Sonographic Features at 12 Weeks’ Gestation

Although the routine second trimester (18- to 20-week) scan is still considered to be the ‘gold standard’ point for anatomical assessment, it is possible to detect major structural abnormalities, including some abdominal defects, at 11 to 13+6 weeks ( Fig. 32.1 ). In a series of 44,859 pregnancies that had a structured sequential anatomical survey completed at the 11- to 13+6-week scan, 488 (1.1%) fetuses had a structural abnormality, and 213 (43.6%) of these were successfully identified. This included all 104 cases of exomphalos, gastroschisis, megacystis and body stalk anomaly. In contrast, none of the three reported cases of bowel atresia were detected at 11 to 13+6 weeks.

A composite of ultrasound images showing normal anatomy in axial and longitudinal section ( A and B ) and demonstrating the presence of gastroschisis ( C ) and exomphalos ( D ) at 12 weeks’ gestation. CRL, Crown–rump length.

The fetus is traditionally assessed in midsagittal section in the first trimester, which allows accurate assessment of crown rump length for confirmation of gestational age and measurement of nuchal translucency thickness, but this is not the most valuable plane for assessment of the abdomen and anterior abdominal wall. This is best achieved by rotating the probe so that the fetus is imaged in an axial section. The probe can then be manipulated to sweep through the abdomen, visualising structures of importance within a few seconds. Moving caudally from the thorax through the diaphragm, in the upper abdomen, the stomach should be visible to the left of the midline. The stomach is normally visible in all cases from 11 weeks’ gestation. Care should be taken to ensure that situs is appropriate. To the right of the midline at this anatomical level, the parenchyma of the liver can be seen, which is typically homogeneous with no echogenic foci. The hepatic portion of the umbilical vein can be used as a second landmark to define the correct plane for measurement of the abdominal circumference.

After the upper abdomen has been assessed, the probe can be swept down to the level of the umbilicus, and care should be taken to define the integrity of the insertion of the umbilical cord. In early pregnancy (i.e. <9 weeks’ gestation), space within the abdominal cavity is limited, and the developing small bowel extrudes through the umbilicus into the base of the cord. This should have returned to the abdominal cavity by 11 weeks’ gestation, and continued herniation of bowel or other intraabdominal viscus should be regarded as evidence of exomphalos. The features of this anomaly are discussed in more detail later. Colour Doppler can also be used to identify the umbilical arteries as they enter the abdomen, dividing around the bladder. A two-vessel cord can be detected if care is taken in this evaluation and has been reported to be associated with an increased risk for aneuploidy as well as with renal anomalies and is therefore a useful marker in the assessment of these disease processes. In addition to focusing on the umbilicus, it is important to check that there is no evidence of bowel herniation to the right of the midline; gastroschisis is also readily detected at 11 to 13+6 weeks when free loops of bowel, which are not contained within a membrane, can be seen floating freely in the amniotic fluid.

Sonographic Features at 20 Weeks’ Gestation

The abdomen and pelvis are traditionally assessed using three axial sections at 18 to 20 weeks ( Fig. 32.2 ). Moving sequentially from the thorax through the diaphragm, the upper abdomen is assessed in an axial section that demonstrates the echolucent stomach, the liver and the mid third of the umbilical vein at the level of the portal sinus. The upper poles of the kidneys should not be visible in this view. An additional cystic structure extending to the right may be visible, representing the fetal gallbladder. The abdominal circumference is measured at this level by placing callipers on the outer surface of the skin line and using either perpendicular linear measures or an ellipse to complete assessment. From an anatomical perspective, this view is used to check that the stomach is visible and to check situs and consistent echogenicity across the liver. In combination with coronal or longitudinal sections, this view is also used to demonstrate the integrity of the left and right hemidiaphragms.

A composite of ultrasound images showing normal anatomy in axial sections through the upper abdomen ( A ), at the level of the umbilicus ( B ) and in the pelvis ( C ) at 20 weeks’ gestation.

Moving caudally, the fetal kidneys can be demonstrated lying either side of the spine. Anteriorly, the cord insertion is assessed to ensure integrity of the anterior abdominal wall. Turning into a longitudinal section, the distance between the insertion of the umbilicus and the genital tubercle can also be assessed. The lumen of the small and large bowel is not typically obvious at this gestation, and dilated loops can be detected if present. The small bowel may be echogenic, which has been associated with a range of pathologies described later. An axial section of the pelvis is used to demonstrate the presence of the bladder, and colour Doppler can be applied to show the bifurcation of the umbilical arteries and therefore the presence of a three-vessel cord. Moving caudally, the external genitalia can also be assessed.

Sonographic Features after 28 Weeks’ Gestation

Although a third trimester scan is not routinely performed in many jurisdictions, the majority of women seen in our practice are referred by their obstetrician for sonographic review on at least one occasion beyond 28 weeks’ gestation. The third trimester scan typically focuses on fetal growth and well-being but presents an opportunity for a brief review of systems that are either difficult to examine at early gestations (e.g. the heart) or of systems in which pathologies may only become apparent with advancing gestation (the urinary system and abdominal defects such as small and large bowel obstruction). Best practice therefore includes assessment of fetal anatomy during a third trimester growth and well-being scan; in the case of the abdomen, it is sensible to specifically assess the stomach and bowel and to look for any other aberrant cystic or solid masses within the abdomen or pelvis.

Gastroschisis

Gastroschisis is an anterior abdominal wall defect in which bowel herniates into the amniotic fluid. In contrast to an exomphalos, the abdominal wall defect lies to the right of the midline, and the herniated bowel is not covered by a peritoneal membrane. If the defect is large, other abdominal organs may be involved. The prevalence is approximately 2 to 4 per 10,000 births. Postnatally, infants with gastroschisis are often described as having simple (isolated) or complex (associated with other bowel and structural anomalies and with a more complex postoperative course) disease. Outcomes for these cohorts can be very different; complex cases are more likely to need a bowel resection and to have ongoing GI complications and have a significantly longer length of stay after birth (37 vs 108 days) and significantly longer requirement for parenteral nutrition (26 vs 71 days).

A number of theories have been extended in an attempt to describe the embryologic derivation of gastroschisis. Amongst the most popular are theories related to vascular disruption during the development of the anterior abdominal wall. It has, for example, been suggested that premature atrophy (before 5 weeks’ gestation) of the right umbilical vein interferes with development of the junction of the somatopleure and the body stalk, resulting in a weakness in the umbilical ring. During the subsequent process of herniation of the midgut into the extraembryonic coelom, the small bowel herniates through this paraumbilical defect and fails to return to the abdominal cavity at the end of this process. This is supported by the fact that gastroschisis is associated with intestinal nonrotation. An alternative theory suggests that at a slightly later gestation (8–9 weeks), by the time the herniated midgut has returned to the peritoneal cavity, there is a vascular accident leading to obliteration of the right omphalomesenteric arteries (the arterial plexus that is the precursor of the superior mesenteric artery), resulting in necrosis of the cord base and herniation of the small bowel.

The cause of such a vascular insult has not been clearly identified. It is recognised that the prevalence of gastroschisis is higher amongst younger pregnant women, and it has therefore been suggested that there may be a teratogenic insult. Epidemiologic studies, many of which are limited by potential population recruitment and reporting bias, have suggested that drugs such as aspirin, pseudoephedrine, organic solvents, alcohol and cocaine have significant associations, although the quality of data is insufficient to correlate levels of exposure and effect. A recent case-control study examining hair samples to identify recreational drug use in the 3 months preceding diagnosis of a fetal abnormality was not able to confirm an association between use of these drugs and gastroschisis.

In clinical practices that include routine ultrasound assessment at 12 and 20 weeks, more than 95% of cases of gastroschisis are detected prenatally, predominantly before the 20-week scan ( Fig. 32.3 ). In one recent study that included 44 cases of gastroschisis detected throughout the Northern region of the Netherlands, 86% of defects were detected at the 12-week scan, and the remainder were all detected at the 20-week morphology scan; the only cases that were detected postnatally involved women who had declined routine ultrasound screening. A total of 12% of affected fetuses had other structural anomalies, although chromosomal abnormalities were uncommon; one fetus was affected by trisomy 18 and another by a genetic mutation in the NF1 gene. After identification of gastroschisis, 14% of parents chose to terminate the pregnancy, and 16% of the remaining infants died in the perinatal period.

Axial section of the upper abdomen at 20 ( A ) and 34 ( B ) weeks. The panel on the left ( A ) shows loops of bowel lying freely (with no membranous covering) within the amniotic cavity. In B , the bowel is slightly more pronounced, although the degree of dilatation is within normal limits for the later gestation.

The contemporary challenge facing clinicians involved in prenatal diagnosis lies in defining the risk that an infant with gastroschisis has complex rather than simple disease and in defining risks of preterm delivery (associated with an increased risk for neonatal mortality and morbidity) or of stillbirth. A number of groups have retrospectively reviewed prenatal sonographic findings in an attempt to distinguish between simple and complex cases and to define risk for perinatal complication and mortality. These findings have been well reviewed and brought together in a meta-analysis published by a European consortium that reviewed 26 studies including 2023 fetuses. They reported that intraabdominal bowel dilation and polyhydramnios were associated with bowel atresia (odds ratio (OR), 5.48; 95% confidence interval (CI), 3.1–9.8 and 3.76; 1.7–8.3, respectively) and that gastric dilation was associated with neonatal death (OR, 5.58; 95% CI, 1.3–24.1). No other ultrasound features were found to be significantly associated with pregnancy outcome.

At the time of antenatal diagnosis, it is important to ensure that a sequential anatomical survey is completed because fetuses that have other structural anomalies are more likely to have a poor outcome and are more likely to have an underlying chromosomal or genetic disorder. Although fetuses with gastroschisis were traditionally thought to be at relatively low risk for chromosomal abnormality, the risk for conventional forms of aneuploidy is significantly higher than that seen in an anatomically normal population and recent studies have shown that use of genomic microarrays for karyotyping is likely to detect pathogenic copy number variations (CNVs) in 5% to 10% of cases. Consequently, karyotyping should be offered to these families.

The nature of the gastroschisis anomaly can be assessed by identifying all herniated structures, measuring diameters of bowel both intra and extraabdominally, measuring the size of the ‘neck’ or abdominal wall defect. Infants with gastroschisis are frequently small for gestational age. Although it is difficult to assess growth using the abdominal circumference, which will be naturally reduced because of herniation of intraabdominal contents, it is possible to look at the bigger picture and include assessment of the amniotic fluid index and maternal, placental and fetal Doppler. Doppler assessment of the superior mesenteric artery has also been advocated, but this is often difficult to perform, and there are no data showing this improves management decisions and pregnancy outcome.

One publication on a large series of 860 infants affected by gastroschisis has reported in utero and postnatal death rates by gestational age ( Fig. 32.4 ). Fetuses affected by gastroschisis that are born very preterm are frequently impacted by infection (either related to prematurity or their surgical morbidity), which has a significant effect on outcomes, and infant death rates fall dramatically after 32 weeks. In contrast, stillbirth rates start to climb late in the third trimester. Sparkes and colleagues used these data to look at the relative risk for expectant management compared with elective delivery after 32 weeks. They showed that by 39 weeks, there was a significant increase in risk for stillbirth through expectant management, and most clinicians now advocate delivery around 38 weeks’ gestation. According to the US-based data set, the number needed to treat involves 17 elective deliveries at 39 weeks to prevent one in utero death.

The impact of gestation on stillbirth and infant death rates (per 1000 ongoing cases) for fetuses affected by gastroschisis. Stillbirth data are shown in circles and infant deaths by squares .

It has been suggested that those cases that have a narrow ‘neck’ and dilated loops of bowel within the abdomen are more likely to be complicated, with areas of bowel stenosis requiring postnatal resection. Other authors have suggested that dilation of external loops of bowel is also a poor prognostic indicator, being associated with an increased risk for complex disease and preterm delivery, but this can be difficult to interpret from a management perspective. Robertson and coworkers recently reported the value of antenatal sonography for prediction of neonatal outcome in a series of 101 fetuses with gastroschisis. The group were able to define that extraabdominal loops (>20 mm diameter) of bowel were associated with an increased risk for complex disease with sensitivity of 73%. However, only 11 of the 38 cases with extraabdominal bowel dilation had complex disease – a positive predictive value of 29% – making it difficult to act with confidence on this finding.

Fetuses that have gastroschisis need surgical intervention shortly after delivery. It is therefore inappropriate to deliver them outside of centres with tertiary neonatal and paediatric surgical facilities. Although diagnosis, counselling about the condition and any invasive testing may be done locally, it is normally best to transfer care in the third trimester so that the obstetrician or fetal medicine specialist who is responsible for coordinating delivery can conduct serial surveillance to assess fetal well-being. This typically involves combinations of ultrasound and cardiotocography, aiming for planned early delivery at 38 weeks’ gestation. One research group recently reported a 58% reduction in stillbirth by formalising this process of assessment.

There is no good evidence that elective preterm delivery by caesarean section improves the outcome of infants with gastroschisis. Induction is always more complex from the perspective of being able to define the precise point of delivery, and for this reason, many clinicians advocate that caesarean section allows better synchronisation with neonatal and paediatric surgical teams. Mothers will, however, be more able to support their infants if they deliver vaginally, and this therefore appears to be a sensible approach in most circumstances.

Gastroschisis may be repaired through primary closure, or if the amount of prolapsed bowel is large, a silo may be constructed and used to reduce the mass over the course of several days or weeks. Infants who have gastroschisis will have delay in normal feeding and are typically given parenteral nutrition for the first few weeks of life. Although many infants will be home after 3 or 4 weeks, more complex lesions may lead to prolonged neonatal admission.

Exomphalos

An exomphalos or omphalocele is an anterior abdominal wall defect that originates centrally at the umbilicus. Another contrasting feature in comparison to gastroschisis is that the defect is bonded by the peritoneal membrane, although this can rupture in some circumstances.

During fetal development, the bowel becomes too large to be contained within the abdominal cavity, and the midgut herniates into the extraembryonic coelom, being externalised from approximately 5 to 9 weeks’ gestation. This feature is clearly visible on early first trimester ultrasound. Typically, before the bowel returns to the abdomen, it undergoes a process of rotation. This process is typically incomplete in fetuses that have an exomphalos. An exomphalos may contain other abdominal organs such as the stomach, large bowel and liver, and the size of the anterior abdominal wall defect may also vary widely because the anterior abdominal wall muscles are often hypoplastic and displaced laterally.

In a recent study reporting accuracy of prenatal diagnosis in a regional Dutch dataset, 141 fetuses with exomphalos were reported over a 4-year period. A total of 136 (96%) were identified prenatally, including 86% of those presenting in pregnancies where women had opted for first trimester (11–13+6 week’ gestation) screening ( Fig. 32.5 ). All 5 cases that were not identified in this series had been missed at either the first ( n = 1) or second ( n = 4) trimester scan. A total of 85% of fetuses that were identified as having an exomphalos had other structural anomalies, and overall, 47% of this cohort had a chromosomal abnormality. The commonest associated aneuploidy was trisomy 18, but trisomy 13, trisomy 21, 45X, triploidy and a variety of microscopic chromosomal abnormalities were also reported. The strong association with lethal aneuploidy and with other major structural abnormalities led to a high rate of termination of pregnancy.

Axial section of the abdomen in two fetuses at 12 ( A ) and 13 ( B ) weeks’ gestation. There is a small exomphalos, which subtle echogenicity of the root of the cord in A . In contrast, B shows a mega-exomphalos with more than 50% of the intraabdominal contents within the herniated sac.

Because an exomphalos is associated with lethal forms of aneuploidy, it is associated with a high spontaneous intrauterine loss rate, and consequently, data on the prevalence of the anomaly depend in part on the gestational age at which screening is performed. In one study, first trimester (11–13+6 week) prevalence was reported to be 0.25%. The risks of aneuploidy will therefore be higher in those cases identified at earlier gestations. The other impact of the difference in prevalence seen through pregnancy and neonatal cohorts is that different clinicians who interact with these fetuses have very different opinions on the likely outcome for the infant. The paediatric surgeon who operates on the stable neonate that is euploid and has an isolated structural defect has a different vision of the spectrum of disease to a fetal medicine specialist, and it is important to make sure that these experiences are appropriately reflected in counselling. The commonest associated structural anomalies that are seen are cardiac and neurologic abnormalities. In some series, about one third of infants who had an apparently isolated exomphalos were found to have other structural anomalies after birth.

Although the prognosis for a fetus affected by exomphalos is primarily affected by associated chromosomal or structural anomalies, other features can be assessed to determine the risk for a complicated postnatal outcome. In an axial section, the circumference of the omphalocele and abdominal cavity can be measured and expressed as a ratio (the OC/AC ratio). A higher ratio is associated with liver herniation, elective caesarean delivery, respiratory compromise and delayed surgical closure. Total lung volumes can be calculated by magnetic resonance imaging (MRI). Giant exomphalos is associated with a 50% reduction in lung volume when fetuses are compared with established normal ranges. These fetuses are likely to need more ventilatory support, have a delay in establishing feeding and require longer hospitalisation compared with who that have better observed-to-expected lung volume ratios.

There is less controversy about ongoing surveillance than is reported in fetuses with gastroschisis because there does not appear to be an association with unanticipated late preterm or term stillbirth. Although there is no good evidence supporting elective caesarean section, many obstetricians prefer this delivery route, particularly for larger lesions, citing the risk for intrapartum rupture of the surrounding membrane, although this is rarely reported. Timed delivery does have an advantage for timing neonatal and surgical assessment and may reduce the likelihood of postnatal complication, although this has not been proven. The goal of surgical repair is to effect complete fascial and abdominal wall closure without causing cardiovascular or respiratory embarrassment or placing the repair under excessive intraabdominal pressure. Although small defects are amenable to primary repair, surgical intervention for larger defects is often delayed. The membranous sac is allowed to form an eschar and epithelialize, a process that may take up to 10 weeks. Some surgical groups use tissue expanders or mesh to facilitate repair.

Bladder and Cloacal Extrophy

Failure of closure of the lower anterior abdominal wall is a rare event, leading to a spectrum of anomalies ranging from epispadias through bladder extrophy to cloacal extrophy. The liveborn prevalence of bladder and cloacal extrophy are 1 in 40,000 and 1 in 250,000, respectively. Most sonographers will only see one or two these defects in their career. Consequently, recognition of this defect is poor, with a 25% (10 of 40 cases) prenatal detection rate reported in one series.

Bladder extrophy involves disruption of the anterior abdominal and bladder wall with exposure of the posterior wall of the bladder and urethra. The upper urinary tract, which has a different embryologic derivation, is typically normal. The distance between the umbilicus and anus is shortened, and the pubic rami are shortened (by about a third in length) so that the pubic symphysis is not closed. This is then associated with epispadias and a shortened penile corpus in boys and vaginal stenosis and a bifid clitoris in girls. Cloacal extrophy is even more extensive and may include herniation of the foregut or exomphalos cranially together with an imperforate anus and spinal anomalies caudally.

The ultrasound features associated with prenatal diagnosis include an inability to define a normal filling bladder within the pelvis, the presence of an anterior lower abdominal mass, low insertion of the umbilical cord, difficulty in defining fetal sex (or the finding of diminutive external genitalia) and widening of the pubic rami. Charts have been produced that define the normal distance between the base of the cord insertion and the genital tubercle, and these can potentially be used to assess umbilical cord insertion in fetuses that appear to have an absent bladder at the time of the routine scan. On occasion, a urachal cyst has been mistaken for a normal bladder. MRI has also been used to assist prenatal diagnosis and potentially has the advantage of being able to assess the fetus in all three orthogonal planes with a more panoramic view of the anomaly. This is particularly useful in identifying shortening (and low umbilical cord insertion) of the anterior abdominal wall or the presence of a mass in this area—features that can be difficult to demonstrate using traditional ultrasound views.

Repair of bladder extrophy requires input from a multidisciplinary surgical team and is typically delayed to 6 months of age. The principle aims of surgery are to close the abdominal wall, secure continence and reconstruct the genitalia. The bladder is repaired for primary closure, and an osteotomy of the pelvic rami is performed simultaneously. Many children subsequently need further bladder augmentation or procedures to secure continence and may have ongoing orthopaedic problems. Counselling is complex, and the intricacies of this for families facing surgical repair of cloacal anomalies have been well described.