilla-Musoles F, Osborne NG. Fetal limb abnormalities: ultrasound diagnosis. Ultrasound Q 2000;16:203-219.
130. Spirt BA, Oliphant M, Gottlieb RH, et al. Prenatal sonographic evaluation of short limbed dwarfism: an algorithmic approach. Radiographics 1990;10:217-236.
131. Bowerman RA. Anomalies of the fetal skeleton: sonographic findings. AJR Am J Roentgenol 1995;164:973-979.
132. Mahony BS. Ultrasound evaluation of the fetal musculoskeletal system. In: Callen PW, ed. Ultrasonography in Obstetrics and Gynecology. 3rd ed. Philadelphia: WB Saunders and Co., 1994:254-290.
133. Kurtz AB, Needleman L, Wapner RJ. Usefulness of a short femur in the in utero detection of skeletal dysplasias. Radiology 1990;177:197-200.
134. Bromley B, Benacerraf B. Abnormalities of the hands and feet in the fetus: sonographic findings. AJR Am J Roentgenol 1995;165:1239-1243.
135. FitzSimmons J, Droste S, Shepard TH, et al. Long-bone growth in fetuses with Down syndrome. Am J Obstet Gynecol 1989;161:1174-1177.
136. Wax JR, Philput C. Fetal intracardiac echogenic foci: does it matter which ventricle? J Ultrasound Med 1998;17:141-144.
137. Manning JE, Ragavendra N, Sayre J, et al. Significance of fetal intracardiac echogenic foci in relation to trisomy 21: a prospective sonographic study of high-risk pregnant women. AJR Am J Roentgenol 1998;170:1083-1084.
138. Benacerraf BR, Mandell J, Estroff JA, et al. Fetal pyelectasis, a possible association with Down syndrome. Obstet Gynecol 1990;76:58-60.