Manual of Obstetrics
7th Edition

31
Obstetric Ultrasound
Alfred Z. Abuhamad
Key Points
  • Since its introduction into clinical obstetrics, obstetric ultrasound imaging (OUI) has become an essential component of prenatal care.
  • OUI requires careful documentation of findings, particularly in regard to confirmation of dates and fetal anatomy.
  • While OUI has assumed important social significance to mothers and their families, it is important that they understand that it is a medical examination with certain limitations.
Background
Applications
  • Biometry
    • Biometry is direct ultrasonic measurements of the fetus.
    • Biometric information is used to estimate gestational age and fetal weight.
    • When combined with appropriate clinical information, biometry is used to document normal or altered growth patterns of the fetus and fetal organ systems.
  • Structural assessment
    • High-resolution ultrasound images can detect
      • Structural abnormalities of the fetal gastrointestinal, skeletal, genitourinary, cardiac, and central nervous systems
      • Multiple gestations
      • Placental abnormalities.
  • Assessment of fetal well-being
    • Real-time ultrasound is used to obtain biophysical profiles (BPPs), amniotic fluid volume assessments, and Doppler studies of the fetal vasculature in the antepartum period.
    • These tests provide information on the status of the fetus and have helped reduce perinatal mortality.
  • Screening for chromosomal abnormalities
    • Ultrasound can be used in the first and second trimester to screen for chromosomal abnormalities, such as Down syndrome.
      • A first-trimester screen involves measuring fetal nuchal translucency between 10 and 14 weeks gestation.
      • A second-trimester screening involves multiple serum analyte and ultrasound markers that correlate with chromosomal abnormalities.
  • Guidance for invasive procedures:
    • Real-time ultrasonography is used to guide needle placement during invasive procedures such as amniocentesis, chorionic villus sampling (CVS), and cordocentesis.
  • Early pregnancy assessment:
    • Ultrasonography is useful for evaluating early pregnancy issues, such as
      P.563

      • Verifying the presence (or absence) of a viable intrauterine gestation when bleeding occurs
      • Excluding ectopic pregnancy when pain is present
      • Assigning accurate dates.
Evaluation
General Principles
  • Safety concerns
    • As a general principle, obstetrical ultrasound studies should be performed only for specific clinical indications.
    • Ultrasound imaging uses focused, high-frequency sound waves to generate images.
    • Ultrasound waves transmit energy and can theoretically cause damage to the fetus through the mechanisms of heat and cavitation (the production and collapse of bubbles).
    • At much higher intensities than are in current clinical use, ultrasound waves have been shown to disrupt biological systems.
    • The relative susceptibility of a given organ system to ultrasound damage is related to the intensity and duration of ultrasound exposure, its distance from the sound source, and the thermal dissipation characteristics of the organ system (related to blood flow through the organ).
  • In a review of the safety of ultrasonography in obstetrics, the U.S. Food and Drug Administration (FDA) stated that, although no definite effects could as yet be documented for current exposure levels, the possibility of long-term side effects could not be excluded. Further, the report warned that such effects might be subtle in nature and not easily detected.
  • A long-term followup study (8 to 9 years) of children exposed to routine ultrasonography in utero showed that the risk of having poor skills in reading and writing was no greater for children whose mothers had routine ultrasonography than for those whose mothers had not had the procedure (1).
Principles of Ultrasound Image Generation
  • Ultrasound images are generated by coordinating a transducer, which is a combined sound generator and receiver, with an electronic processor.
  • Timed high-frequency pulses of sound (usually in the range 2–8 MHz) are sent out and then reflected back to the transducer by objects in the field.
  • The time delay between signal generation and return is calculated by the electronic processor and, because the average speed of sound waves in human tissues is known (1540 m per second), this delay in signal return can be displayed as depth.
  • By simultaneously obtaining images from adjacent points, the electronic processor can assemble a real-time cross-sectional image of structures within the sound field. This image can be oriented in a linear, curvilinear, or radial manner, depending on the shape of the transducer apparatus.
  • Transabdominal and transvaginal transducers rely on the same technology and are commonly used in obstetric and gynecologic ultrasound.
    • Imaging energy frequencies
      • Multiple-energy frequencies are available; however, the range that is used for obstetrical imaging is typically between 2 and 9 MHz.
      • The exact frequency used for imaging depends on the clinical setting and variables such as tissue density and required depth of penetration. Although fixed frequency transducers can be used, the transducers that are currently in use are variable over a preset energy range with continuous adjustment provided by the electronic processor. Typical energy ranges are:
        P.564

        • Abdominal curvilinear probe: 2 to 7 MHz
        • Vaginal probe: 5 to 9 MHz
        • 3D/4D probe: 4 to 8 MHz.
      • Generally, 5 or 6 MHz provides the best resolution. However, these high-frequency sound waves are easily attenuated by bodily tissues and do not adequately image fetuses more than 6 to 8 cm from the transducer. When this is a problem, lower frequencies of 3 to 4 MHz are used.
    • Tissue interfaces
      • Highly dense structures such as bones are hyperechoic, meaning they reflect a large portion of an incident sound signal, sometimes resulting in a shadowing of structures lying behind them.
      • Fluid-filled structures are hypoechoic, meaning they generate few return images and appear empty on the display.
      • Interfaces between areas of differing tissue densities (e.g., fluid–tissue, tissue–bone) are the most easily visualized.
      • Fetal imaging is more difficult if little difference exists between the structure of interest and surrounding tissues (e.g., distinguishing fetal abdominal circumference from uterus if oligohydramnios is present, distinguishing renal parenchyma from surrounding retroperitoneal structures if calyceal structures are not well developed).
    • Doppler ultrasound
      • Ultrasound can be used to measure the direction and velocity of fluid flow by means of the Doppler principle.
      • The Doppler effect, which is a change in frequency of sound with motion, means that predictable changes in the frequency of a sound wave occur when it is reflected by moving red cells. Cells moving toward a sound wave source will reflect sound waves back at a higher frequency; cells traveling away from a source will reflect sound at a diminished frequency. Furthermore, blood cells moving with a higher velocity reflect sound waves back at a higher frequency than cells moving with a slower velocity. By comparing initial and returning sound frequencies, a Doppler shift is calculated. This information can then be combined (“duplexed”) with simultaneous standard ultrasound images to provide information regarding blood flow in a given area.
      • Blood flow direction:
        • In color Doppler imaging, color converters are added to assign color codes to the directions of blood flow.
        • By convention, flow toward the transducer is colored red, and flow away from the transducer is colored blue. Such information is superimposed on a standard sonographic image.
        • Color flow Doppler ultrasound is particularly useful for evaluating the fetal cardiovascular system and for improving the efficiency of pulsed Doppler measurements.
        • Color Doppler ultrasound is pulsed energy transmission. A relatively large amount of sound energy is required to generate these images. The FDA has approved these energy levels for use in pregnancy. But pulsed Doppler ultrasound should not be used continuously in first-trimester scanning due to the total amount of energy to which the fetus could be exposed.
      • Blood flow velocity
        • Doppler ultrasound can also be used to calculate blood flow velocity either
          • In direct terms as centimeters per second or
          • As the Pourcelot index (RI) or pulsatility index (PI), which are modified ratios of frequency shifts during systole and diastole.
        • These measurements have been performed for various fetal vessels, including the umbilical artery, the aorta, the carotids, the renals, the splenic, the uterine, and the middle cerebral artery, in an attempt to facilitate diagnosis of fetal disease states (2).
P.565

Types of Ultrasonographic Studies
TABLE 31-1 Essential Elements of Fetal Anatomic Ultrasound Survey
Head and neck
  • Cerebellum
  • Choroid plexus
  • Cisterna magna
  • Lateral cerebral ventricles
  • Midline flax
  • Cavum septi pellucidi
Chest
  • The basic cardiac examination includes a four-chamber view of the fetal heart. If technically feasible, an extended basic cardiac examination also can be attempted to evaluate both outflow tracts.
Abdomen
  • Stomach (presence, size, and situs)
  • Kidneys
  • Bladder
  • Umbilical cord insertion site into the fetal abdomen
  • Umbilical cord vessel number
Spine
  • Cervical, thoracic, lumbar, and sacral spine
Extremities
  • Legs and arms (presence or absence)
Sex
  • For evaluation of multiple gestations
From American College of Obstetricians and Gynecologists. ACOG Tech Bull. 2004;58; and American College of Radiology. ACR practice guideline for the performance of antepartum obstetrical ultrasound. In: ACR practice guidelines and technical standards. Philadelphia: American College of Radiology; 2003:625–631.
The American College of Obstetricians and Gynecologists (ACOG), in its technical bulletin on ultrasonography in pregnancy (number 58, December 2004), divided obstetric ultrasound examinations into three types: standard, limited, and specialized.
  • Standard examination: The standard examination is performed during the second and/or third trimesters of pregnancy.
    • It includes an evaluation of fetal presentation, amniotic fluid volume, cardiac activity, placental position, fetal biometry, and an anatomic survey of the fetus. If technically feasible, the uterus and the adnexa should be evaluated.
    • The essential elements of the standard ultrasound examination fetal anatomic survey, as defined by the ACOG, are listed in Table 31-1.
    • Details of the standard examination include
      • A general description of the intrauterine contents, including the number and orientation of fetuses, the placement of the placenta with relation to the uterine cavity and the cervix, and estimation of the amniotic fluid volume (individual estimations for all fetuses in multiple gestations).
      • Measurement of the following fetal features: s
        • Biparietal diameter (BPD)
        • Head circumference (HC)
        • P.566

        • Abdominal circumference (AC)
        • Femur length (FL).
      • For all fetuses beyond 22 weeks of gestation, an estimated fetal weight (EFW) should be calculated from proved regression equations or by using suitable fetal weight normograms. This estimated weight should then be interpreted as a percentile for gestational age (e.g., “The EFW based on a BPD-AC table was 1720 g, placing the fetus at the 25th percentile for gestational age”).
      • A survey of the fetal anatomy (Table 31-1).
      • Comment about the fetal heart rate and rhythm.
      • A cursory evaluation should be carried out for other problems such as an abnormally thickened (hydropic) placenta, an overly distended fetal bladder, cystic dilation of a renal pelvis, evidence of fetal ascites or other effusions, or uterine abnormalities (such as leiomyomas); a survey of the maternal pelvic organs should also be performed.
      • In multifetal gestations, the presence of a dividing membrane should always be sought (because this effectively excludes the possibility of monochorionic/monoamniotic twins). Multifetal gestations beyond 24 weeks of gestation should be evaluated for growth retardation and discordance. Discordance in twins has been defined as a difference in EFWs of more than 25%.
  • Limited examination
    • When a specific question requires investigation, a limited ultrasound examination may be performed.
    • Limited examinations focus on a specific ultrasonic finding that is usually being surveyed serially, such as the assessment of amniotic fluid volume, confirmation of fetal viability, localization of placenta, and confirmation of fetal presentation.
    • If a limited study is selected, a previous standard examination should have been performed and the determination made that a repeat standard examination is not warranted.
  • Specialized examination
    • A specialized examination (detailed or targeted) is a more extensive examination of the entire fetus, often with special attention on a specific fetal organ system. It may be indicated for a fetus with a suspected congenital anomaly or in pregnancies with severe growth abnormalities.
    • It must be emphasized that the accuracy of ultrasonography, even in the most expert hands, does not approach 100%. Patients should be informed of this limitation.
    • The specialized study is best performed under the direct supervision of an experienced sonographer as a real-time examination.
  • Biophysical profile
    • The BPP is a specialized type of ultrasound examination that was originally described by Manning and colleagues (3).
    • The BPP is a scoring system that has proved to be a valuable method of fetal antepartum assessment. Its principal advantage over other methods of fetal evaluation is that it retains sensitivity (i.e., the ability to diagnose impending fetal compromise) but offers improved specificity (fewer false-positive” or abnormal findings).
    • There are four sonographic criteria for the BPP (fetal breathing movements, gross body–limb movements, general body tone, and amniotic fluid volume) and a fetal heart rate nonstress test (NST) component. Each is scored as either normal or abnormal. Two points are assigned for normal findings and no points if a category is judged abnormal. Note that there are no 1-point awards).
    • These points are added to give the total score:
      • 8 or 10 is normal
      • 6 is equivocal
      • 4 or less is abnormal
      • 0 or 2 is ominous.
    • Remember that the BPP and other similar tests are clinical applications of available technology. They must be interpreted in light of all relevant data pertaining to a
      P.567

      given patient. Caution should be used in inferring a particular prognosis from test results viewed outside the clinical context.
Figure 31-1. Intracranial anatomy for the biparietal diameter. A: ultrasound picture. CSP, cavum septum pellucidum; 3V, third ventricle. B: diagram of the ultrasound picture.
Overview of Technique for Various Fetal Measurements
  • Four fetal measurements are essential to every fetal ultrasound examination.
    • Biparietal diameter
      • The fetal BPD is measured in a plane transverse to the long axis of the head, which allows visualization of the midline falx cerebri, the cavum septum pellucidum, and the thalamus (which can be seen straddling the midline centrally) (Fig. 31-1). If structures such as the cerebellum, orbits, or basal skull (the petrous ridges and wings of the sphenoid give an X-shaped appearance) are seen, a more accurate plane should be sought before measurements of the head are taken.
      • Technically difficult studies: With an engaged head it will occasionally be impossible to obtain an accurate BPD. In such circumstances, it is usually best to report that the measurement was “not technically possible” rather than reporting and according clinical significance to a suspect measurement that may be erroneous.
      • Imaging landmarks: The BPD is traditionally measured from the external surface of the cranium anteriorly to the internal surface of the cranium posteriorly. The cranium is most often found to be slightly elliptic in shape. Some fetuses will present with cranial shapes that are either overly rounded (brachycephalic) or an exaggerated narrowed, longitudinally lengthened, elliptical shape (dolichocephaly).
      • The cephalic index has been developed to assess the degree of alteration in fetal head shape. It is obtained by dividing the transverse cranial diameter by the occipitofrontal diameter. Cephalic indices of 0.74 to 0.83 are within one standard deviation of the mean.
      • Brachycephaly and dolichocephaly are not intrinsically abnormal but can result in false estimates of gestational age (EGA) or EFW. Infants with brachycephaly will tend to have overestimated EFW and EGA determinations because of the exaggerated width of the BPD measurement relative to the overall fetal size. The converse is true for dolichocephalic infants. This problem can be overcome by using other parameters to assess gestational age (such as FL and HC) and by using EFW normograms based on HC or FL, or both, in combination with AC.
  • Head circumference
    • The HC is measured in the same plane as the BPD. It can be measured directly with the ultrasound computer’s calipers, or its length can be accurately estimated by measuring the diameter of the head in the occipitofrontal dimension and in the transverse dimension. All measures can be obtained from the external surface of the relevant portions of the cranium or at a midpoint within the cranial bones.
      P.568

      • If the occipitofrontal dimension and transverse dimension are used to estimate the HC, the following formula can be used: HC = 1/2 (OFD + TD) x 3.1416. This formula is modeled for a circle, but it closely approximates the results obtained if formulas for elliptic circumference are used.
    • The HC has the advantage of being unaffected by alterations in the shape of the fetal head. Thus, it is an ideal parameter for estimating fetal weight. Unfortunately, HC offers a much wider range of possible values than BPD, and normograms relating HC to EFW are less often used clinically. If computers or hand calculators are used to directly compute EFW, regression equations using HC are good options.
    • HC is also used in diagnosing microcephaly. Typically, measurements below two standard deviations for a corresponding gestational age are required for a diagnosis of microcephaly.
  • Abdominal circumference
    • The fetal AC is measured in a plane perpendicular to the long axis of the torso. It should contain the stomach bubble and the midportion (not umbilical insertion) of the umbilical vein (Fig. 31-2). The superior poles of the kidneys usually lie slightly caudal to this plane.
    • To obtain this cross-section, it is usually best to orient the transducer parallel to the spine and slightly caudal to the heart. The transducer should then be rotated 90°, taking care to avoid the oblique transabdominal planes. A full fetal rib should be visible on either side when an accurate transverse plane of the abdomen is obtained. The AC should then be measured several times until two or three consistent values have been obtained. Care should be taken to include the skin in these measurements. As with the HC, if computed planimetry is unavailable, two diameters can be obtained and an estimated AC calculated with the following formula: AC = 1/2 3 (D1 + D2) x 3.1416.
    • The AC measurement is very sensitive to intrauterine growth retardation (IUGR), as evidenced by the use of FL/AC and HC/AC ratios in screening for IUGR.
    • The AC measurement is also the first biometric measurement in the fetus to be affected by growth abnormalities.
    Figure 31-2. Anatomic landmarks of the abdominal circumference. A: ultrasound picture. B: diagram of the ultrasound picture.
  • Femur length
    • The part of the femur measured by ultrasound is the ossified portion of the diaphysis. The epiphyseal cartilages are hypoechoic and not easily visualized by ultrasound because they are not well calcified in utero. The femur itself is somewhat bowed, but the proper FL should be the linear distance between the proximal and distal diaphyses.
    • P.569

    • Embryologically, the distal femoral epiphysis becomes ossified at 28 to 35 weeks of gestation. At times, this observation can be used to help assess fetal gestational age (4,5).
  • Other fetal measures
    • For a more complete discussion of fetal biometry, see the references and “Selected Readings” at the end of this chapter. These publications provide appropriate normograms for estimation of fetal weight and gestational-age-specific tables of fetal measurements.
Indications for Obstetric Ultrasound Imaging Evaluation
  • The ACOG and the National Institutes of Health special study groups do not consider routine OUI) to be standard care in the United States. Rather, its use should be reserved for specific medical indications.
  • As experience with OUI has accumulated, the indications for such imaging have expanded broadly.
  • Routine performance of ultrasound examinations around 18 weeks of gestation is widely practiced. Ultrasound examination provides important information regarding confirmation of gestational age and fetal anatomic survey. The value of this routine ultrasound examination, although not clearly demonstrated in the United States, is directly related to the expertise of the operator. We recommend routine ultrasound examinations around 18 weeks of fetal gestation if expertise in performing and interpreting this examination is available.
  • The National Institutes of Health’s 1984 Consensus Report on Safety of Ultrasound reported a number of indications for OUI:
    • Estimation of gestational age for confirming clinical dating among patients who are to undergo elective repeat cesarean delivery, induction of labor, or elective pregnancy termination.
    • Evaluation of fetal growth among patients with suspected or known medical complications associated with either IUGR or macrosomia; these conditions include severe preeclampsia, chronic hypertension, chronic renal disease, and severe diabetes mellitus.
    • Vaginal bleeding of undetermined cause in pregnancy.
    • Determination of fetal presentation when the fetal presentation cannot be adequately assessed in labor or when the fetal presentation is variable in late pregnancy.
    • Suspected multiple gestation if multiple fetal heart rate patterns are present, if the fundal height is larger than expected for gestational age, or if fertility-enhancing medications have been taken.
    • OUI helps the operator avoid the fetus and placenta during amniocentesis, thus decreasing the risk to the fetus.
    • For uterine size–dates discrepancy, OUI facilitates accurate assessment of gestational age and detection of conditions such as oligohydramnios and polyhydramnios.
    • For evaluating a pelvic mass noted on the clinical examination.
    • Suspected hydatidiform mole based on symptoms such as hypertension, proteinuria, or ovarian cysts, or a combination, or the absence of fetal heart tones by Doppler ultrasound after 12 weeks of gestation.
    • As an adjunct to timing of cervical cerclage and placement of the cerclage suture.
    • Suspected ectopic pregnancy or pregnancy in a patient at high risk for ectopic pregnancy.
    • Suspected intrauterine fetal death.
    • As an adjunct to special procedures such as fetoscopy, intrauterine transfusion, percutaneous fetal blood sampling, shunt catheter placement, or CVS.
    • Suspected uterine anomaly such as clinically significant leiomyomas, didelphic uterus, or bicornuate uterus.
    • Localization of intrauterine contraceptive device.
    • Surveillance of ovarian follicle development.
    • Biophysical profile for fetal well-being after 28 weeks of gestation.
    • P.570

    • Observation of intrapartum events such as the version or extraction of the second twin.
    • Suspected polyhydramnios or oligohydramnios.
    • Suspected abruptio placentae.
    • As an adjunct to external breech version.
    • Estimation of fetal weight and presentation in premature rupture of membranes and preterm labor.
    • Abnormal maternal serum a-fetoprotein (AFP) value for clinical gestational age when drawn. Obstetric ultrasound imaging provides an accurate assessment of gestational age and detects several conditions that may cause elevations of maternal serum AFP, such as multiple gestations, anencephaly, and neural tube defects.
    • Serial evaluation of identified fetal anomaly.
    • History of congenital anomaly.
    • Serial evaluation of fetal growth in multiple gestations.
    • Estimation of gestational age in patients presenting late for prenatal care.
Application of Obstetric Ultrasound Imaging to Several Common Clinical Problems
Discordant Uterine Size for Gestational Age
  • Uterine fundal height inappropriate for expected for dates.
    • If gestational age is well established, the fundal height approximately equals the gestational age in weeks.
    • A fundal height measurement that is 2 to 3 cm more or less than expected for gestational age is considered abnormal and should be evaluated by OUI to determine fetal biometry and EFW.
  • Uncertain pregnancy dating.
    • In pregnancies with poor or uncertain dates, fetal measures of BPD, HC, and FL can be used to estimate gestational age and, in combination with the fetal AC, can be used to estimate fetal weight and serially evaluate fetal growth.
  • Good pregnancy dating with fundal height less than expected.
    • If the gestational age is known with reasonable certainty, the EFW can be compared to the expected EFW for gestational age.
    • Fetuses below the 10th percentile for gestational age are considered growth restricted.
    • Ratios of HC/AC and FL/AC can also be used to assess possible IUGR. Interpretation of the HC/AC ratio depends somewhat on fetal gestational age (1.12 at 24.0 weeks, 1.05 at 32 weeks, and 0. 98 at 40 weeks); the FL/AC ratio (normally approximately 0.20–0.22) does not vary during pregnancy. Together, these measures and ratios provide a relatively sensitive and specific means of assessing gestational age and fetal growth.
    • The two general categories of IUGR are asymmetric growth retardation and symmetric growth retardation.
    • Asymmetric IUGR is most common and results in “sparing” of fetal brain growth with consequently normal BPD and HC with small AC and EFW estimates.
    • Symmetric IUGR is characterized by abnormally decreased EFW with paradoxically normal FL/AC and HC/AC ratios. Symmetric IUGR can also be falsely diagnosed in infants who are growing at rates that are individually normal but below the 10th percentile for the general population.
    • Although a distinction has been made between symmetrical and asymmetrical IUGR with regard to the timing of the insult and the pathogenesis of the disease, in clinical practice the distinction is not that evident; ultrasound may not be specific, and management is uniform for both types.
  • Good pregnancy dating with fundal height more than expected.
    • A fundal height of 3 cm or more, higher than expected for gestational age, is considered large for dates. Potential causes of a fundal height more than expected for age include
      P.571

      • Incorrect pregnancy dating. If no prior ultrasonic evaluations have been performed to establish pregnancy dating, the possibility of incorrect pregnancy dates should be considered.
      • Large-for-gestational-age fetus.
      • Multiple gestations.
      • Polyhydramnios.
    • Repeat evaluation should be considered near term to assess interval growth and evaluate for possible fetal macrosomia.
High-risk Pregnancy
Evaluation
  • Serial evaluations of fetal growth in most cases are best initiated at 24 to 26 weeks of gestation and then repeated at monthly intervals.
  • Fetuses not progressing normally for gestational age (using population-specific percentile growth tables) should be monitored with other methods of antepartum assessment (such as contraction stress tests, nonstress tests, Doppler velocimetry, or BPPs).
  • In multifetal gestations, normal growth tables are not well established because a population of “normal” twin gestations has not been easily defined. In general, twin gestations at term yield fetuses weighing approximately 10% less than matched singleton fetuses. Serial assessments of EFW should be evaluated for discordant growth. Fetuses with differences of more than 25% in EFW or with BPDs varying by more than 4 mm exhibit abnormal discordance and should be placed into an antepartum assessment protocol as described before.
  • Patients thought to be at risk for preterm labor may benefit from early ultrasonic confirmation of pregnancy dating because accurate pregnancy dating helps in the management of preterm labor. These patients do not require serial sonographic evaluation unless other indications of IUGR are noted.
Complications
  • Obstetric ultrasound imaging facilitates the management of pregnancies at risk for complications. Accurate pregnancy dating allows more precise evaluation of fetal growth. Precise knowledge of obstetric dating may also affect decisions regarding certain therapeutic interventions (choice of tocolytic agents, use of fetal lung-maturing agents) or timing of delivery in complicated pregnancies.
  • The following pregnancy complications are best managed by an ultrasound evaluation obtained at 14 to 18 weeks to confirm pregnancy dating, followed by serial sonographic evaluation of fetal growth and EFW:
    • Chronic renal disease
    • Collagen-vascular disease
    • Diabetes or prior gestational diabetes
    • Hypertension or prior preeclampsia
    • Isoimmunization (rhesus or other)
    • Lupus anticoagulant
    • Maternal malignancy
    • Multiple gestations.
Suspected Fetal Anomaly
Background
Obstetric ultrasound imaging is an effective method for diagnosing major fetal anomalies, with a sensitivity of approximately 50% and close to 100% specificity for major anomalies.
P.572

Evaluation
History
  • Patients with family or obstetric histories suggestive of fetal anomalies should undergo a thorough ultrasound evaluation to screen for fetal anomalies. This evaluation should initially survey all fetal organ systems and then focus on the index organ system (i.e., the system with a history of prior anomaly).
  • The timing of this study is sometimes difficult because major central nervous system abnormalities (such as anencephaly) can often be detected by 15 weeks of gestation, whereas cardiac structure is much more easily assessed at 22 weeks of gestation.
  • Maternal serum AFP abnormalities
    • Obstetric ultrasound imaging is a valuable adjunct for evaluating fetal anatomy and potential viability when abnormal maternal serum AFP values are present on quadruple screen results (see below).
    • Elevated maternal serum AFP values are often associated with open neural tube defects as well as ventral abdominal wall defects, multiple gestations, fetal death (in multifetal gestations), and other less common conditions.
    • An ultrasound evaluation should always be performed to establish the correct gestational age. If the confirmed or corrected gestational age places the maternal serum AFP in the normal range, no further evaluation is necessary. Otherwise, a thorough fetal ultrasonic evaluation is required, with emphasis on visualizing intracranial structure and the entire neural tube. A negative ultrasonic evaluation does not negate the significance of an elevated maternal serum AFP. Small neural tube defects can potentially be missed by a thorough ultrasound evaluation.
    • If the cause for elevated maternal serum AFP is not obvious, ultrasound-guided amniocentesis may be performed to directly measure the amniotic fluid AFP), to measure the amniotic fluid acetylcholinesterase, and to perform a fetal karyotype. Acetylcholinesterase is found in high concentrations within the central nervous system and is present in the amniotic fluid very early in pregnancy. If present after 14 weeks of gestation, amniotic fluid acetylcholinesterase is considered direct evidence of an open neural tube defect. Contamination of amniotic fluid with fetal blood can raise the level of amniotic fluid AFP but does not affect the level of amniotic fluid acetylcholinesterase. Note that many patients elect not to have an amniocentesis when the targeted ultrasound is normal in association with an elevated maternal serum AFP.
  • Abnormal quadruple screen. The quadruple (“quad”) screen is a biochemical test performed on maternal blood between 14 and 22 weeks. It is for the detection of fetal chromosomal abnormalities, mainly Down syndrome, and includes maternal serum AFP to screen for risk of open neural tube and other defects as discussed above. Patients with an abnormal quad screen are at increased risk for having infants with chromosomal abnormalities such as trisomies 21 and 18. Such syndromes cannot be reliably excluded by ultrasound evaluation. Thus, amniocentesis for fetal karyotype should be offered in these circumstances. A modification of a patient’s risk for Down syndrome can be accomplished by a quad screen in combination with a comprehensive ultrasound examination between 18 and 22 weeks of gestation. Markers of Down syndrome on ultrasound include thickened nuchal fold, short femurs and humerii, echogenic intracardiac foci, fetal pyelectasis, and echogenic fetal bowel. The absence of these markers may reduce the patient’s risk profile significantly. Several prenatal diagnosis and ultrasound centers are combining the quad screen with the ultrasound findings in modifying a patient’s risk for Down syndrome.
  • Abnormalities of amniotic fluid volume
    • Measurement of amniotic fluid volumes
      • Abnormalities of amniotic fluid volume may be indicators of functional abnormalities within the renal, gastrointestinal, or central nervous systems.
      • Amniotic fluid index (AFI) is a measure of amniotic fluid volume. Derivation of the index involves measuring, in a longitudinal plane, the largest fluid pocket in each of the four quadrants of the uterus and adding the four measurements. The pockets are measured when no fetal parts or umbilical cord is contained within.
      • Normal AFI values are between 10 and 20 cm (4,6).
P.573

Polyhydramnios
Background
  • No exact definition for polyhydramnios exists, but two suggested definitions are as follows:
    • Subjectively increased amniotic fluid volume and a largest pocket diameter >7 cm.
    • AFI (the sum of largest vertical amniotic fluid pocket depths from the four uterine quadrants) of 20 or greater (4,5).
Evaluation
  • Polyhydramnios is often seen in association with diabetic pregnancies, isoimmunization syndromes, open neural tube defects, and fetal anomalies affecting the fetal swallowing mechanism (esophageal atresia, tracheo-esophageal fistula) or the gastrointestinal tract (duodenal atresia).
  • If polyhydramnios is judged to be severe, fetal chromosomal evaluation should be considered.
Oligohydramnios
Background
  • Oligohydramnios is defined as an amniotic fluid volume less than expected for gestational age.
  • Oligohydramnios is of great clinical significance because severe reductions in amniotic fluid volume may alter fetal mobility and predispose to abnormal facies, limb contractures, and pulmonary hypoplasia.
  • In the term pregnancy, oligohydramnios (with intact membranes) is highly associated with adverse fetal outcomes and requires careful management.
  • Oligohydramnios can be caused by rupture of membranes or decreased production of amniotic fluid. After 18 weeks of gestation, the fetal kidneys produce most amniotic fluid. Renal agenesis, dysplasia, or obstruction of the urinary system may cause oligohydramnios.
Evaluation
  • Two commonly used methods of quantifying oligohydramnios are the AFI (6,7) and the largest-pocket-size method.
  • The AFI (6,7) was been described above. A four-quadrant total of 5 mm or less is defined as oligohydramnios; values >10 cm are normal, while values between 5 and 10 mm are considered to be decreased amniotic fluid volume.
  • The largest-pocket method defines oligohydramnios as a largest amniotic fluid pocket less than 1 x 1 cm. Pockets of 1 x 1 cm to 2 x 2 cm are also considered to be borderline oligohydramnios.
  • Patients with diminished amniotic fluid volumes should be carefully monitored for the onset of oligohydramnios with serial sonographic evaluation.
Single Umbilical Artery
  • In a normal pregnancy, three umbilical blood vessels (two arteries and one vein) are present. This should be confirmed by OUI.
  • Two-vessel umbilical cords (single umbilical artery) occur in up to 1% of pregnancies.
  • Two-vessel cords are common associations with other fetal abnormalities.
  • P.574

  • There is some controversy as to whether fetuses with two-vessel umbilical cords are at risk for intrauterine growth restriction.
  • Some studies have also reported an increased incidence of congenital heart defects in fetuses with a single umbilical artery (8). Therefore, a fetal echocardiogram is indicated in the pregnancy with a single umbilical artery.
  • If definite fetal anomalies are identified by OUI, amniocentesis for fetal karyotype or percutaneous fetal blood sampling should be considered. The incidence of karyotypic abnormalities increases if one anomaly is present. The presence of two or more anomalies is much more strongly associated with fetal karyotypic abnormalities.
Uncertain Dating of Pregnancy
Background
  • Patients who do not have well-established menstrual dating (i.e., history confirmed by early physical examination) should have an ultrasound evaluation to confirm or establish the gestational age and estimated date of confinement.
Evaluation
History
  • The earliest available sonographic study is generally used to establish obstetric dates, provided it was performed before 24 weeks of gestation.
  • Patients should be questioned as to prior ultrasound evaluations at other facilities, and every effort should be made to obtain existing information. If the examination was performed by suitably experienced practitioners, it should be used as the basis for establishing the pregnancy dates.
  • Estimated gestational age
    • The margin of error associated with ultrasonic determination of EGA is approximately 10% of the determined gestational age (±1-week margin of error before 12 weeks of gestation, ±1 to ±2 weeks error from 12 to 20 weeks of gestation, and ±2 to ±3 weeks error after 20 weeks of gestation).
    • If a scan is performed between 7 and 10 weeks of gestation, the fetal crown–rump length should be used to determine the EGA.
    • For scans obtained later in pregnancy, the EGAs associated with the fetal BPD, HC, and FL should be obtained and (as available) averaged.
    • If a scan is obtained before 6 weeks of gestation and no fetal pole is clearly visible, the gestational sac measure should not be used to establish the EGA. The scan should be repeated several weeks later to measure the crown-rump length (CRL) and to document fetal cardiac activity within the uterus (excluding the possibility of ectopic gestation). The pregnancy is then dated using the crown–rump length from the later scan.
    • If the ultrasound estimates an EGA that agrees with the menstrual dates (within the procedural error limits), the menstrual dates are considered confirmed.
    • For pregnancies that are less than 24 weeks of gestation by ultrasound with discrepancies between the menstrual dates and ultrasound measurements of more than 2 to 3 weeks, it is generally safe to accept the ultrasound EGA as the correct obstetric EGA.
Growth Restriction
  • In pregnancies of more than 24 weeks’ gestation as estimated by ultrasound with reported menstrual date-derived EGAs that are 3 weeks or more ahead of the ultrasound measurements, the possibility of growth restriction should be considered. If underlying maternal disease is present, this diagnosis is more likely.
  • P.575

  • If the AC is disproportionately decreased relative to the ultrasound gestational age, then asymmetric IUGR may be present. Conversely, if FL/AC and HC/AC ratios are within normal limits (i.e., if all fetal measures are symmetric), then the possibility of symmetric IUGR should be considered.
  • When growth restriction is believed to be a reasonable possibility, the fetus should be evaluated with ongoing antepartum testing, and a repeat sonogram for growth should be conducted in approximately 3 to 4 weeks. If the repeat examination shows normal growth for assigned ultrasonic gestational age, the obstetric EGA should be accepted, and antepartum testing can be discontinued. If little or no growth occurs, the fetus should be managed as a high-risk pregnancy (see Chapter 33).
Changing the Estimated Date of Delivery
  • Once properly established, the gestational age and estimated date of delivery should be strictly adhered to.
  • If better information becomes available and it is judged necessary to change the estimated date of delivery, this information should be carefully documented in the patient’s chart.
Serial Scans
  • Obstetric ultrasound imaging allows accurate interval assessment of fetal growth throughout pregnancy. Pregnancies at risk for IUGR (maternal disease states, multiple gestations) should initially be scanned early in pregnancy and then at established intervals throughout pregnancy.
  • A commonly used scheme is
    • Twelve-week scan to confirm gestational age
    • Eighteen- to twenty-week scan for fetal anatomy and confirmation of appropriate growth
    • Repeat scans at 4- to 6-week intervals.
The fetus can also be surveyed at other intervals if findings necessitate therapeutic intervention or early delivery. Anomalies such as hydrocephalus or hydronephrosis or fetal risk for hydrops or cardiac compromise may require individualized schedules for serially scans.
Unexplained Abdominal Pain or Vaginal Bleeding
Ectopic Pregnancy
  • The management of unexplained abdominal pain and vaginal bleeding in pregnancy has been greatly aided by abdominal and vaginal ultrasonography.
  • If an ectopic pregnancy is suspected, ultrasonic evaluation of the uterine cavity should be performed, and a serum β-human chorionic gonadotropin (β-hCG) titer obtained.
  • If the β-hCG titer is >1500 IU, an intrauterine gestation should be seen by endovaginal ultrasound examination and, if not, an ectopic gestation is likely (9, 10).
Abnormalities of Placentation
  • Abnormal placentation is strongly associated with bleeding complications in all stages of pregnancy.
  • Ultrasonic studies allow relatively precise delineation of the site of placentation. Of greatest concern is establishing or excluding the diagnosis of placenta previa, although there are numerous other placental abnormalities that can be identified by ultrasound.
  • Before 20 weeks of gestation, the placenta is frequently seen to be low lying. In 60% to 90% of second-trimester marginal placenta previas, normal placentation will be noted at term.
  • P.576

  • Placentas found to be implanted both anteriorly and posteriorly over the internal cervical os (e.g., central placenta previa) most commonly do not resolve as pregnancy progresses.
Patient Education
  • For more information to provide to patients about ultrasound, please refer to the American Institute of Ultrasound in Medicine website at www.AIUM.org.
References
1. Salvesen KA, Bakketeig LS, Eik-Nes SH, et al. Routine ultrasonography in utero and school performance at age 8–9 years. Lancet. 1992;339:85–89.
2. Zacko A, Neilson JP. Doppler ultrasonography in high-risk pregnancies: systematic review with meta-analysis. Am J Obstet Gynecol. 1995;172:1379–1387.
3. Manning FA, Baskett TF, Morrison I, Lange I. Fetal biophysical profile scoring: a prospective study of 1,184 high risk patients. Am J Obstet Gynecol. 1981;140: 289–294.
4. Mahony BS, Bowie JD, Killam AP, Kay HH, Cooper C. Epiphyseal ossification centers in the assessment of fetal lung maturity: sonographic correlations with the amniocentesis lung profile. Radiology. 1986;159:521–524.
5. McLeary RD, Kuhns LR. Sonographic evlauation of the distal femoral epiphyseal ossification center. J Ultrasound Med. 1983;2:437–438.
6. Phelan JP, Ahn MO, Smith CV, et al. Amniotic fluid index measurements during pregnancy. J Reprod Med. 1987;32:601–604.
7. Phelan JP, Smith CV, Broussard P, et al. Amniotic fluid volume assessment with the four-quadrant technique at 36–42 weeks’ gestation. J Reprod Med. 1987;32:540–542.
8. Abuhamad AZ, Shaffer W, Mari G, et al. Single umbilical artery: does it matter which artery is missing? Am J Obstet Gynecol. 1995;173:728–732.
9. Nyberg DA, Filly RA, Filho DL, Laing FC, Mahoney BS. Abnormal pregnancy: early diagnosis by ultrasound and serum chorionic gonadotropin levels. Radiology. 1986;158:393–396.
10. Nyberg DA, Filly RA, Laing FC, Mack LA, Zarutskie PW. Ectopic pregnancy: diagnosis by sonography correlated with quantitative HCG levels. J Ultrasound Med. 1987;6:145–150.
Selected Readings
Abuhamad A. A practical guide to fetal echocardiography. Philadelphia: Lippincott-Raven;. 1997.
American College of Obstetricians and Gynecologists. Ultrasonography in pregnancy. ACOG Tech Bull. 2004;58.
Callen PW, ed. Ultrasonography in obstetrics and gynecology. 4th ed. Philadelphia: W.B. Saunders, 2000.
Nyberg DA, Mahony BS, Pretorius DH, eds. Diagnostic ultrasound of fetal anomalies: text and atlas. Chicago: Year Book; 1990.
Romero R, et al. Prenatal diagnosis of congenital anomalies. East Norwalk, CT: Appleton & Lange; 1988.
Sabbagha RE, ed. Diagnostic ultrasound applied to obstetrics and gynecology. 3rd ed. Philadelphia: J.B. Lippincott; 1993.