Core Curriculum, The: Ultrasound
1st Edition

6
Obstetric Ultrasound First Trimester
The first trimester covers the period from conception to the end of the 13th menstrual week. This is a time of dynamic growth and the differentiation and development of most organ systems. The embryo has the greatest risk of maldevelopment, injury, and death during this period because of external factors (infection, drugs, radiation) or chromosome abnormalities. Approximately 40% of implanted zygotes are lost as a result of unrecognized abortion with menstruation, whereas another 25-35% of the surviving embryos will threaten to abort during the first trimester. Approximately 1-2% of pregnancies will be ectopic, and these pregnancies are a major cause of pregnancy-related morbidity and mortality. Patients presenting with vaginal bleeding or pelvic pain in early pregnancy require urgent and accurate diagnosis (Box 6.1).
Imaging Technique
US remains the imaging method of first choice for diagnostic evaluations in obstetrics. Transvaginal (TV) US is particularly valuable in the first trimester because of its capability to demonstrate in high resolution the detailed anatomic changes of early pregnancy.
Examinations are usually begun using a transabdominal (TA) approach scanning the pelvis through a full bladder with a 3.5-5.0-MHz sector transducer. In our practice, if this examination demonstrates a normal-appearing intrauterine pregnancy with no adnexal abnormalities, the examination is considered satisfactory and is terminated. If the TA examination yields any other result, then TV examination is performed whenever possible. The patient is asked to empty her bladder and the examination continues utilizing a condom-covered 5.0-7.5-MHz intravaginal transducer.
Guidelines for first trimester sonography are provided by the American Institute of Ultrasound in Medicine and endorsed by the American College of Radiology [1]. The location of the gestational sac (GS) is documented. The embryo is identified and its crownrump
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length (CRL) measured. If no embryo is present the mean diameter of the GS is measured. Gestational age (GA) is estimated by reference of the measurements made to appropriate tables. The presence or absence of embryonic life is determined by real-time observation. M-mode US is utilized to document cardiac activity and to measure cardiac rate. Doppler confirmation of embryonic cardiac activity is not recommended because of the higher US energy involved. Fetal number is documented. The uterus, cervix, and adnexa are carefully examined for any abnormalities.
Normal First Trimester Pregnancy
Gestational Dating
In clinical obstetrics GA is determined from the first day of the mother’s last menstrual period, assuming a regular menstrual cycle of 28 days. In embryology, GA is dated from conception providing a 2-week difference in embryologic dating compared with menstrual dating. In clinical practice and in this text all dating parameters will refer to menstrual dating. For example, 13 weeks GA means that 13 weeks have elapsed since the first day of the mother’s last menstrual period. Technically, the developing human is considered an embryo up to 10 weeks menstrual age and a fetus thereafter until birth. Normal pregnancies last 40 weeks with a range of 37-42 weeks.
In the first trimester, GA is determined by measurement of mean sac diameter (MSD) before the embryo is visualized and by measurement of CRL from when the embryo is apparent to 12 weeks GA. MSD is determined by measuring the length (L), width (W), and height (H) of the GS in perpendicular planes, adding the results together and dividing by 3 (MSD = [L + W + H]/3). Measurements of MSD are made from the fluid/tissue interface of the sac and do not include any portion of the wall of the sac (Fig. 6.1). CRL is determined by measuring the maximum length of the visualized embryo (Fig. 6.2). No portion of the yolk sac, umbilical cord, or limbs is included in the measurement. A straight-line measurement is always used even if the torso of the embryo is flexed. Appropriate charts relate the measurements to menstrual age (Table 6.1) [2,3]. Both MSD and CRL measurements are accurate to ±1 week. These early pregnancy charts are acceptably accurate in the first trimester without correction for race, parental size, altitude, or other parameters known to affect birth weight.
Normal Gestation
Understanding the physiology and detailed anatomy of early gestation as shown by TV US is essential to understanding the pathologic changes that occur in early pregnancy.
Ovulation and Fertilization
In a “normal” menstrual cycle of 28 days, days 1-14 are the follicular phase during which the ovary responds to follicle stimulating hormone (FSH) secreted by the pituitary gland [4]. FSH promotes development of a group of follicles that enlarge and produce and release estrogen.
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Rising estrogen levels suppress secretion of FSH, resulting in atresia of all but one of the stimulated follicles. This dominant follicle (graafian follicle) continues to mature and enlarge and will be the source of ovulation. Ovulation occurs around day 14 (range, day 10-18) when the dominant follicle ruptures, releasing the ovum and a small amount of fluid into the peritoneal cavity. The corpus luteum develops at the site of the dominant follicle, forming a highly vascular mass that often becomes cystic as it fills with blood and lymph. The corpus luteum produces progesterone that maintains the secretory phase endometrium for successful implantation during the second half of the menstrual cycle. If fertilization of the ovum fails to occur, the corpus luteum will degenerate within 14 days. If fertilization occurs, human chorionic gonadotropin (hCG) from the fertilized ovum stimulates the corpus luteum to continue hormone secretion to support the developing pregnancy. The corpus luteum continues to function through the tenth week of gestation when the placenta replaces its hormone production. The corpus luteum may persist as a visible structure throughout pregnancy and must be recognized and differentiated from pathologic structures.
Figure 6. 1 Measurement of Mean Sac Diameter. The diameter of the gestational sac is measured between fluid-soft tissue interfaces (between pairs of closed arrows). No portion of the wall of the sac is included. Three orthogonal measurements are made in perpendicular imaging planes. These three measurements are averaged to calculate mean sac diameter. Standardized tables (Table 6.1) are consulted to relate mean sac diameter to gestational age. Most US scanners provide these tables in the computer memory. The choriodecidual reaction (open arrow) that defines the wall of the gestational sac is well defined and uniformly echogenic.
Figure 6. 2 Measurement of Crown-Rump Length. The crown-rump length is measured as the maximum length of the visualized embryo (between cursors, +) when the embryo is small (A). When the embryo is larger (B), crown-rump length is measured as the straight-line distance from the top of the cranium to the end of the torso (between cursors, +) of the embryo. No compensation is made for curvature of the embryo. Care must be taken to avoid including limbs or cord in the measurement. Standardized tables (Table 6.1) relate the crown-rump length measurement to gestational age. The umbilical cord (arrow) is visualized. y, yolk sac.
Table 6.1: Gestational Age Estimation
Gestational Sac Size Mean Sac Diameter (mm) Crown-Rump Length (mm) [3] Gestational Age Menstrual Dating (weeks) [2]
3 4.6
4 4.7
5 4.9
5.5 5.0
6 5.1
7 5.3
8 5.4
9 5.6
10 2 5.7
11 3 5.9
12 3.5 6.0
13 4 6.1
14 5 6.3
15 6 6.4
16 7 6.6
17 8 6.7
18 9 6.9
19 9.5 7.0
20 10 7.1
21 11 7.3
22 12 7.4
23 13 7.6
24 14 7.7
25 15 7.9
26 16 8.0
26.5 17 8.1
27 18 8.3
28 19 8.4
29 20 8.6
30 21 8.7
31 22 8.9
32 23 9.0
33 24 9.1
34 25 9.3
35 26 9.4
36 28 9.6
37 29 9.7
38 30 9.9
39 31 10.0
40 32 10.1
41 34 10.3
42 35 10.4
43 37 10.6
44 38 10.7
45 40 10.9
46 40 11.0
47 42 11.1
48 44 11.3
49 46 11.4
50 48 11.6
51 50 11.7
52 52 11.9
53 54 12.0
Adapted from: Robinson H. “Gestational sac” volumes as determined by sonar in the first trimester of pregnancy. Br J Obstet Gynaecol 1975;82:100-107; Hadlock F, Shah Y, Kanon D, et al. Fetal crown rump length: reevaluation of relation to menstrual age (5-18 weeks) with high resolution real time US. Radiology 1992;182:501-505.
  • Primordial follicles are too small to see with US [4].
  • Developing follicles appear as thin-walled, anechoic cysts that may enlarge up to 14 mm. These follicles are randomly distributed over the surface of the ovary (see Figs. 5.6 and 5.29). They are seen most prominently during cycle days 5-7.
  • A follicle larger than 14 mm is likely to be the dominant follicle, which continues to enlarge up to 25-30 mm prior to ovulation. In 5-11% of menstrual cycles, dominant follicles develop on each ovary.
  • Rupture of the dominant follicle with ovulation results in partial or complete collapse of its cystic structure (Fig. 6.3). Clotted blood and invading fibroblasts occupy the site, which appears as a partially collapsed cyst with wrinkled walls or as an echogenic mass. Fluid volume in the cul-de-sac averages 15-25 mL following ovulation [4].
  • The corpus luteum initially appears as small irregular cyst containing echogenic blood products (the corpus hemorrhagicum). In most patients the corpus luteum disappears with menstruation. In some patients it continues to enlarge through menstruation up to approximately 40-mm size. This persistent corpus luteum appears as a cystic structure containing anechoic fluid or fluid with particulate matter. It usually disappears within 1 or 2 subsequent menstrual cycles.
  • The corpus luteum of pregnancy is nearly always identified (98%) with TV US but has a variety of appearances [5,6]:
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  • Identification of origin of the corpus luteum on the ovary is key to its recognition and differentiation from other similar appearing structures.
  • Size of the corpus luteum averages 2 cm and varies up to 5-6 cm.
  • A solid-appearing hypoechoic structure on the ovary is most common (34%) (Fig. 6.4).
  • A cyst containing anechoic fluid and having a thick wall is seen in 27% (Fig. 6.5).
  • A cyst with floating debris and particulate matter is present in 23%.
  • A thin-walled cyst with anechoic fluid is seen in 15% (Fig. 6.6).
  • Intense blood flow surrounds the corpus luteum on color Doppler and may resemble the intense trophoblastic flow surrounding a GS (see Fig. 6.22). Spectral Doppler shows a low-resistance, spectral pattern with mean resistance index of 0.50.
Figure 6. 3 Early Corpus Luteum. The site of rupture of the dominant follicle soon after ovulation appears as a collapsed cystic structure (arrow) on the ovary (o). u, uterus.
Figure 6. 4 Corpus Luteum–Hypoechoic Solid Appearance. The corpus luteum appears as a hypoechoic solid mass (arrow) on the right ovary (o) on this transvaginal image.
Implantation and Early Appearance of the Gestational Sac
A zygote is formed when a sperm fertilizes the ovum, usually in the ampullary portion of the fallopian tube. Cell division occurs as the zygote migrates to the endometrial cavity. A
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cystic structure called the blastocyst is formed with two cell layers present. The outer cell layer is the trophoblast that forms the chorion and the fetal components of the placenta. The inner cell layer forms the embryo, umbilical cord, amnion and secondary yolk sac. The blastocyst implants 7-10 days after fertilization (around menstrual day 23) by burrowing into the endometrium. The thickened, secretory-phase endometrium, characteristic of the second half of the menstrual cycle, begins to secrete glycogen rich in mucin to support the pregnancy. The thickening and functional change of the endometrium in response to pregnancy is called decidual reaction, and the endometrium is now called decidua. The blastocyst continues to develop forming a recognizable GS completely covered by decidua.
Figure 6. 5 Corpus Luteum–Thick-Walled Cyst Appearance. Transvaginal scan shows an anechoic ovarian cyst (between calipers, +, x) with moderately thick walls.
Figure 6. 6 Corpus Luteum–Thin-Walled Cyst Appearance. This corpus luteum (arrow, between cursors, +, x) has a thin wall and contains anechoic fluid.
  • The early GS is first visible by TV US at approximately 4.5 weeks menstrual age. A small cystic structure 2-3 mm in size is seen burrowed into and completely covered by echogenic decidua, giving it the appearance of a thick-walled cyst (Fig. 6.7). The appearance of this very early GS is called the intradecidual sign [7]. This appearance is highly characteristic of early intrauterine pregnancy, although care must be taken to avoid mistaking fluid in the uterine cavity or an endometrial cyst for an early GS [8]. The thin
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    echogenic line of the uterine cavity must be recognized and observed to pass by, rather than meet, the sac burrowed in the decidual lining [9]. A fluid collection that meets or forms a beak with the uterine cavity line represents fluid in the uterine cavity and not an intradecidual GS. Endometrial cysts are found close to the basal layer of the endometrium and have a thin wall (Fig. 6.8), rather than the thick wall characteristic of a GS.
  • The secondary yolk sac is the first structure visible within the GS and is a finding that unequivocally confirms identification of the GS (Fig. 6.9). The yolk sac is usually visible by TV US at the start of the sixth menstrual week when the GS measures 8-10 mm. It serves a primary source of nutrients for the embryo before placental function is established. The yolk sac should always be visible on TA US when the GS measures 20-mm MSD [10]. It should always be seen with TV US when the GS measures 8-mm diameter [11]. The normal yolk sac is a thin-walled, fluid-filled cyst that does not normally exceed 6 mm in diameter [12]. The yolk sac is located in the chorionic cavity (Fig. 6.10). It is connected to the umbilicus of the embryo by the vitelline duct (omphalomesenteric duct).
  • The double bleb sign describes the early appearance of the amniotic cavity and the yolk sac seen as early as 5.5 weeks [13]. The yolk sac and amniotic cavity appear as two fluid-filled, thin-walled, spherical structures approximately equal in size within the GS (Fig. 6.11).
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  • The embryo is first visualized as an echogenic, disk-like structure approximately 2 mm long within the amniotic cavity. The embryo is normally seen by the end of the fifth menstrual week (Figs. 6.2A, 6.11). The normal embryo grows approximately 1 mm per day in length.
  • Embryonic cardiac activity may be seen with TV US when the embryo is as small as 1-2 mm. All normal embryos should have cardiac activity visible on TV US when the embryo measures 5 mm or more in length [14]. Initial visible embryonic heart rate at 5-6 weeks GA is 100 beats/minute. The rate increases over the next 2-3 weeks to 140 beats/minute [15].
Figure 6. 7 Intradecidual Sign. A 3-mm gestational sign (white arrow) is seen burrowed within the decidua. An echogenic line (black arrow) represents the uterine cavity.
Figure 6. 8 Endometrial Cyst. An endometrial cyst (arrow) in a non-pregnant patient has an appearance very similar to the early gestational sac shown in Figure 6.7. Note the location of the endometrial cyst in the basal layer of the endometrium near the myometrium.
Figure 6. 9 Yolk Sac in Early Pregnancy. Magnified transvaginal image shows a tiny cystic structure that contains a yolk sac (white arrow). Visualization of the yolk sac is diagnostic of this cystic structure being a gestational sac.
Figure 6. 10 Yolk Sac in Chorionic Cavity. In a more advanced gestation with an embryo (e) present, the yolk sac (long arrow) is a prominent cystic structure in the chorionic cavity outside of the amnion (short arrow).
Normal Structure of the Gestational Sac
The normal anatomy of the GS must be recognized to correctly interpret early pregnancy US examinations (Fig. 6.12). The maternal decidua consists of three layers (Fig. 6.13). The decidua capsularis covers the surface of the expanding GS as it protrudes into the uterine cavity. The decidua basalis combines with chorion frondosum to form the placenta at the site of implantation. The decidua vera (also called decidua parietalis) lines the remainder of the uterine cavity away from the placenta. Because the GS is implanted within and is covered
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by decidua, the uterine cavity remains open to the cervix. Bleeding associated with the pregnancy occurs into the uterine cavity and exits the uterus via the cervix. The outer membrane of the GS is the chorion. The inner membrane is the amnion (Fig. 6.10), which defines the amniotic cavity containing the developing embryo. The yolk sac is in the chorionic cavity, between amnion and chorion (Fig. 6.10). Low-level echoes are commonly seen within the fluid of the chorionic cavity. As the embryo grows, the amnion expands and becomes adherent to the chorion, obliterating the chorionic cavity at 15-16 weeks. Occasionally the amnion and chorion remain unfused throughout pregnancy. US features of the normal GS are listed below:
Figure 6. 11 Double Bleb Sign. The yolk sac (long white arrow) and amniotic sac (short white arro w) appear as two cystic structures of equal size within the fluid of the chorionic cavity. The embryo (black arrow) is seen within the amniotic cavity.
Figure 6. 12 Anatomy of Early Pregnancy. A line drawing illustrates the anatomic structures of early pregnancy.
  • The normal GS is round or oval in shape, smooth in contour, and is positioned in the fundus of the uterus. The choriodecidual reaction produces a smooth echogenic rim >2 mm in thickness around the GS (Fig. 6.1) [10].
  • The double decidual sign describes the appearance of two echogenic arcs of decidual tissue separated by a hypoechoic arc of fluid in the endometrial cavity (Fig. 6.14) [16].
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    The visualized decidual arcs consist of decidua vera and decidua capsularis. The double decidual sign partially encircles the GS. The decidual arcs are incomplete because of development of the placenta at the site of implantation.
  • Normal growth of the GS is 1.1 mm/day MSD for the first 8 weeks.
  • Doppler examination of the trophoblastic tissue that surrounds a pregnancy shows intense vascularity with a high-velocity, low-resistance spectral waveform. This vascular pattern has been characterized as a “ring of fire.” The presence of this rim of high vascularity can be used to differentiate intrauterine pregnancy (IUP) from the intrauterine pseudogestational sac of ectopic pregnancy [17]. The use of Doppler is controversial in early pregnancy because of the higher US energies utilized and concern for potential, but unproven, adverse effects on a normal embryo.
Figure 6. 13 Three Layers of Decidua. Transvaginal image illustrates the three layers of decidua. The amniotic sac and a normal embryo were visible elsewhere within this gestational sac. The placenta (P) forms from decidua basalis and chorion frondosum at the site of implantation of the blastocyst. Decidua capsularis (c) covers the portion of the gestational sac that protrudes into the uterine cavity (u). Decidua vera (v) lines the remainder of the uterine cavity. This uterine cavity contains a small amount of blood. The chorion is a very thin membrane defined only by the sharp fluid-tissue interface (arrow) of the gestational sac.
Figure 6. 14 Double Decidual Sign. Transabdominal image illustrates the two layers of echogenic decidua separated by a thin echolucent line that make up the double decidual sign (black arro w). The double decidual appearance extends only part way around the circumference of the sac. It is not present at the site of placental development (white arrow).
Role of Quantitative β-hCG Determinations
Radioimmunoassay (RIA) for the serum beta subunit of human chorionic gonadotropin (β-hCG) provides a specific and sensitive laboratory test for the presence of a fertilized ovum. If the RIA is negative, the patient is not pregnant. If RIA is positive, serial quantitative measurements are useful in diagnosis of normal and abnormal pregnancy. The problem comes in interpretation of results. Numerous papers have been written touting the value of “discriminatory levels” of β-hCG. For example, one study states that if serum β-hCG exceeds 1000 mIU/mL, then an intrauterine GS should always be identified in a normal pregnancy on TV US [18]. To apply this criterion, the first pitfall is to correlate the reporting standard used in the published study with the reporting standard used by the laboratory where you practice. Since the 1960s, a number of reporting standards have been utilized, including the First International Standard, the Second International Standard, the Third International Standard, and the International Reference Preparation. Reported values differ significantly depending on which standard is used. The second pitfall is that different laboratories using the same standard will report different results depending on which laboratory test kit they use. The range of normal values for a given week of pregnancy varies widely from one laboratory to another. These pitfalls make the use of reported discriminatory levels hazardous in any given practice unless specific correlation has been individually studied for a given laboratory and patient population. Further caution is warranted because even if these studies are correlated at each practice location, hospitals commonly change laboratories and test kits for cost considerations without informing practicing physicians.
In view of these difficulties with using numerical reported values, several uses of β-hCG are unequivocally valuable in any setting. First, if the qualitative β-hCG is positive, a pregnancy “event” has unequivocally occurred. The differential diagnosis may include all of normal and abnormal pregnancy. Second, for a given patient following serial β-hCG is useful in determining normal development. In normal pregnancy, β-hCG levels double every 48 hours. Failure to double in value every 48 hours is strong evidence of a non-viable, possibly ectopic, pregnancy.
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Abnormal First Trimester Pregnancy
Abnormal Intrauterine Pregnancy
Threatened Abortion
Bleeding and cramping in the first trimester of pregnancy while the cervix is closed is clinically diagnosed as a threatened abortion. US is utilized to make a specific and accurate diagnosis on which to base appropriate therapy. Threatened abortion affects 25% of all clinically apparent pregnancies. Up to 50% of these patients will abort their pregnancy. The differential diagnosis of threatened abortion is listed in Box 6.1.
Signs of Failed Pregnancy
Approximately 12% of all clinically apparent pregnancies will fail. These pregnancies develop to an early stage and then become non-viable. The following are diagnostic signs of a failed pregnancy (Box 6.2). Each sign depends upon visualization of a given structure of a minimum “discriminatory size” before the sign can be considered diagnostic. In every instance, TV US allows accurate diagnosis at an earlier stage of pregnancy. When measurements are close to the discriminatory size, follow-up examination should be considered to give the benefit of the doubt to the pregnancy [19]. Failed pregnancies may be classified an anembryonic pregnancy if no embryo is visualized, or as embryonic demise or fetal demise if an embryo or fetus is present.
  • Empty GS. Visualization of an “empty” GS, exceeding a discriminatory size that contains neither yolk sac nor embryo, is evidence of failed pregnancy (Fig. 6.15). Discriminatory sac size for non-visualization of a yolk sac is 8-mm MSD for TV US and 20-mm MSD for TA US [10,11].
  • Absent embryo. Discriminatory sac size for non-visualization of an embryo is 16 mm for TV US and 25 mm for TA US [10,11]. A failed pregnancy with absence of a visualized embryo is termed an anembryonic pregnancy or blighted ovum (Fig. 6.15).
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  • Empty amnion. Visualization of the amnion without visualization of an embryo is a related sign of failed pregnancy (Fig. 6.16). Between 6.5 and 10 weeks gestation (GS-MSD of 14-36 mm), the diameter of the amniotic cavity is normally equal to the CRL of the fetus [20]. An embryo should always be visualized if the amniotic cavity measures 6-mm diameter or above. The amnion is a thin membrane that separates the amniotic cavity from the chorionic cavity. Visualization requires high-quality technique.
  • Dead embryo. Visualization of an embryo without cardiac activity is proof of a failed pregnancy (Fig. 6.17). The discriminatory embryonic size to unequivocally diagnose absent heartbeat is 4-5-mm CRL for TV US and 9-mm CRL for TA US [21,22,23]. Absent
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    heartbeat is distinguished from bradycardia by observing the cardiac area for at least 2 minutes, preferably by two US examiners. Most early embryonic demise is caused by fatal chromosome abnormalities [24].
  • Abortion in progress. A distorted GS presenting in the lower uterine segment with an open cervical os is classified as an abortion in progress or inevitable abortion (Fig. 6.18). Complete expulsion of the GS soon follows.
  • Blighted twin. Demise of one twin in a twin pregnancy is a relatively common event in the first trimester. The dead embryo will usually be reabsorbed while the live embryo usually develops normally (Fig. 6.19). US reveals no embryo or a dead embryo in a usually smaller or partially collapsed GS. Alternatively, one GS contains an embryo and the second GS is empty. The GS of a blighted twin may be mistaken for a subchorionic hemorrhage.
Figure 6. 15 Empty Gestational Sac. Transvaginal image shows a large, misshapen gestational sac. This sac measured 31 mm mean sac diameter. At this size both a yolk sac and an embryo should be visualized on a transabdominal US examination. The sac is keyhole-shaped rather than round or oval. The choriodecidual reaction is thin and only weakly echogenic. This appearance is diagnostic of an anembryonic pregnancy.
Figure 6. 16 Empty Amnion. Visualization of the amnion (arrow) without the presence of an embryo is evidence of a failed pregnancy. The size of the amniotic sac (~12 mm) far exceeds the normal size of the yolk sac (~6 mm) and confirms identification of this membrane as amnion. Cursors (+) measure the diameter of the gestational sac.
Figure 6. 17 Dead Embryo–Calcified Yolk Sac. A 6.3-week embryo (between cursors, +, crown-rump length = 5.8 mm) without a heartbeat is seen within the amniotic cavity. The yolk sac (arrow) is calcified. Calcification of the yolk sac is associated with embryonic demise.
Figure 6. 18 Abortion in Progress. Transabdominal image shows a gestational sac (black arrow) containing a dead embryo (between cursors, +) presenting at an open cervical os. Expulsion of the pregnancy is inevitable. The balloon (white arrow) of a Foley catheter is evident in the bladder.
Signs That Predict a Poor Outcome of Pregnancy
The following US signs are predictive of poor outcome (failure) of a first trimester pregnancy (Box 6.3).
  • Bradycardia. An embryonic heart rate below 85 beats per minute is strong evidence of impending embryonic demise [25].
  • Oligohydramnios in the first trimester results in small size of the GS relative to the size of the embryo. The MSD of the GS normally exceeds the CRL by 5 mm or more between
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    5.5 and 9 weeks GA. A small sac (less than 5 mm larger than CRL) is an indicator of incipient spontaneous abortion [26].
  • Subchorionic hemorrhage is a common cause of vaginal bleeding in the first trimester affecting up to 18% of women who present with bleeding [27]. Subchorionic hemorrhage is believed to be a mild form of placental abruption with venous bleeding arising from the edge of the placenta (Fig. 6.20). Hemorrhage strips chorion from the myometrium and extends into the uterine cavity. US shows the hemorrhage as a crescent-shaped collection of fluid between the GS and the uterine wall. The appearance of hemorrhage depends upon its age. Acute clotted blood is isoechoic or hyperechoic to the placenta. As the clots dissolve the collection becomes progressively hypoechoic to anechoic. Small subchorionic hemorrhages are of no clinical significance. However, large hemorrhages exceeding 40% of the volume of the GS are associated with fetal loss rates of up to 50% [28,29]. Risk of spontaneous abortion increases with the size of the hemorrhage, older age of the mother (>35 years), and with earlier gestations (8 weeks or less) [30].
  • Abnormal yolk sac. An abnormal appearance of the yolk sac correlates with early pregnancy failure. The yolk sac may be too large (>6 mm) (see Fig. 6.28), irregular in shape, have a thick wall, or be calcified (Fig. 6.17) [12,31,32]. Differentiation of an enlarged yolk sac from the amniotic cavity is difficult but irrelevant because any empty cystic structure within the GS larger than 6-mm diameter predicts a failed pregnancy.
  • Thin (<2 mm), poorly echogenic decidua is a poor prognostic sign for first trimester pregnancy but is difficult to recognize with certainty (Fig. 6.15). It adds to the evidence of impending failure of pregnancy when other signs are present [10].
  • Abnormal position of the GS correlates with poor prognosis (Fig. 6.21). Any position of the GS except within the fundus of the uterus is abnormal.
  • Uterine anomalies are associated with an increased rate of pregnancy loss (Fig. 6.22).
Figure 6. 19 Blighted Twin. Transvaginal image shows two gestational sacs (black arrows). The larger sac (large black arrow) contained a live embryo. The smaller, deformed sac (small black arrow) contained no embryo. Note the double decidual sign (white arrow).
Retained Products of Conception
Retention of products of conception (POC) within the uterus following spontaneous abortion or delivery is associated with risk of bleeding, uterine infection, and synechiae formation. Following death of the embryo, the pregnancy is usually retained in the uterus for a week or longer until hormone levels decrease and vascular support of the pregnancy atrophies. Decidual necrosis causes uterine irritability that expels the pregnancy. Patients who present with bleeding or signs of infection following spontaneous or assisted abortion are examined for retained POC.
  • Thin endometrium (<2 mm) is indicative of the absence of retained POC. The decidua becomes necrotic following demise of the pregnancy and should be completely sloughed with expulsion of the pregnancy [33].
  • Any cystic or echogenic space-occupying collection in the uterine cavity should be considered to be POC (Fig. 6.23).
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  • Thickening of the hyperechoic endometrium >5 mm is suggestive of POC [33].
  • A thin layer of hypoechoic material in the uterine cavity is more likely to be blood than POC.
  • Gas in the endometrial cavity produces focal bright echoes with ring down or acoustic shadowing. Gas is found in the uterine cavity in 15% of normal pregnancies postpartum and cannot by itself be considered evidence of endometritis [34].
Figure 6. 20 Subchorionic Hemorrhage. A. A subchorionic hemorrhage results from bleeding from the margin (curved black arrow) of the placenta (P) into the uterine cavity (h). The amniotic cavity containing the developing embryo (long white arrow) is separated from the hemorrhage by fused amnion/chorion (short white arrow) covered by decidua capsularis. B. A larger subchorionic hemorrhage is seen in another patient nearly completely encircling the gestational sac. The sac is anchored to the uterine wall at its placental attachment (arrow). C. Another hemorrhage shows clotted blood (arrow) in the uterine cavity.
Figure 6.21 Low Position of Gestational Sac. This small deformed gestational sac (arrow) is positioned low in the uterine body. Implantation of the gestational sac anywhere except in the uterine fundus is a poor prognostic sign.
Figure 6. 22 Pregnancy in Septate Uterus. Transvaginal image shows an empty gestational sac (curved arrow) in the left horn of a septate uterus. Echogenic decidual reaction is seen in the right horn (fat arrow). A thin muscular septum (short arrow) separates the two chambers of the uterus. A small septum, such as this one, can usually be resected hysteroscopically and result in improved prognosis for successful completion of subsequent pregnancies.
Ectopic Pregnancy
An ectopic pregnancy is implantation of a fertilized ovum outside of the fundus or body of the uterine cavity. Any pregnant woman may have an ectopic pregnancy but the risk is increased when there is a past history of pelvic inflammatory disease, tubal surgery, previous ectopic pregnancy, use of intrauterine device, ovulation induction, or in vitro fertilization. The ectopic pregnancy is prone to rupture with hemorrhage that may be fatal. Ectopic
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pregnancy is responsible for approximately 15% of maternal deaths. Patients present with pelvic pain, cramping, or vaginal bleeding.
Figure 6. 23 Retained Products of Conception. Longitudinal (A) and transverse (B) transabdominal images show an enlarged uterine cavity containing amorphous echogenic material (between arrows).
US currently plays a critical role in the diagnosis or exclusion of ectopic pregnancy. US findings most often provide an assessment of the risk of ectopic pregnancy, rather than a specific diagnosis. The initial goal of US examination is to demonstrate evidence of an IUP. When an IUP is definitely present, the risk of coexisting ectopic pregnancy is small, estimated at 1 in 30,000 for the general population and 1 in 6,000-7,000 for the high-risk population [35]. Patients at higher risk for heterotopic pregnancy (simultaneous intrauterine and ectopic pregnancy) are those who are under treatment for infertility. Detailed evaluation of the adnexa is mandatory even if an IUP is present.
Most (95%) of ectopic pregnancies implant within the ampullary or isthmic portions of the fallopian tube. Approximately 2-4% of ectopics implant within the intramural (interstitial) portion of the tube as it traverses the uterine wall. Cervical, abdominal, and ovarian implantations are rare with each accounting for less than 1% of ectopic pregnancies.
A definitive diagnosis of ectopic pregnancy is made only when a live embryo or GS containing a yolk sac is identified clearly outside of the uterus (Figs. 6.24, 6.25). Unfortunately this result is present in only 20% of ectopic pregnancies.
The following is a list of US findings in ectopic pregnancy. The approximate risk of ectopic pregnancy associated with each finding is stated. This risk is stated with the caveat that no IUP is identified in association with the extrauterine findings.
  • IUP is confirmed by demonstration of intrauterine GS containing a yolk sac or living embryo. Risk = 1 in 7,000 to 1 in 30,000 for general population [35]. However, it is reported as high as 1 in 100 for infertility patients treated with ovulation induction or in vitro fertilization [36].
  • Intradecidual sac. This sign is highly but not perfectly predictive of IUP. Follow-up US to identify a yolk sac or embryo and serial serum β-hCG to confirm appropriately increasing levels are needed to confirm IUP. Risk = very low, follow-up needed [8].
  • Extrauterine GS containing yolk sac is diagnostic of ectopic pregnancy (Fig. 6.24). Risk = 100%.
  • Extrauterine GS containing living embryo is diagnostic of ectopic pregnancy (Fig. 6.25). Risk = 100%.
  • Extrauterine “tubal ring,” an echogenic thick-walled, ring-like mass separate from the ovary, represents a GS with surrounding trophoblastic reaction (Figs. 6.24, 6.25A). Risk = ~95% [37].
  • Complex cystic or solid adnexal mass without distinguishing features. This finding is consistent with tubal rupture and clotted blood (Fig. 6.26). Risk = ~86% [38].
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  • Small volume of anechoic-free intraperitoneal pelvic fluid. Risk = normal finding, not predictive of ectopic pregnancy.
  • Moderate or large volume of free intraperitoneal fluid, particularly if the fluid is echogenic (Fig. 6.27). Risk = ~70% [37].
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  • The presence of both complex or solid adnexal mass and echogenic fluid is virtually diagnostic of ectopic pregnancy. Risk = ~98%.
  • Ovarian masses are unlikely to represent an ectopic pregnancy. Most ovarian masses are corpus luteum cysts. The wide range of appearance of corpus luteum cysts has been previously described (Figs. 6.3, 6.4, 6.5 and 6.6). Thick-walled cysts may closely resemble the tubal ring of an ectopic pregnancy. Clot within a hemorrhagic cyst may closely resemble a dead embryo. Blood flow within or adjacent to the cyst may resemble the ring of fire on color flow US. Risk = physiologic finding not predictive of ectopic pregnancy.
  • Thin-walled ovarian cyst containing anechoic fluid is likely the corpus luteum. Risk = normal finding, not predictive of ectopic pregnancy.
  • Double decidual sign (Fig. 6.14) is highly predictive of IUP, although the IUP may not be viable. Risk = very low.
  • Intrauterine fluid collection may represent the “pseudogestational sac” seen with ectopic pregnancy or may be retained fluid associated with recent failed pregnancy or spontaneous abortion (Figs. 6.27, 6.28). The intrauterine fluid collection must be differentiated from a true GS by careful attention to detail. Pseudosac fluid is commonly echogenic and may show dependent layering. The shape of the fluid collection is usually non-spherical. No double decidual sign is present. Doppler shows minimal low-velocity flow or no flow. Risk depends upon coexisting findings. An IUP is not confirmed.
  • Normal US examination. Because the patient is pregnant an ectopic pregnancy is not excluded. Differential diagnosis includes early normal IUP, early abnormal IUP, completed abortion, and ectopic pregnancy. Risk = ~5% [38].
  • Doppler findings on adnexal masses are usually noncontributory. The prominent low-impedance “ring of fire” blood flow (Fig. 6.29) seen in trophoblastic tissue surrounding the GS is also seen with tubo-ovarian abscess, corpus luteum cyst, malignant ovarian tumors, and pedunculated leiomyomas. Risk = not affected.
Figure 6. 24 Ectopic Gestational Sac Containing Yolk Sac. An extrauterine sac (between cursors, +, x) shows a tubal ring sign with thick echogenic wall and contains a yolk sac (arrow). The presence of the yolk sac is diagnostic of extrauterine gestation.
Figure 6. 25 Ectopic Pregnancy–Live Embryo. A. A transabdominal image in transverse plane shows an empty uterus (u) with thickened endometrium (white arrow) representing decidual reaction. Posterior to the uterus is a thick-walled cystic mass (solid black arrow), a “tubal ring sign.” Fluid is seen in the cul-de-sac (open black arrow). b, urine-filled bladder. B. Transvaginal image in the same patient shows a small living embryo (between cursors, +) with cardiac motion easily visible during real-time US examination. The amnion (arrow) is also seen within this ectopic gestational sac.
Figure 6. 26 Ectopic Pregnancy–Amorphous Adnexal Mass. Transvaginal image shows a lobulated heterogeneous mass (between cursors, +, x) posterior to the uterus (u). This mass proved to be a tubal ectopic pregnancy adjacent to the right ovary (o). A small volume of fluid is present in the uterine cavity (arrow).
Figure 6. 27 Ectopic Pregnancy–Echogenic Fluid in Cul-de-Sac. Longitudinal transabdominal image shows a moderate volume of echogenic fluid in the cul-de-sac (fat black arrow). A round, amorphous, solid-appearing mass (long black arrow) is evident superior to the uterus (u). Within the uterus is an unusually shaped fluid collection (white arrow), a “pseudosac” of ectopic pregnancy. The combination of adnexal mass and echogenic cul-de-sac fluid makes this patient very high risk (~98%) for ectopic pregnancy. b, bladder.
Interstitial Ectopic Pregnancy
An interstitial ectopic pregnancy implants in the intramural portion of the tube as it traverses the uterine wall. It tends to present clinically later in the course of development with the GS and its blood supply is significantly larger. Rupture then presents a greater risk of massive hemorrhage.
Figure 6. 28 Pseudogestational Sac of Ectopic Pregnancy. A. Transabdominal image shows a small fluid collection (white arrow) in the uterine cavity. Transvaginal US is needed to characterize the appearance of the fluid collection. Echogenic fluid is noted in the cul-de-sac (black arrow). B. Transvaginal image in the same patient shows the fluid collection (arrow) is oblong, not round or oval, and lacks the well-defined echogenic rim of choriodecidual reaction. No double decidua sign is present. This patient had a small tubal pregnancy in the right adnexa. C. In another patient, transvaginal US shows a pseudosac (long white arrow) containing layering echogenic fluid (short white arrow). No double decidual sign is present. Fluid is also evident in the cul-de-sac (black arrow).
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  • Asymmetric thickness of myometrium around the GS (Fig. 6.30). The myometrial mantle may be partially absent [39].
  • GS is eccentric to empty uterine cavity (Fig. 6.30).
  • Many cases do not have a recognizable GS. An inhomogeneous mass is seen in the cornual region [40].
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  • The interstitial line sign has been reported as a reliable sign of interstitial ectopic pregnancy. An echogenic line extends from the uterine cavity to abut the center of the eccentric mass or GS. This line represent the interstitial portion of the fallopian tube [40].
  • Leiomyomas may displace the GS and simulate an interstitial ectopic pregnancy (Fig. 6.31).
  • Uterine anomalies, such as septate uterus, may also be mistaken for interstitial ectopic pregnancy (Fig. 6.32).
Figure 6. 29 Ring of Fire. This ring of fire surrounds a corpus luteum. Similar intense color depicting hypervascularity may be seen surrounding a normal pregnancy, ectopic pregnancy, tubo-ovarian abscess, malignant ovarian tumor, or pedunculated leiomyoma (see Color Figure 6.29).
Figure 6. 30 Interstitial Ectopic Pregnancy. Transvaginal image shows an eccentric intrauterine gestational sac (long arrow) with well-defined echogenic rim. The endometrium (e) of the fundal region was ill defined, and the uterine cavity was empty. Note the asymmetric and thin myometrium (short arrow) surrounding a portion of the sac. Surgery confirmed a gestational sac implanted in the interstitial portion of the fallopian tube.
Ovarian Ectopic Pregnancy
Ectopic pregnancy implantation on the ovary is rare. Most ovarian masses discovered in a pregnant woman are corpus luteum cysts or preexisting ovarian lesions such as benign cystic teratoma or cystadenoma.
  • A GS is implanted on the ovary.
Figure 6. 31 Leiomyoma Simulates Interstitial Pregnancy. The gestational sac is eccentrically positioned in the uterus and is surrounded by an asymmetrically thinned mantle of myometrium (black arrow). An interstitial ectopic pregnancy was suspected. At surgery, a leiomyoma was found to be displacing the gestational sac. The leiomyoma was seen in retrospect as a heterogeneous area of myometrium (white arrow).
Figure 6. 32 Eccentric Gestational Sac in a Septate Uterus. An intrauterine pregnancy in a septate uterus also demonstrates an eccentric gestational sac (white arrow). An empty smaller horn can be recognized as the cause of the eccentricity. The endometrium of the empty horn (long black arrow) is thickened and echogenic, representing decidual reaction. The septum (short black arrow) is visualized.
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Cervical Ectopic Pregnancy
Patients usually present with painless vaginal bleeding at 6-12 weeks of pregnancy. Implantation in the less vascular cervical tissue provides insufficient blood supply for the pregnancy to progress.
  • The GS is implanted abnormally low within or near the cervical canal.
  • Differentiation from an aborting GS in the cervix may be difficult. A live embryo within a normal-appearing GS is found with a cervical ectopic pregnancy. An aborting sac appears misshapen and contains an embryo without a heartbeat.
  • A large nabothian cyst may simulate a cervical ectopic pregnancy.
Abdominal Pregnancy
Abdominal pregnancies are associated with fetal mortality as high as 90% and maternal mortality of 6-14%. Despite its importance this diagnosis is commonly missed. The pregnancy may implant anywhere in the abdominal cavity but is most common in the pouch of Douglas, the posterior uterine wall, and the anterior abdominal wall.
  • No myometrium is present around the fetus or the GS. However, this seemingly obvious finding may be difficult to recognize because trophoblastic tissue and the mother’s abdominal wall may simulate the myometrium (Fig. 6.33).
  • An empty uterus is identified. The uterus may be squashed deep in the pelvis and is difficult to identify, particularly if the pregnancy is large. The endometrium is thickened due to decidual reaction.
  • The location of the fetus is unusual.
  • The presentation of the fetus is unusual. Persistent transverse lie is common.
  • The lower uterine segment and cervix are not clearly identified.
  • Magnetic resonance imaging may be definitive when the US diagnosis is uncertain.
Heterotopic Pregnancy
The simultaneous presence of intrauterine and ectopic pregnancy has become increasingly common with the use of in vitro fertilization as treatment for infertility. In some reports, heterotopic pregnancy is as common as 1-in-100 pregnancies in this population [36]. This fact stresses the need for detailed examination of the adnexa, even when an IUP is documented.
  • An IUP is present.
  • Signs of one or more ectopic pregnancies are also present (Fig. 6.34).
Gestational Trophoblastic Disease
Gestational trophoblastic disease (GTD) is a proliferative disease of the trophoblast that ranges from benign and highly curable to aggressively malignant [41,42]. The trophoblast
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is the functional unit of the placenta, originating as the outer covering of the blastocyst. The normal trophoblast has vigorous invasive and proliferative properties, needed for normal placental development, that are accentuated in this disease. GTD tissue usually has an abnormal karyotype. Patients present in the first trimester with hyperemesis, toxemia, or bleeding. Uterine size is usually larger than expected for gestational age. Serum β-hCG is always substantially elevated. The spectrum of GTD includes hydatidiform mole (complete and partial), invasive mole, and choriocarcinoma.
Figure 6. 33 Abdominal Pregnancy. A. Transabdominal image in transverse plane demonstrates an empty uterus (arrows) and a complex cystic mass (between cursors, +) in the cul-de-sac. B. Sagittal MR image clearly shows the empty uterus (arrow) and the advanced pregnancy in the abdominal cavity.
Complete Hydatidiform Mole
Complete (classic) mole is the benign, non-invasive, most common (80% of cases) type of GTD. Placental villi show excessive proliferation and hydropic swelling. Karyotype is diploid
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(46, XX) with all chromosomes of paternal origin. The ovum is believed to lose its haploid (23, X) chromosomes. Fertilization by a 23, X sperm follows and these paternal chromosomes are duplicated. US is usually diagnostic. Treatment is dilatation and suction curettage with evacuation of all POC [41,42].
Figure 6. 34 Heterotopic Pregnancy. Transabdominal image documents a live intrauterine pregnancy (IUP) and two ectopic pregnancies (1, 2) in this patient who had recently undergone in vitro fertilization.
Figure 6. 35 Snowstorm Appearance of First Trimester Molar Pregnancy. Transvaginal images (A, B) of two patients show the echogenic “snowstorm” appearance characteristic of molar pregnancy in the first trimester. In B, tiny cysts are barely visualized.
  • The uterus is larger than expected for dates.
  • In the first trimester, the US appearance is more variable. A “snowstorm” appearance of a coarse granular echogenic mass without discrete cysts filling the uterine cavity is often seen (Fig. 6.35). A variant appearance seen in early gestation is a large central fluid collection indistinguishable from anembryonic pregnancy. Theca lutein ovarian cysts are rarely present in first trimester molar pregnancy [43].
  • In the second trimester, the US appearance is usually classic and diagnostic. The uterine cavity is distended and filled with a moderately echogenic heterogeneous mass. TV US resolves innumerable small cystic spaces within the heterogeneous mass (Fig. 6.36). These small cysts correspond to the hydropic chorionic villi seen pathologically. The vesicles increase in size up to 30 mm with increasing gestational age [41,42].
  • In the second trimester the ovaries commonly (40% of cases) demonstrate enlargement with multiple bilateral theca lutein cysts (Fig. 6.37). These result from ovarian hyperstimulation caused by high circulating levels of β-hCG. Hemorrhage or rupture occasionally complicates these cysts. They resolve within a few months of effective treatment of the mole. Persistence or enlargement of the cysts post-treatment is evidence of continued disease.
  • Coexistence of a fetus with a complete mole is rare (1-2% of cases) and occurs only with dizygotic twinning. The karyotype of the fetus is normal while that of the mole is abnormal. The fetus is supplied by a normal placenta. The “twin” is a complete mole. Survival of the fetus is unlikely because of complications of treatment of the molar twin [44].
  • Interpretation must be made in coordination with complete clinical evaluation. The differential diagnosis of cystic mass in the uterus of a pregnant patient is given in Box 6.4.
Figure 6. 36 Classic Molar Pregnancy in Second Trimester. Transabdominal images (A, B) in two patients demonstrate the classic appearance of molar pregnancy seen in the second trimester. The uterine cavity is expanded and filled with an echogenic mass with innumerable cysts of varying size. No embryo is present.
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Partial Hydatidiform Mole
In distinction from complete mole in which all placental villi are hydropic, in partial mole normal villi are mixed in with hydropic villi and trophoblastic proliferation is much less pronounced. The karyotype is triploid with fertilization of an ovum by two sperm (69, XXX; 69, XXY; or 69, XYY). Partial moles have an abnormal fetus present. The fetus is triploid
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with the same karyotype as the molar tissue and does not survive. Partial moles commonly present with spontaneous abortion or fetal demise.
Figure 6. 37 Theca Lutein Cysts. The ovary is massively enlarged by the presence of numerous cysts. In this patient, the enlargement of the ovary with theca lutein cysts 7 weeks after evacuation of the uterus for molar pregnancy was evidence of persistent gestational trophoblastic disease.
  • The placental changes of mole are less pronounced and may be focal with areas of more normal-appearing placenta (Fig. 6.38) [42].
  • The fetus is usually grossly abnormal with multiple anomalies and growth retardation. Fetal demise is frequent.
Invasive Mole
Invasive mole is a form of persistent GTD that occurs in approximately 10% of patients with complete mole and less frequently in patients with partial mole. Patients present with bleeding and persistent elevation of β-hCG after treatment for molar pregnancy. In invasive mole the trophoblasts invade the myometrium and blood vessels and may metastasize to the lungs and other tissue. The diagnosis is made by clinical findings and elevated β-hCG levels. US is used primarily to exclude pregnancy as a cause of elevated β-hCG [41,42]. Treatment is chemotherapy.
  • US shows the findings of complete mole. Invasion of the myometrium by the echogenic mass is sometimes evident. MR is more sensitive in demonstrating invasion of the junctional zone myometrium but has little impact on therapy.
Choriocarcinoma
Choriocarcinoma is a malignant neoplasm of the chorionic epithelium that complicates 2-5% of molar pregnancy, but may also occur following normal pregnancy, ectopic pregnancy, or spontaneous abortion. Prognosis is best when choriocarcinoma follows molar pregnancy
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with nearly all cases curable. Pathology shows invasive trophoblasts without formation of villi. Extensive necrosis and hemorrhage are usually present. As with invasive mole, patients present with bleeding and persistent elevation of β-hCG. Treatment is chemotherapy [41,42].
Figure 6. 38 Partial Molar Pregnancy. The placenta (black arrow) is enlarged, misshapen, and contains small cysts throughout. A yolk sac (small white arrow) and portion of a dead embryo (large white arrow) are visualized. The yolk sac is abnormally large, measuring 11 mm. The patient’s serum β-hCG was elevated well above normal for gestational age.
Figure 6. 39 Embryonic Limb Buds. Primitive limb buds (short arrows) are seen in this 9.5-week embryo. The amnion (long arrow) is also visible.
  • US shows a heterogeneous, necrotic, hemorrhagic, infiltrative mass enlarging the uterus. The mass may extend through the uterine wall into the parametrium [41,42].
  • Large, frequently hemorrhagic metastases may be found in the lungs (75%), brain, vagina, kidneys, bowel, and liver. Metastases to lymph nodes and bone are uncommon.
Normal Developmental Anatomy of the Embryo
Normal embryologic structures developing in the first trimester must be recognized to avoid mistaking them for anomalies.
  • Primitive limb buds are visible by 8 weeks menstrual age (Fig. 6.39).
  • The rhombencephalon is seen as a normal but prominent cystic structure in the posterior cranial fossa at 6-8 weeks (Fig. 6.40). This structure must not be mistaken for Dandy-Walker or other cranial anomaly. The rhombencephalon will form the fourth ventricle [45].
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  • The midgut herniates into the base of the umbilical cord between 9 and 11 weeks. This physiological event must not be mistaken for omphalocele. Normal herniated bowel forms a small echogenic mass 6-9 mm in size on the midline anterior abdominal wall (Fig. 6.41) [46].
  • The choroid plexus becomes visible as prominent bilateral echogenic structures in the brain by 9-10 weeks. The choroid is normally very large compared to the size of the cranial fossa (Fig. 6.42). As the brain and cranium grow, the choroid changes little in size and appears relatively smaller later in pregnancy.
  • Nuchal lucency. Between 9 and 12 weeks, an anechoic area between the neck or occiput and the skin of the embryo that measures 3 mm or greater in thickness is indicative of increased risk of the fetus having trisomy 21 (Down’s syndrome) or other chromosome anomaly (Fig. 6.43A) [47]. The risk is approximately 7% in women over age 35. Amniocentesis
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    or chorionic villus sampling for karyotype is usually recommended. Care must be taken to ensure the nuchal lucency is not caused by covering amnion (Fig. 6.43B).
Figure 6. 40 Normal Rhombencephalon. Transvaginal image of an 8-week embryo shows a prominent, but normal, cystic rhombencephalon (black arrow) as the dominant structure in the cranium. The amnion is evident (white arrow).
Figure 6. 41 Physiologic Herniation of the Midgut. Embryo at 8 weeks shows normal herniation of the midgut into the base of the umbilical cord as an echogenic mass (arrow) less than 10-mm size on the midline anterior abdominal wall. The abdomen (A) of the embryo is seen in transverse plane.
Figure 6. 42 Normal Choroid Plexus. The normal choroid plexus is seen as a prominent echogenic structure in the brain of the embryo. The choroid plexus fills the lateral ventricle (between cursors, +). Note that brain anatomy is usually obscured by reverberation artifact in the portion of the cranium nearest the transducer.
Figure 6. 43 Nuchal Lucency. A. Prominent nuchal lucency extends over the head (H), neck (N), and thorax (T) of this 11.5-week fetus. In the neck region, the nuchal lucency (arrow) measured 7 mm, exceeding the normal upper limit of 3 mm in the first trimester. Amniocentesis confirmed the presence of trisomy 21. B. Apparent nuchal lucency (arrow) was confirmed by careful examination to be caused by the amnion lying in close proximity to the neck of the embryo. This is the major pitfall in recognizing abnormal nuchal lucency in the first trimester.
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