Etiology: Interruption of the fetal GI tract can occur at any point from the esophagus to the anus, and the etiology for the loss of continuity is not the same at each site. The cause of esophageal atresia (EA) with or without a fistula (communicating between the trachea and the esophagus distal to the atretic level) is not known but is believed to be a result of failure of the tracheal bud to develop normally from the primitive foregut (see Chapter 97).^3,4 EA occurs in approximately 1 in 4000 live births and can be associated with a number of additional abnormalities in the fetus. Most commonly these abnormalities include those comprising the VACTERL complex (vertebral, anal, cardiac, tracheoesophageal, renal, and limb anomalies) and those seen in Down syndrome, such as common atrioventricular canal, absent or hypoplastic nasal bone, and nuchal thickening. EA occurs most commonly with a distal fistula (84%).⁵

Gastric atresia is the rarest atresia and is almost always pyloric with a distended proximal stomach. It likely results from vascular impairment during fetal development and is associated with epidermolysis bullosa.6,7

In contrast to pyloric atresia, duodenal atresia (DA) is relatively common, and along with EA is associated with Down syndrome. The etiology of DA may be exceptional in that it is believed to be due to dysfunction in embryonic recannulation of this segment of the proximal small bowel (see Chapter 103).⁸

Atresias distal to the duodenum are thought to be the result of ischemia or other focal insult to the bowel and may be multiple. Thus it is prudent not to assume that the most proximal obstruction is the only site. Small bowel atresias are the most common cause of fetal and neonatal bowel obstructions, with colonic segments affected much more rarely (in 10% of all atresias).9,10 Although most atresias are sporadic, some populations are prone to multiple atresia syndromes.^11,12 Meconium ileus in persons with cystic fibrosis also can have small bowel atresia, and colonic atresia can be seen in persons with Hirschsprung disease.^13,14 Anorectal atresia/malformation deserves special consideration because it often is seen in combination with other malformations, such as VACTERL, caudal regression, cloacal malformation, or the Currarino association.

Imaging of the Esophagus: Despite the likelihood of a distal fistula that might allow for flow of fluid into the gut, the most consistent clues to the diagnosis of EA on ultrasound are a persistently small-volume stomach and polyhydramnios. Distension of a proximal esophageal pouch typically is intermittent and therefore unreliably witnessed either on ultrasound or MRI, although in the rare cases where it is vital to make a prenatal diagnosis, serial midline sagittal MRI with cine SSFP sequences may prove useful (Fig. 86-1).¹⁵

Imaging of the Stomach: A lack of visualization of the stomach is most likely EA with or without a distal fistula. Other considerations in the differential diagnosis of a small stomach at fetal imaging include congenital microgastria; oligohydramnios/anhydramnios, with a lack of fluid to swallow and distend the stomach; diminished swallowing function (e.g., in arthrogryposis, with little to no fetal movement); or obstruction to normal swallowing, including from increased intrathoracic pressure (Fig. 86-2). A markedly distended stomach, on the other hand, might raise concern for outlet obstruction, as with pyloric atresia. Gastric volvulus rarely has been reported in utero and is more of a risk when the stomach is not in its normal location.¹⁶ A right-sided stomach in situs ambiguous or inversus should be evident as long as one is careful to establish the orientation of the fetus to the mother during the examination (Fig. 86-3).

Imaging of the Duodenum: DA appears on imaging as the classic fluid-filled double bubble of dilated stomach and duodenal bulb (Fig. 86-4). Duodenal stenosis sometimes can be distinguished by hyperperistalsis of the dilated proximal duodenum, whereas in DA, no peristalsis may exist at all. DA is associated with malrotation of the bowel in approximately 29% of cases.⁸ Other causes for proximal obstruction include an adhesive Ladd band in a patient with primary midgut malrotation, or annular pancreas. As with other obstructions to the GI tract above the level of the distal jejunum, a fetus with duodenal obstruction typically has polyhydramnios; the more proximal the atresia, the earlier and the more frequently an abnormally large amniotic fluid volume tends to present.¹⁷

Imaging of the Small Bowel: The small bowel is considered to be dilated if it is larger than 7 mm; the colon is usually up to 15 mm in the third trimester, while the normal rectum, which should be the largest segment, can be larger18–20 (Fig. 86-5). In proximal jejunal atresia, more loops will be dilated than just the double bubble seen with DA (“triple bubble” in very proximal jejunal atresia) (Fig. 86-6); distal jejunal atresia and ileal atresia, on the other hand, may be more difficult to distinguish prospectively because multiple dilated, fluid-filled loops are seen in both conditions, and typically a normal amniotic fluid volume is present. Other considerations for a distal bowel obstruction parallel those in newborn imaging and include meconium ileus in a fetus with cystic fibrosis; midgut volvulus; volvulus around a residual of the omphalomesenteric duct; and total colonic aganglionosis (Hirschsprung disease). One clue to the diagnosis of meconium ileus might be the presence of hyperechoic debris filling distended loops near the point of obstruction in the distal small bowel on ultrasound; MRI shows that the internal contents are hyperintense on T1-weighted images but intermediate on T2 imaging, consistent with meconium. One would expect low-signal T2 contents with the more simple intraluminal fluid in the fetus with ileal atresia.^21–23 Vigorous hyperperistalsis of the bowel is indicative of obstruction (Fig. 86-7 and Video 86-1). Cases of in utero volvulus, although uncommon, have been prospectively identified when the dilated ischemic segments are thick walled, nonperistaltic, and have internal echogenicity from hemorrhage.²⁴

Imaging of the Colon: Colonic atresias are difficult to diagnose specifically unless the obstructed segment can be identified with certainty in the expected location of the colon, or interhaustral notches can be separated from valvulae conniventes on ultrasound. Meconium typically is seen in the rectum by 20 weeks’ gestation, in the left colon by 24 weeks’ gestation, and in the right colon by 31 weeks’ gestation, but this pattern may be altered by the pathology that is present.²⁵ On FSE T2-weighted sequences, meconium has a very low signal, whereas it shows a high signal on T1 weighting.

In general, when dilated loops are discovered, ultrasound is sufficient to diagnose the level of obstruction as proximal versus distal and to generate a focused differential diagnosis. Fetal MRI is reserved for complex cases or for those in which other abnormalities are suspected and might be confirmed by additional imaging.²⁶ The advantage of MRI is in its larger field of view and reproducible multiplanar capability, showing the anatomy relative to other landmarks.²⁷ This advantage usually is demonstrated well with single-shot T2-weighted sequences in multiple planes, whereas the addition of GRE T1-weighted sequences to show the distribution of high-signal meconium can provide useful additional information (Fig. 86-8). It is crucial to understand that meconium is not exclusively restricted to the colon, can normally be seen in the distal ileum, and may be in dilated loops proximal to distal obstructions (Fig. 86-9).²⁸ Box 86-1 lists causes of nonvisualization of meconium in the colon.

Although one might anticipate that bowel loops would be dilated in cases of anorectal atresia, such dilation is uncommon, and the diagnosis often is made on the basis of a high level of suspicion because of associated features.²⁹ On ultrasound and MRI, the diagnosis may be suspected if the characteristic hypoechoic configuration of the anal musculature (Fig. 86-10) is absent. Also, a cord or ridge of abnormal fibrous tissue at the perineum may be imaged by ultrasound or MRI (Fig. 86-11). If a fistula between the colon and the urinary tract allows mixing of meconium and urine, intraluminal precipitant calcifications may be detected (Fig. 86-12).

Congenital diarrhea syndromes have been reported as showing dilation of the bowel to the rectum but without meconium signal on T1-weighted imaging, reflecting the loss of normal meconium.³⁰ Therefore lack of colonic meconium is not exclusive to mechanical small bowel obstructions. Differential considerations include secretory sodium or chloride diarrheas, megacystis-microcolon-hypoperistalsis syndrome, pseudoobstruction, and total colonic aganglionosis (Hirschsprung disease).^25,28,30,31

Treatment and Follow-up: At this time, no criteria have been established for intervention in the fetus based on bowel obstruction unrelated to ventral wall defects, aside from amnioreduction for polyhydramnios. Selected patients may have amniocentesis to evaluate for the possibility of chromosomal abnormalities or cystic fibrosis. Follow-up is performed with ultrasonography, with a plan to have the newborn evaluated at birth by the appropriate pediatric specialists.

Etiology: Normal herniation of the midgut into the proximal umbilical cord occurs in the eighth gestational week, and after undergoing 270 degrees of rotation, the midgut returns to the abdominal cavity for fixation by the end of the twelfth week. Visualization of either the liver or bowel in the umbilical cord after 12 weeks is abnormal. Ventral wall defects include all abnormalities in which extrusion of internal anatomic structures to the outside of the fetal abdomen and/or pelvis occurs. In general, these abnormalities are thought to occur from a failure of the normal infolding and fusion of the embryonic disc.³² If craniad infolding fails, the resulting defect would be to the abdominal wall and lower chest, resulting in findings in the spectrum of the pentalogy of Cantrell (omphalocele, sternal defect, ventral mediastinal diaphragmatic hernia, pericardial defect, and ectopia cordis) (Fig. 86-13). Lateral fold defects lead to an omphalocele, which is the most common isolated ventral wall defect and is contained by a membrane (parietal peritoneum and amnion) unless ruptured (Fig. 86-14).³³ Caudal fold failure results in bladder or cloacal exstrophy, which primarily affect the genitourinary tract. In bladder exstrophy, the ventral wall of the bladder, inferior rectus musculature, and skin are absent, and the residual dorsal wall of the bladder is continuous with the abdominal wall. Cloacal exstrophy is more complex; persistence of the infraumbilical cloacal membrane is thought to prevent normal anterior abdominal wall closure and separation of the urogenital system from the rectum, and as a result, two hemibladders are separated by bowel. Closed spinal defects, renal and genital malformations, and club feet may occur. Both bladder and cloacal exstrophy include diastasis of the pubic symphysis and may have more complex pelvic bone deformities (Fig. 86-15). OEIS complex describes the particular association of omphalocele, bladder exstrophy, imperforate anus, and spinal defect,³⁴ but this complex is believed by many persons to be a synonym for cloacal exstrophy.³⁵

Body stalk anomaly is a severe form of ventral wall defect that results from failure of folding along multiple planes and is not compatible with extrauterine life (Fig. 86-16).³⁶

Gastroschisis is a type of ventral wall defect that may be unrelated to embryonic disc folding. It is possible that a vascular insult results in a through-and-through defect in the layers of the abdominal wall adjacent to the umbilicus, almost universally on the right, through which the bowel herniates without a covering membrane; other hypotheses have been proposed, but the cause is not yet known (Fig. 86-17).^37,38 The risk of bearing a fetus with gastroschisis is reportedly increased in young primigravida women and also has been associated with the use of agents that may cause vasoconstriction, such as tobacco and salicylates.^39,40

Because of the lack of skin covering in this constellation of defects, the maternal alpha-fetoprotein levels can be elevated, and further subsequent screening with ultrasound leads to the prenatal diagnosis.

Imaging: Visualization of liver or bowel within the umbilical cord beyond the twelfth week of gestation is evidence of an omphalocele. A giant omphalocele is defined when more than 50% of the liver is extracorporeal or the defect is greater than 5 cm wide.41,42 Wharton’s jelly pseudocysts may be found in association with the defect, located near the cord insertion site on the omphalocele.⁴³ When the defect is small, the differentiation of a small, skin-covered umbilical hernia from omphalocele is difficult. Ultrasound typically can diagnose these defects readily and accurately define the extruded anatomy (Fig. 86-18). When ultrasound fails to reveal the pertinent details, MRI can be useful and relies on standard single-shot FSE T2-weighted imaging; the liver tends to have a higher signal than other abdominal structures on T1-weighted sequences, which is useful for identification of the position of the liver in questionable cases. Characteristically, however, when fetal MRI is requested in cases of omphalocele, it is because of an interest in assessing for additional abnormalities or a need to calculate MR-derived lung volumes. Newborns with giant omphalocele have a tendency to need long-term ventilatory support and may have pulmonary hypoplasia.^42,44,45 Echocardiography is indicated, because 50% of fetuses with omphalocele have congenital heart disease. Ascites may be present but tends to decrease through gestation. Note that if a normal bladder is not seen over time or in repeated studies, exstrophy must be considered.

Gastroschisis is differentiated from omphalocele by the lack of a covering membrane of the herniated viscera and the normal insertion of the umbilical vessels at the abdominal wall (Fig. 86-19). MRI generally is not necessary for evaluation of this diagnosis because gastroschisis almost always is an isolated defect without additional fetal malformations or lung hypoplasia.³² Closure of the abdominal wall defect in utero with subsequent severing of the extruded bowel, thus leading to extensive atresia/congenital short-gut syndrome, has been reported.⁴⁶ Additionally, on rare occasions the membrane covering an omphalocele may rupture, thus simulating gastroschisis on imaging because of the free-floating appearance of the bowel. Differentiation is made by examining the relationship of the umbilical vessels to the extruded bowel. If the liver is extruded, it is an omphalocele (Fig. 86-20).

Treatment and Follow-up: Fetuses in whom omphalocele is diagnosed are tested for chromosomal abnormalities because the incidence of aneuploidy is 40% to 60%, especially if only bowel is herniated.⁴⁷ The outcome for patients with omphalocele is usually dependent on the severity of other anomalies, including cardiac defects.⁴⁸ Repair of omphaloceles in the newborn period has a high rate of success, especially with surgical techniques emphasizing gradual reduction for larger lesions.^33,49,50

Gastroschisis, unlike omphalocele, is not associated with chromosomal abnormalities, although the occurrence of extraintestinal abnormalities is approximately 6%.⁵¹ Fetuses with gastroschisis experience complications that occur in the part of the bowel that is directly exposed to amniotic fluid during pregnancy; perforation or atresia occurs in up to 20% of cases.⁵¹ Although dilation of the herniated loops is poorly correlated to worsened prognosis, sustained distension of intraabdominal bowel loops is of concern.⁵² Postnatally, these patients may require resection of portions of their bowel, and short-gut syndrome may develop. These children are at risk for presentation with necrotizing enterocolitis and perforation even months after undergoing repair.^53,54

In fetuses with either omphalocele or gastroschisis, bowel rotation does not occur, and these patients, by definition, have midgut malrotation.

Etiology: Echogenic bowel is a nonspecific ultrasound finding that should not be ignored, although in most cases no subsequent fetal or neonatal problem develops.⁵⁵ In cases in which this finding is a marker for an abnormality, the problem could be as varied in etiology as infection (e.g., cytomegalovirus or parvovirus), intrauterine growth retardation, or aneuploidy (e.g., trisomy 21), or the direct cause of the increased conspicuity of the bowel might remain unclear. In other cases, the etiology may be more obvious; for example, in persons with cystic fibrosis, the meconium is more viscous and may present as hyperechoic contents within the bowel lumen. If bleeding has occurred into either the amniotic fluid or GI tract, swallowed intraluminal fluid can appear bright on ultrasound.

Abnormalities of the peritoneum in the fetus include ascites and calcifications. The etiology of fetal ascites can be classified under five general categories, ranging from serous through chylous, hemorrhagic, bilious, and related to the urinary tract (Box 86-2).^56–65

Imaging: When the bowel is as echogenic as the fetal skeleton, it is considered abnormal (Fig. 86-21).⁶⁶ It has been recommended that transducer frequencies of 5 MHz or less be used for confirmation, because higher frequencies may make the bowel appear unnecessarily bright.⁶⁷ Although most of these patients are healthy, the finding of abnormally bright bowel in the fetus should prompt a careful ultrasound search for additional abnormalities, with consideration of further imaging by MRI for evaluation of the neural axis if infection is suspected.

Imaging of fetal ascites should be directed toward determining whether the fluid can be related to a particular organ system. Anatomy should be examined carefully, whether on ultrasound or MRI, for abnormalities of the urinary or reproductive tracts, biliary system, chest, and bowel. The presence of septations or loculation might suggest protein contamination from blood products, bowel pathology, or abdominal lymphangiomatosis. In some case, ascites may be a physiological consequence of a condition (e.g., hypoproteinemia, hydrops, or infection) not directly related to an anatomic abnormality. It is important to look for any evidence of calcifications in the peritoneum, especially along the liver capsule, that would indicate meconium peritonitis and the implied disruption of bowel integrity, as from perforations secondary to obstruction or ischemia. Calcifications in the parenchyma of the liver or spleen would be a clue to the presence of aneuploidy or infection, or rarely may be related to portal emboli.68,69 Calcification inside the bowel lumen is an indication of GI-genitourinary fistula (see Fig. 86-12).⁷⁰

Treatment and Follow-up: Treatment and follow-up are directed toward the cause of the finding and could include in utero intervention (e.g., intrauterine transfusion) and postnatal surgery, depending on the etiology. Fetuses who have isolated fetal ascites with a normal karyotype and a negative infection screen have a good prognosis.71,72

Etiology: The etiology of masses in the fetal abdomen may be related to abnormally enlarged or infiltrated viscera or to the presence or development of an abnormal structure.

Visceromegaly includes hepatosplenomegaly, which may be the result of viral infection, fetal hydrops, glycogen or lysosomal storage disorders, Beckwith-Wiedemann syndrome, or the presentation in utero of anemia or hematologic malignancy (particularly for trisomy 21), and amniotic fluid testing may be necessary.57,65,73–75 Considerations for isolated hepatomegaly include increased right heart pressure; the liver also might appear enlarged if it lies transversely across the abdomen, as in heterotaxy. Neoplasia also may present with visceromegaly, with the organ being either a repository for metastatic disease, such as neuroblastoma diffusely metastatic to the liver,^76,77 or the site of a primary intraperitoneal tumor, most frequently in the fetal liver (Fig. 86-22). Intrauterine torsion also can present with visceromegaly from edema.^78–80 A special note should be made of megacystis-microcolon-hypoperistalsis syndrome, which can be considered in patients, more commonly female, with gross bladder enlargement and no colonic meconium on T1-weighted MRI (Fig. 86-23).^25,81

The category of abnormal structures presenting as mass lesions includes intraperitoneal cysts or fluid collections, such as meconium peritonitis as a result of perforation of the bowel, or, as recently reported, as a result of reflux of meconium into the peritoneum through the uterus in a fetus with cloacal malformation (Figs. 86-24 and 86-25).⁸² Box 86-3 lists the origins of cystic masses in the abdomen and pelvis, and Box 86-4 describes solid masses that have been reported. Occasionally a “pseudomass” is described inside of a bowel lumen, such as the stomach; in this particular case, it is thought to represent debris such as blood products and may be seen after a history of intervention (e.g., amniocentesis).⁸³ Note that most commonly, masses in the fetal abdomen are related to the genitourinary tract; these masses are discussed in greater detail in Chapter 113.

Some abnormalities are related to the absence of expected structures. For example, the spleen may be absent, and the gallbladder may be in abnormal position or absent; these findings suggest heterotaxy, which would mandate careful evaluation of the cardiac axis and configuration and other viscerovascular structures. The gallbladder usually can be detected on serial ultrasound⁸⁴ and is identified consistently on MRI.⁸⁵ Its absence would suggest biliary atresia and polysplenia if found within the heterotaxy complex. A gallbladder also may be indistinguishable in a fetus with cystic fibrosis.

Calcifications and/or debris in the gallbladder can be a sign of fetal hemolysis, but most are incidental and do not cause complications.⁸⁶ Small stones typically resolve without intervention. Finally, organs may be displaced from their normal position in persons with splenogonadal fusion, the so-called wandering spleen, and of course with diaphragmatic hernias.

Imaging: Careful delineation of the organ of involvement and of the pertinent anatomy can suggest the correct diagnosis (Fig. 86-26). When calcifications are identified, infection should be considered.⁸⁷ The use of a high-resolution linear transducer in the appropriate patient can be extremely advantageous in delineating fine detail, such as subtle calcifications, the presence within a large cyst of a daughter cyst indicative of ovarian origin, or the characteristic bowel signature of a duplication cyst. For all abnormalities, in addition to identifying the organ of origin, Doppler evaluation will help to understand whether a primary vascular lesion (increased flow) or torsion/ischemia (decreased flow) should be advanced in the differential diagnosis (Fig. 86-27).

Ultrasound generally delineates well the origin of fetal abdominal masses from solid organs. Masses are investigated by MRI when the organ of origin or the extent of involvement is imprecisely detailed by ultrasonography. In addition to the use of GRE, T1-weighted and FSE T2-weighted sequences, SSFP T2-weighted sequences are useful because of their ability to delineate adjacent structures and high-signal blood flow. This effect can be very helpful in identifying primary vascular lesions (Fig. 86-28) or enlarged vessels supplying tumors. In addition, fetal MRI may be indicated in the evaluation of the liver in conditions of potential iron overload, in which the signal from the liver is abnormally low.^88,89

Umbilical venous varix is a vascular lesion that is variably associated with in utero complications.⁹⁰ Other vascular anomalies of the venous system, including absence of the ductus venosus or agenesis of the portal veins, have been described and require meticulous delineation of the associated portosystemic shunt pathway by ultrasound Doppler or bright-blood (SSFP) MRI for accurate diagnosis.^91,92.

Treatment and Follow-up: Depending on the diagnosis, alterations in planning and timing of delivery may be indicated; amniocentesis and in utero therapies may be considered (e.g., intrauterine transfusion for fetal anemia). In general, therapy is targeted at providing the safest environment in which appropriate treatment can be directed for the fetus or neonate. To date, there are no clear indications for fetal surgery for gastrointestinal anomalies, although imaging evaluation can alter both the timing and mode of delivery. Open fetal surgery has been reserved for conditions determined to be terminal without intervention in a previable fetus, and has been limited to excisions of malformations and tumors of the chest, head and neck, and coccyx (e.g., sacrococcygeal teratoma).93–95