Prenatal diagnosis of esophageal, gastrointestinal, and anorectal atresia

INTRODUCTION — Congenital atresia of the esophagus, small or large bowel, or anorectum results in partial or complete obstruction, which may have clinical manifestations in utero. The ability to diagnose these atresias prenatally is influenced by the site of obstruction, the presence of associated anomalies, and the gestational age at the time of imaging.

This topic will discuss prenatal evaluation and obstetric management of fetal esophageal, small/large bowel, and anorectal atresia. Postnatal clinical manifestations, diagnosis, evaluation, and management are reviewed separately. (See "Congenital anomalies of the intrathoracic airways and tracheoesophageal fistula" and "Intestinal atresia" and "Congenital aganglionic megacolon (Hirschsprung disease)".)

IMAGING THE FETAL GASTROINTESTINAL TRACT

Normal ultrasound findings — Ultrasound findings of the various parts of the normal fetal gastrointestinal system depend on the gestational age at examination.

●Esophagus — The normal fetal esophagus is collapsed and typically not visualized throughout pregnancy. The amount of fluid in a swallow is usually too small to be seen in the esophagus unless fluid accumulates due to an atresia or stricture.

●Stomach — A fluid-filled stomach should be detectable sonographically in most second- and third-trimester fetuses since the fetus begins to swallow amniotic fluid by 11 to 14 weeks of gestation. Nomograms for stomach size across gestation are available [1]. An enlarged stomach without any associated abnormalities, such as a dilated duodenum, is typically an incidental finding with a normal outcome (ie, no intestinal obstruction) [2].

Nonvisualization of the stomach on a single examination may be due to gastric emptying, but persistent nonvisualization on serial fetal ultrasound examinations is generally due to an abnormality (image 1). (See 'Differential diagnosis of a nonvisualized/small stomach' below.)

●Bowel — Rapid growth of the bowel and liver occurs during the fourth to fifth postconceptional week of development. During the sixth week of postconceptional development (or eight weeks from the first day of the last menstrual period [ie, 8^th postmenstrual week]), the abdominal cavity temporarily becomes too small to accommodate all of its contents, resulting in protrusion of the bowel into the residual extraembryonic coelom at the base of the umbilical cord. This temporary herniation is called physiologic midgut herniation and is sonographically evident between the 9^th and 11^th postmenstrual weeks (image 2). Reduction of this hernia occurs by the 12^th postmenstrual week; beyond the 12^th week, a midgut herniation of bowel is no longer normal [3].

The bowel lumen appears collapsed in the first trimester. Fluid may be seen in the lumen beginning at approximately 13 weeks of gestation and is usually seen by 20 weeks. In the second and third trimesters, normal small bowel loops generally do not exceed 7 mm in diameter or 15 mm in length [4]. Colonic diameter increases with advancing gestation, achieving diameters up to 23 mm filled with meconium at term [5,6]. Tables of normal lumen diameters across gestation are available [5,7]. Dilated loops suggest some degree of obstruction.

The colonic portion of the bowel is best visualized after 24 weeks of gestation as hypoechoic regions along the periphery of the abdomen. Separate loops of small bowel are distinguishable after 28 weeks of gestation.

Peristalsis can be observed as early as 18 weeks of gestation. Obstruction may be accompanied by vigorous peristalsis.

The bowel is variably echogenic, at times similar in echogenicity to adjacent liver, spleen, and kidneys. High-resolution linear transducers can be helpful in differentiating bowel from these organs. Echogenicity may be due to meconium, which accumulates in the bowel throughout the second and third trimesters [5,7-10]. Excessively echogenic bowel (defined as bright as bone) in the second trimester can be a normal finding or a marker of an abnormality, such as cystic fibrosis, trisomy 21 (Down syndrome), congenital infection with cytomegalovirus or parvovirus, or severe intrauterine growth restriction. (See "Fetal echogenic bowel".)

Diagnostic performance of ultrasound imaging — Prenatal sonographic diagnosis of gastrointestinal atresia is challenging since obstruction may not become evident sonographically until the late second trimester, after the typical time of a routine fetal anatomic survey (18 to 20 or 22 weeks of gestation). It can also be difficult to differentiate dilated small bowel loops from normal colon or megaureters sonographically.

The performance of ultrasound for prenatal diagnosis of gastrointestinal obstruction was illustrated in a study of routine ultrasonographic examination of an unselected population [11]. The overall prenatal detection rate of gastrointestinal obstruction (atresia, stenosis, absence, or fistula) was 34 percent, of which 40 percent was detected at ≤24 weeks of gestation. The detection rate was 52 percent (33 out of 64) for duodenal obstruction, 40 percent (27 out of 68) for small intestine obstruction, 29 percent (28 out of 95) for large intestine obstruction, and 25 percent (31 out of 122) for esophageal obstruction. The detection rate for anal atresia is much lower: 6 to 8 percent [11-13].

Others have also reported poor sensitivity of prenatal ultrasonographic diagnosis of large bowel lesions and misdiagnosis between small and large bowel obstruction [14]. In addition to low sensitivity, ultrasound is not able to accurately identify the number and location of obstructions and has limited ability for assessing the viability of unobstructed distal bowel [8].

Indications for fetal magnetic resonance imaging — Magnetic resonance imaging (MRI) may be used to confirm or clarify suspected gastrointestinal abnormalities on ultrasound examination if this information is important for managing the pregnancy. Fetal bowel is well visualized by MRI (image 3) and easily differentiated from adjacent liver, spleen, kidneys, bladder, and gallbladder. Meconium is also well visualized on T1-weighted signals [15,16]. The normal esophagus, stomach, and duodenum should always be filled with hyperintense fluid (amniotic fluid) on T2-weighted signals. The jejunum typically has a high T2-weighted and low T1-weighted signal after 33 weeks of gestation [17,18]. The distal small bowel has a more variable appearance, dependent on gestational age. Before 32 weeks, 50 percent of distal small bowel remains high T1-weighted signal due to slow protein-rich meconium progression. T1-weighted high signal intensity may remain in proximal dilated loops because of lack of progression of meconium if bowel atresia is present or peristalsis is decreased [17,18].

GENERAL PRINCIPLES OF PREGNANCY AND DELIVERY MANAGEMENT — It is unclear whether prenatal diagnosis of esophageal, gastrointestinal, or anorectal atresia improves outcome. However, early prenatal diagnosis provides an opportunity for parental counseling and preparation, screening for associated anomalies, and the option for pregnancy termination or birth at a setting with appropriate personnel and facilities for optimal newborn care.

After the initial diagnosis, periodic ultrasound examinations should be performed to look for any change in the appearance of the atresia or associated anomalies and to assess interval fetal growth and amniotic fluid volume. Nonstress tests or biophysical profiles are reasonable in pregnancies in which the risk of antepartum fetal demise is increased, such as a fetal anomaly associated with growth restriction. (See "Overview of antepartum fetal assessment".)

Many pregnancies with gastrointestinal atresia are complicated by polyhydramnios due to impaired fetal swallowing. The management of pregnancies complicated by polyhydramnios varies according to the severity and gestational age and is discussed separately. (See "Polyhydramnios: Etiology, diagnosis, and management in singleton gestations".)

Atresia alone is not an indication for cesarean birth in the absence of a standard obstetric indication. However, if the abdominal circumference is much larger than the head circumference, cesarean birth should be considered due to the risk of fetal abdominal dystocia. Delivery should be planned at a center that has an appropriate level of neonatal support.

ESOPHAGEAL ATRESIA — Esophageal atresia refers to a congenitally interrupted esophagus; one or more fistulae may occur between the malformed esophagus and the trachea. There are five types of tracheoesophageal anomalies [19]:

●Type A – Esophageal atresia without tracheoesophageal fistula (TEF; 10 percent)

●Type B – Esophageal atresia with a TEF to the proximal esophageal segment (<1 percent)

●Type C – Esophageal atresia with a TEF to the distal esophageal segment (85 percent)

●Type D – Esophageal atresia with TEF to both the proximal and distal esophageal segments (<1 percent)

●Type E – TEF with no esophageal atresia (4 percent)

(See "Congenital anomalies of the intrathoracic airways and tracheoesophageal fistula", section on 'Tracheoesophageal fistula and esophageal atresia'.)

When to suspect esophageal atresia — A standard obstetric sonogram in the second or third trimester includes an evaluation of amniotic fluid volume and a fetal anatomic survey, with assessment of the presence, size, and site of the stomach [20]. Polyhydramnios and a nonvisualized stomach (image 1) or a small stomach (resulting from partial obstruction or gastric secretions) are the two key abnormalities that should prompt consideration of esophageal atresia.

Ultrasound findings

●Esophageal atresia without TEF (Type A esophageal atresia) is characterized by the following three findings [21,22]:

•Polyhydramnios – Polyhydramnios may not develop until the late second trimester but is present in 100 percent of cases by the third trimester

•Nonvisualized stomach or collapsed stomach (two parallel echogenic lines in the upper abdomen [23])

•Intermittently dilated proximal esophageal pouch in the neck or mediastinum

●Esophageal atresia with TEF can be difficult to diagnose prenatally because the fistula allows fluid to flow into the stomach; therefore, the amniotic fluid and stomach volumes may be normal, and the proximal esophageal pouch may be normal or only transiently dilated (image 4). Polyhydramnios occurs in approximately one-third of fetuses with esophageal atresia plus distal TEF [24].

In a systematic review including 20 studies and 1760 fetuses with esophageal atresia, prenatal ultrasound identified 77.9 percent of fetuses with esophageal atresia and 21.9 percent of fetuses with esophageal atresia with TEF [25]. Polyhydramnios was diagnosed in 56.3 percent of cases, and a small or absent stomach was identified in 50.0 percent of cases.

Differential diagnosis of a nonvisualized/small stomach — Nonvisualization of the fetal stomach can be due to a recently emptied stomach, so repeated examination is necessary. Failure to visualize the fetal stomach over serial examinations strongly suggests esophageal atresia [24]. The diagnosis should still be suspected if the stomach is visualized but collapsed or small, especially if polyhydramnios is present, since gastric juices can accumulate in and distend the stomach [11,23,24].

Other causes of a nonvisualized or small stomach include [10]:

●Mechanical obstruction to swallowing. The neck and pharynx should be examined for masses that could compress the esophagus or throat and thus obstruct swallowing.

●Neuromuscular dysfunction that impairs swallowing. Sonographic observation of swallowing and normal fetal tone help to exclude a neuromuscular cause.

●Lack of amniotic fluid (anhydramnios) for the fetus to swallow. (See "Oligohydramnios: Etiology, diagnosis, and management in singleton gestations", section on 'Etiology'.)

●Abnormal stomach location. A diaphragmatic defect can displace the stomach into the chest, an abdominal wall defect can displace the stomach externally, and situs inversus displaces the stomach to the right upper quadrant. (See "Congenital diaphragmatic hernia: Prenatal issues" and "Omphalocele: Prenatal diagnosis and pregnancy management" and "Gastroschisis".)

●Congenital microgastria. This is a rare malformation that can be isolated or part of multiple anomalies (syndromic microgastria) [26].

Associated anomalies — The fetus should be assessed for associated anomalies, which are present in up to 50 percent of fetuses with esophageal atresia [19,27].

Cardiac malformations are the most common (25 percent) associated abnormality and result in the highest morbidity and mortality [27].

Esophageal atresia with a TEF is often related to the VACTERL association (vertebral, anal atresia, cardiac, TEF, renal, limb abnormalities); a two-vessel umbilical cord is common [24,28].

Prenatal postdiagnostic evaluation

●Detailed anatomic survey to look for associated abnormalities (See 'Associated anomalies' above.)

●Echocardiography – We suggest echocardiography in all cases of suspected esophageal atresia, given the increased prevalence of cardiac malformations. The finding of a ventricular septal defect or pulmonary stenosis, which may not be seen on a four-chamber view, helps to confirm that VACTERL association is present when the diagnosis is unclear [27].

●Microarray – Diagnostic genetic testing of amniocytes with microarray should be offered as 6 to 10 percent of fetuses with esophageal atresia will have chromosomal abnormalities, particularly trisomy 21 or 18 [29]. (See "Prenatal genetic evaluation of the fetus with anomalies or soft markers".)

In addition:

●Magnetic resonance imaging – Due to the low sensitivity in identifying a dilated proximal esophageal pouch in cases of TEF, magnetic resonance imaging (MRI) has been used as an adjunct for further assessment. At times, the dilated esophageal pouch can be seen on sagittal T2-weighted sequences or during multiphase cine imaging of the fetus when swallowing [30]. In a systematic review, MRI had sensitivity of 94.7 percent and specificity of 89.3 percent for identifying esophageal atresia when performed following a suspicious ultrasound examination [25]. MRI (image 5) can also be useful in selected cases to help identify associated anomalies that are suspected but not definitively diagnosed by ultrasound or echocardiography, such as pulmonary atresia or cranial anomalies, that may be important for managing the pregnancy [31].

●Gamma-glutamyl transpeptidase – Gamma-glutamyl transpeptidase (GGTP) is a digestive enzyme secreted by microvilli. An index derived by multiplying GGTP and alpha-fetoprotein (alpha-fetoprotein [MoM] × GGTP [MoM]) in amniotic fluid has been used to predict esophageal atresia in a few centers; a level ≥3 is suggestive of the anomaly [32]. Sensitivity and specificity of 89.9 and 99.6 percent, respectively, have been reported [25]. However, few studies have evaluated this marker, they were performed in fetuses already identified to have high suspicion of the anomaly on ultrasound, and the test is not widely available, so its role in prenatal diagnosis of esophageal atresia is unclear.

Postnatal management and outcome — (See "Congenital anomalies of the intrathoracic airways and tracheoesophageal fistula".)

DUODENAL ATRESIA — Duodenal atresia and stenosis are the most common types of intestinal obstruction detected in the fetus [33]. Duodenal atresia accounts for up to 75 percent of intestinal obstructions and has three phenotypes [34]:

●Type 1 – Membranous mucosal atresia with an intact muscular wall (most common type, accounting for 69 percent of cases)

●Type 2 – Short fibrous cord which connects the two ends of the atretic duodenum

●Type 3 – Complete separation of the two ends of the duodenum plus biliary tract anomalies

When to suspect duodenal atresia — A standard obstetric sonogram in the second or third trimester includes an evaluation of the presence, size, and site of the stomach [20]. Observation of two fluid-filled structures in the upper abdomen is the key abnormality that should prompt consideration of duodenal atresia.

The diagnosis is likely to be missed if the duodenum does not become dilated because it is stenotic (ie, incompletely obstructed) rather than atretic or if fetal swallowing is inhibited, such as with coexisting esophageal atresia [21,35-37].

Ultrasound findings — Duodenal atresia is characterized by a dilated fluid-filled stomach adjacent to a dilated proximal intestinal segment (termed "double bubble") (image 6); the distal intestinal segment is empty [38].

The diagnosis is most commonly made in the third trimester when both the stomach and the duodenum are likely to be dilated. Early in the second trimester, dilation of both structures may not be present, which could prevent early prenatal diagnosis.

Polyhydramnios develops in up to 50 percent of cases, typically in the mid to late second trimester [39-43].

Differential diagnosis of a "double bubble" — Differential diagnosis includes annular pancreas with extrinsic compression of the duodenum, intestinal malrotation with Ladd's bands, gastrointestinal duplication cysts, preduodenal portal vein, and choledochal cyst. Jejunal atresia should typically have additional fluid-filled loops of bowel suggesting that the obstruction is more distal, but could also be considered in the differential if the obstruction is in the most proximal portion of the duodenum [43,44].

Documenting that the second fluid-filled structure is contiguous with the fluid-filled stomach helps to differentiate duodenal atresia from a duplication or choledochal cyst, which would not be contiguous and may be located far from the stomach [42]. The double mucosa sign noted by ultrasound postnatally in duplication cysts may not be visualized prenatally. In a true double bubble, the two bubbles interconnect (stomach and proximal portion of duodenum, and the second bubble is across midline).

Associated anomalies — The fetus should be assessed for associated anomalies, which are present in more than 50 percent of cases [45].

●Trisomy 21 – Trisomy 21 is the most frequent cause of duodenal atresia, occurring in up to 30 percent of cases [46,47].

●Congenital heart disease – Twenty to 30 percent of fetuses with duodenal atresia have congenital heart disease [41].

●VACTERL – Duodenal atresia can be part of the VACTERL association (vertebral, anal atresia, cardiac, tracheoesophageal fistula, renal, limb).

●Annular pancreas – Duodenal atresia can be associated with annular pancreas.

●Umbilical cord ulcers – Rarely, congenital duodenal and jejunal obstruction has been associated with one or more umbilical cord ulcers 1 to 5 cm in diameter [48]. Rupture of umbilical vessels in an ulcer with severe hemorrhage into the amniotic cavity leading to fetal or neonatal death has been reported.

Prenatal postdiagnostic evaluation

●Detailed anatomic survey to look for associated abnormalities (See 'Associated anomalies' above.)

●Echocardiography – We suggest echocardiography in all cases of suspected duodenal atresia, given the increased prevalence of cardiac malformations. The finding of a ventricular septal defect or pulmonary stenosis, which may not be seen on a four-chamber view, helps to confirm that VACTERL association is present when the diagnosis is unclear.

●Microarray – Diagnostic genetic testing of amniocytes with microarray should be offered given the high incidence (up to 30 percent of cases [46]) of trisomy 21. (See "Prenatal genetic evaluation of the fetus with anomalies or soft markers".)

In addition:

●Magnetic resonance imaging – Magnetic resonance imaging can be useful in selected cases to identify associated anomalies that are suspected but not definitively diagnosed by ultrasound, such as pulmonary or cranial anomalies, that may be important for managing the pregnancy [37].

Postnatal management and outcome — (See "Intestinal atresia".)

JEJUNAL AND ILEAL ATRESIA — Jejunal and ileal atresias are complete obstructions of the small bowel lumen; they are more common than small bowel stenosis. Atresias can occur anywhere along the small bowel, but the most common sites are the proximal jejunum (30 percent) and distal ileum (35 percent). Multiple atretic sites occur in up to 6 percent of cases [49]. They may be caused by ischemia, with secondary complications of volvulus and/or meconium peritonitis [50]. (See "Intestinal atresia".)

Intestinal atresias are classified as:

●Type 1 – Intraluminal diaphragm in continuity with the muscular coats of the proximal and distal segments (32 percent).

●Type 2 – Fibrotic cord connecting two blind ending bowel segments (25 percent).

●Type 3a – Complete separation of blind ending loops (15 percent).

●Type 3b – Mesenteric defect and associated apple peel deformity. The terminal ileum is perfused from single ileocolic artery (11 percent).

●Type 4 – Multiple atresias (6 percent).

When to suspect small bowel atresia — A standard obstetric sonogram in the second or third trimester includes an evaluation of amniotic fluid volume and a fetal anatomic survey with assessment of the fetal abdomen (stomach, kidneys, bladder, umbilical cord insertion site) [20]. Examination of the bowel is not a focus of the routine anatomy scan; however, dilated bowel loops and polyhydramnios may be noted and should prompt consideration of a small bowel atresia.

Findings of small bowel obstruction are rarely seen before 18 weeks of gestation and can be difficult to image before 24 weeks of gestation [51]. In the third trimester, small bowel loops become progressively more dilated and have vigorous peristalsis; thus, they are easier to visualize.

Ultrasound findings — Fetal bowel obstruction (image 7) is suggested by the following findings, and particularly when all are present [10]:

●Dilated bowel loops (>15 mm in length and 7 mm in diameter)

●Mural thickness greater than 3 mm

●Polyhydramnios

The abdomen may be distended, resulting in a reduced head circumference to abdominal circumference ratio (HC/AC, normal ratio is approximately 1:1 before 36 weeks). If meconium ileus is present, increased bowel echogenicity may be noted [12].

A few dilated loops of bowel suggest a jejunal obstruction whereas multiple dilated bowel loops suggest a more distal ileal obstruction. It is difficult to differentiate an isolated proximal atresia from multiple atresias that involve both the jejunum and ileum.

Polyhydramnios is seen in up to 50 percent of cases of jejunal obstruction and rarely with more distal obstruction. In a series of 28 patients with a prenatal diagnosis of small bowel atresia, 17 required delayed anastomosis with a prolonged hospital course while 11 had direct anastomosis with shorter length of stay [52]. In the absence of other malformations, the association of dilated bowel loops and polyhydramnios was highly predictive of severe small bowel atresia requiring prolonged hospitalization (64.7 versus 9 percent).

Although dilated loops of bowel and polyhydramnios are diagnostic signs for small bowel atresia, a systematic review found wide variation in both the sensitivity and specificity of prenatal ultrasound in detecting jejunal and ileal atresia [53]. As an example, in one retrospective study of 58 cases of jejunal or ileal atresia, sensitivity was 50 percent (95% CI 26.0-74.0) and specificity was 70.6 percent (95% CI 52.5-84.9) [54]. The presence of both bowel dilatation ≥17 mm and polyhydramnios after 32 weeks of gestation slightly increased sensitivity (66.7 percent, 95% CI 34.9-90.1) and specificity (80.0 percent, 95% CI 44.4-97.5).

Differential diagnosis of dilated abdominal tubular loops — The differential diagnosis for dilated tubular loops within the fetal abdomen includes megaureters and abdominal mesenteric cysts [15,55]. In addition, a variety of intestinal abnormalities can occur in association with jejunal or ileal atresia (see 'Associated anomalies' below) or mimic atresias. These include volvulus, meconium ileus/peritonitis, total colonic aganglionosis, intestinal duplication cysts, and congenital chloride diarrhea [56,57].

Dilated ureters can mimic dilated bowel loops. A dilated renal pelvis and calyces is a clue that the loops are ureters rather than bowel. In addition, a connection between the dilated ureter and the kidney or bladder may be seen.

Mesenteric cysts tend to be circular or oval and do not show peristalsis, which helps to distinguish them from a dilated bowel loop.

The whirlpool sign (clockwise wrapping of the superior mesenteric vein and the mesentery around the superior mesenteric artery suggesting a whirlpool), fluid levels, and/or coffee bean sign (dilated bowel loops with the appearance of a coffee bean) suggest a volvulus, which can be the reason for dilated bowel loops [58,59].

Extravasation of meconium from a perforation at the site of atresia can lead to ascites, meconium peritonitis, and the development of meconium pseudocysts (image 8). In these cases, scattered peritoneal calcifications with posterior acoustical shadowing may be noted [60,61]. (See "Fetal abdomen: Differential diagnosis of abnormal echogenicity and calcification", section on 'Meconium pseudocyst' and "Fetal abdomen: Differential diagnosis of abnormal echogenicity and calcification", section on 'Meconium peritonitis'.)

Congenital chloride diarrhea is a rare autosomal recessive cause of dilated bowel. It is characterized by polyhydramnios and multiple dilated bowel loops which show peristalsis [55,62]. The dilated bowel loops tend to be of uniform size and can be seen throughout the fetal abdomen. By magnetic resonance imaging (MRI) low signal T1-weighted fluid-filled loops of bowel extend to the rectum [63]. Other fetal anomalies, ascites, and intraperitoneal calcifications are not present.

Utility of magnetic resonance imaging in differential diagnosis — Because meconium and bowel have different signal characteristics, MRI is useful for distinguishing diagnoses related to meconium from other bowel and nonbowel disorders [18,64]. For example, assessment of the size and meconium content of the sigmoid colon and rectum can help differentiate dilated obstructed bowel from intestinal duplication cysts, mesenteric cysts, and hydroureters. Meconium peritonitis can be diagnosed by visualization of extraluminal fluid collections scattered through the abdomen. Peritoneal calcifications may be present and are best noted by ultrasound [65]. Small bowel dilatation with decreased meconium in the rectum suggests a more complex diagnosis, such as cystic fibrosis [66,67]. In a review of 34 fetuses with small bowel or anorectal obstruction who underwent both ultrasound and MRI examinations, diagnostic accuracy was 84.4 percent [67,68].

In bowel obstruction, bowel loops proximal to the obstruction are dilated (13 to 30 mm) [15,69,70]. The signal characteristics are variable depending on the gestational age and location of obstruction. The bowel can be fluid-filled high signal T2-weighted, meconium-filled high signal T1-weighted, or intermediate in signal. Distally, the rectum may contain less meconium and thus decrease in size (2 to 7 mm) (image 9) [66,67,71].

The level of obstruction is suggested by the number of dilated loops and the number of nondilated loops proximal to the dilated loops. The absence of loops distal to the level of obstruction suggests multiple atresias [15]. One report described dilated loops with two different signals in a case with two levels of atresia [72].

The authors of a series of three atresias commented on the increasing echogenicity of bowel contents with more distal obstruction [73]. In a series of 12 small bowel obstructions evaluated by ultrasound and MRI, a trend of increasing complexity of bowel contents (increasing ultrasound echogenicity and high T1-weighted signal on MRI) corresponded to a distal level of obstruction [66]. Polyhydramnios was present in 9 of 12 cases.

The size of the meconium-filled rectum is another clue as to the etiology of the obstruction. All seven jejunal atresias and one ileal atresia in another series had small-caliber, meconium-filled colons (microcolon) but normal-sized rectums [67]. By contrast, three fetuses with cystic fibrosis and one fetus with jejunal and anal atresia had microcolon and paucity of rectal meconium.

Associated anomalies — The fetus should be assessed for associated anomalies, as additional gastrointestinal anomalies occur in up to 45 percent of cases and may be the cause of the atresia [74]. In one study, these anomalies included malrotation (23 percent), meconium peritonitis (8 percent), microcolon (3 percent), duplication cysts (3 percent), and esophageal atresia (3 percent). Volvulus has also been associated with atresias [50,58,59].

Small bowel atresia is secondary to meconium ileus in 10 percent of cases. In these cases, impacted thick meconium obstructs the terminal ileum, resulting in perforation with secondary ileal atresia and meconium peritonitis. Up to 15 percent of fetuses with cystic fibrosis present with meconium ileus and up to 50 percent of these cases will be complicated by an atresia, volvulus, perforation, and/or meconium pseudocysts [67,75,76]. (See "Cystic fibrosis: Clinical manifestations and diagnosis", section on 'Meconium ileus and distal ileal obstruction'.)

Gastroschisis may result in bowel kinking, which may cause ischemic bowel injury resulting in atresia [77,78] (see "Gastroschisis"). Likewise, placental vascular abnormalities may lead to fetal ischemic injury resulting in small bowel atresia [79].

In rare cases, congenital duodenal and jejunal obstruction have been associated with one or more umbilical cord ulcers that have bled, leading to fetal or neonatal death [48].

Prenatal postdiagnostic evaluation

●Detailed anatomic survey to look for associated abnormalities (See 'Associated anomalies' above.)

●Cystic fibrosis – If echogenic bowel is present, testing for cystic fibrosis should be performed. (See "Cystic fibrosis: Carrier screening", section on 'Prenatal diagnosis'.)

●Microarray – While aneuploidy is unusual with isolated small bowel atresia, diagnostic genetic testing (microarray) is recommended if another fetal structural anomaly is present or the patient requests such testing. (See "Prenatal genetic evaluation of the fetus with anomalies or soft markers".)

●Magnetic resonance imaging – MRI can be a useful in differential diagnosis if additional information is important for establishing a diagnosis and managing the pregnancy. (See 'Differential diagnosis of dilated abdominal tubular loops' above.)

●Echocardiography – Echocardiography is not performed routinely because the incidence of cardiac anomalies is not increased relative to the general obstetric population [49].

Postnatal management and outcome — (See "Intestinal atresia".)

COLONIC OBSTRUCTION — Colonic obstruction may be due to Hirschsprung disease, anorectal malformations, or colonic atresia. It is often missed prenatally because fluid is resorbed by the small bowel and colonic loops, which allows the small bowel and colon to retain normal diameters despite distal obstruction.

Hirschsprung disease — Hirschsprung disease is one of the most common causes of intestinal obstruction in the newborn. Aganglionosis of a segment of colon results in a functionally obstructed distal segment with a dilated proximal segment that tapers at a transition zone. (See "Congenital aganglionic megacolon (Hirschsprung disease)".)

Prenatal diagnosis is rare as obstructive symptoms and constipation typically develop only after birth [80]. However, the disease may present in utero if the entire colon rather than a segment is involved.

Total aganglionosis has been diagnosed in the third trimester based on polyhydramnios and small bowel dilation [81-83]. While intraluminal calcifications (enteroliths) may develop due to functional obstruction and precipitation of urates within the lumen and echogenic bowel may be present, neither finding is specific for this diagnosis. (See "Fetal echogenic bowel".)

Up to 25 percent of affected fetuses have associated anomalies, with a strong association with trisomy 21 [83,84].

Anorectal malformations — Abnormalities of the rectum and anus are thought to be due to arrest of the caudal descent of the urorectal septum to the cloacal membrane. The resulting malformations range from isolated imperforate anus to persistent cloaca. These malformations are classified as "high" supralevator lesions, which end above the levator sling and are typically associated with fistulas, and "low" infralevator lesions, which end below the levator sling and are not associated with fistulas.

Isolated transient bowel dilation has been observed as early as 12 weeks of gestation in a fetus with anorectal atresia [85,86]; the distal colon may be dilated prenatally [87]. The anal canal may appear small (anal canal diameter <95% CI of the normal range), or the anal canal and rectum may be absent [88].

Associated anomalies occur in up to 50 percent of cases. Anal atresia is most commonly associated with the VACTERL association (vertebral, anal atresia, cardiac, tracheoesophageal fistula, renal, limb) and occurs with increased frequency in trisomy 21 [89].

If a vesicorectal fistula is present, urine may mix with meconium, causing it to calcify and form enteroliths [90]. Therefore, if intraluminal calcifications are identified, anal atresia with vesicorectal fistula should be suspected (image 10).

Assessment of the fetal cul-de-sac by magnetic resonance imaging (MRI) can be helpful in distinguishing between a high anorectal malformation and cloaca [9,15,91]. MRI may reveal dilated meconium-filled rectum in cases of anal atresia [91], and detection of abnormal fluid in the rectum suggests vesicorectal fistula (image 11) [18].

Colonic atresia — Colonic atresia is a rare cause of intestinal obstruction and accounts for less than 15 percent of all bowel atresias [51]. Similar to jejunal and ileal atresia, colonic atresia is thought to be secondary to a vascular event or a mechanical event, such as intestinal volvulus. The majority of colonic atresias occur proximal to the splenic flexure with a distal microcolon.

Distal colonic obstruction is often missed prenatally because fluid is resorbed in the small bowel so colonic loops retain their normal caliber rather than dilating. Polyhydramnios is rare and, when present, should raise the suspicion that a more proximal obstruction is present. Perforation may occur with resulting ascites and meconium peritonitis.

In two-thirds of cases, colonic atresia occurs as an isolated defect without associated abnormalities. In the remaining cases, associated anomalies may include gastroschisis, omphalocele, Hirschsprung disease, or ocular and skeletal anomalies [92]. Cardiac anomalies and genetic defects are rare.

Differential diagnosis — Differential diagnosis of colonic obstruction includes meconium ileus, persistent cloaca, meconium plug syndrome, fetal diarrhea [55,63], and megacystis microcolon hypoperistalsis syndrome [93,94].

Intraluminal calcifications suggest a rectovesical fistula or Hirschsprung disease, while extraluminal calcifications may be secondary to meconium peritonitis. (See "Fetal abdomen: Differential diagnosis of abnormal echogenicity and calcification", section on 'Peritoneal calcification' and "Fetal abdomen: Differential diagnosis of abnormal echogenicity and calcification", section on 'Solitary or multifocal calcification in bowel'.)

When megacystis is noted in the second trimester with increased amniotic fluid in a female fetus, megacystis, microcolon, or intestinal hypoperistalsis syndrome should be considered.

Prenatal postdiagnostic evaluation

●Detailed anatomic survey to look for associated abnormalities, including VACTERL association.

●Echocardiography – We suggest echocardiography, given the increased prevalence of cardiac malformations with anorectal malformations.

●Microarray – The finding of a fetal structural anomaly increases the possibility of a chromosome abnormality, therefore, offering diagnostic genetic testing (microarray) is reasonable (see "Prenatal genetic evaluation of the fetus with anomalies or soft markers"). Five percent of children with trisomy 21 have a gastrointestinal tract anomaly. (See "Down syndrome: Clinical features and diagnosis", section on 'Gastrointestinal abnormalities'.)

Postnatal management and outcome — (See "Congenital aganglionic megacolon (Hirschsprung disease)" and "Intestinal atresia".)

SUMMARY AND RECOMMENDATIONS

●General principles

•Prenatal diagnosis – Early prenatal diagnosis provides an opportunity for parental counseling and preparation, screening for associated anomalies, pregnancy termination, or delivery at an appropriate setting. (See 'General principles of pregnancy and delivery management' above.)

The diagnosis of bowel atresias prenatally can be difficult, particularly those of the esophagus or distally in the colon/anus. Diagnosis is typically made in the second or third trimester. Proximal small bowel obstruction is more readily diagnosed than distal obstruction because bowel dilatation and/or polyhydramnios are more likely to be present. Polyhydramnios often develops with proximal esophageal and duodenal obstructions, but typically does not develop in more distal atresias or develops late in the third trimester. (See 'Esophageal atresia' above and 'Duodenal atresia' above and 'Jejunal and ileal atresia' above.)

When obstruction is suspected in the second trimester, follow-up ultrasound after 32 weeks of gestation is recommended. (See 'Imaging the fetal gastrointestinal tract' above.)

Magnetic resonance imaging (MRI) can be a useful adjunct in delineating the size and contents of normal and abnormal bowel and can aid in the evaluation of associated anomalies. (See 'Indications for fetal magnetic resonance imaging' above.)

•Delivery – Delivery should be planned at a center that has appropriate neonatal support. Gastrointestinal abnormalities alone are not an indication for altering the route of delivery unless there is significant enlargement of the abdominal circumference as compared with the head circumference. (See 'General principles of pregnancy and delivery management' above.)

●Esophageal atresia

•Visualization of the fetal stomach is important in screening fetuses for esophageal atresia. Nonvisualization can be due to a recently emptied stomach so repeated examination is necessary. Failure to visualize the fetal stomach despite serial examinations strongly suggests the diagnosis. (See 'Esophageal atresia' above.)

•Isolated esophageal atresia is readily diagnosed prenatally by the presence of polyhydramnios, absent stomach, and a dilated proximal esophageal pouch in the neck or mediastinum. However, over 85 percent of esophageal atresias are associated with a tracheoesophageal fistula (TEF), and the diagnosis in these cases is often missed prenatally because fluid can course into the stomach via the fistula. (See 'Esophageal atresia' above.)

●Duodenal atresia – Sonographic findings suggestive of duodenal atresia include a dilated fluid-filled stomach next to a dilated proximal intestinal segment (fetal "double bubble"); the distal bowel is collapsed. Duodenal atresia is the most common atresia identified prenatally and is associated with other anomalies, particularly trisomy 21. The diagnosis can be missed prior to 24 weeks if the duodenum has not yet significantly dilated or if a TEF is also present. (See 'Duodenal atresia' above.)

●Colonic obstruction/atresia

•Prenatal ultrasound has low sensitivity for detection of colonic obstructions including Hirschsprung disease, anorectal malformations, and colonic atresia. A dilated meconium-filled rectum above the bladder outlet, lack of meconium in the rectum, intraluminal calcifications, and/or the presence of VACTERL association (vertebral, anal atresia, cardiac, TEF, renal, limb) suggest the diagnosis. (See 'Colonic obstruction' above.)

•When calcifications are noted, it is important to determine if they are intraluminal or extraluminal. Intraluminal calcifications suggest a rectovesical fistula or Hirschsprung disease. Extraluminal may be secondary to meconium peritonitis. Echogenic bowel with meconium peritonitis may be related to cystic fibrosis. (See 'Colonic atresia' above.)

•Anal atresia may be associated with rectovesical fistula. In these cases, urine can mix with meconium resulting in a dilated fluid-filled rectum with intraluminal calcifications, which is easily identified by ultrasound. On MRI, the rectum may be fluid-filled with high signal on T2w. (See 'Anorectal malformations' above.)