Fetal Gastrointestinal Tract

Stomach and Esophagus

The stomach is depicted as a fluid-filled structure in the left upper quadrant ( Fig. 17-1 ). It is routinely visualized by 13 to 14 weeks, usually earlier ( Fig. 17-2 ). Gastric size increases with advancing gestation and is assessed subjectively. Published nomograms of stomach size are available, although the measurement is complicated by variability in gastric shape and fluctuations in the size of the stomach during the course of an examination. Visualization of fluid in the fetal stomach depends on the swallowing of amniotic fluid by the fetus. The stomach varies in size as it fills in response to fetal swallowing and subsequently empties. Failure to identify the stomach or visualization of a small stomach is occasionally normal, attributable to imaging during a period of physiologic emptying ( Fig. 17-3 ). When the stomach appears small or is not seen, assessment should be repeated several times during the course of the examination. If gastric size is still in question, an additional attempt can be made to visualize the stomach a few days later. A large stomach may be seen in a normal fetus or secondary to gastrointestinal obstruction further distally ( Fig. 17-4 ). Gastric outlet obstruction such as in pyloric stenosis is rarely identified in utero .

Persistent failure to identify a normal-size stomach is associated with a high risk of fetal abnormalities. Esophageal atresia should be considered when the stomach is persistently nonvisualized, particularly in the setting of polyhydramnios ( Fig. 17-5A and B ). The combination of a small or nonvisualized stomach and polyhydramnios is concerning for esophageal atresia. Visualization of a fluid-filled dilated proximal esophageal pouch in the fetal neck or thorax increases the specificity of antenatal ultrasound for esophageal atresia but is relatively infrequent. The sensitivity of antenatal ultrasound for esophageal atresia is low, partly because most fetuses with esophageal atresia have a tracheoesophageal fistula. Fluid traversing a tracheoesophageal fistula can pass into the distal esophagus and stomach, facilitating gastric visualization, although often the visualized stomach is relatively small in this setting (see Fig. 17-5C and D ). Infrequently the stomach is detected in the absence of a tracheoesophageal fistula due to intrinsic gastric secretions.

Ultrasound of the fetus with esophageal atresia is often normal early in the second trimester, with signs of esophageal atresia not visualized until the later stages of pregnancy. The incidence of additional anomalies is high, estimated to occur in appro­ximately 50% to 70% of affected fetuses. Associated abnormalities comprise a wide range of anomalies, aneuploidy, and syndromes such as VACTERL ( v ertebral, a nal, c ardiac, t racheal, e sophageal, r enal, and l imb) anomalies.

A small or nonvisualized stomach is not specific for esophageal atresia ( Fig. 17-6 ). Multiple conditions are associated with nonvisualization of a normal-size stomach in its expected location, including abnormally located stomach (e.g., congenital diaphragmatic hernia or situs abnormality), esophageal obstruction by a mass such as a goiter or a mediastinal tumor, aneuploidy, and inability of the fetus to swallow normally, which usually occurs in the setting of a facial cleft or neuromuscular disorder. Another common cause of failure to depict a normal-size stomach is oligohydramnios, owing to the decreased volume of amniotic fluid available for the fetus to swallow ( Video 17-1 ).

Internal echoes are frequently visualized in the fetal stomach and correspond to swallowed material such as sloughed skin cells in the amniotic fluid. Echoes in the stomach occasionally conglomerate into a rounded collection resembling a mass, termed a gastric pseudomass ( Fig. 17-7 ). The likelihood of visualizing a gastric pseudomass or internal echoes in the stomach may be increased in fetuses with slowed transit through the gastrointestinal tract (as can occur with bowel obstruction) or with blood in the amniotic fluid (e.g., in the setting of a subchorionic hematoma or placental abruption, or following amniocentesis) ( Fig. 17-8 ). Despite this, gastric pseudomass and internal gastric debris are seen so frequently with current ultrasound equipment that they are generally considered normal when seen as an isolated finding.

Small Bowel

Duodenum

A fluid-filled duodenum is not typically seen in normal fetuses. The most common cause of a dilated fluid-filled duodenum is duodenal atresia. Less common etiologies include duodenal stenosis, webs and extrinsic obstruction from annular pancreas, malrotation, or bands. Ultrasound of the fetus with duodenal obstruction reveals the double-bubble sign consisting of two fluid-filled upper abdominal structures, the stomach in the left upper quadrant and the dilated proximal duodenum in the right mid abdomen ( Fig. 17-9 ). Polyhydramnios is frequent in fetuses with duodenal obstruction. The overall detection rate for duodenal obstruction is low, particularly when the fetus is scanned during the early or mid second trimester, when duodenal dilatation and polyhydramnios are frequently subtle or not yet present ( Fig. 17-10A and B ). It is important to demonstrate that the two cystic structures connect to confirm they are due to stomach and duodenal bulb (see Fig. 17-10C and D ; Video 17-2 ; e-Figs. 17-1 and 17-2 ). There are numerous other possible etiologies for an extra bubble in the fetal abdomen that do not exhibit a connection to the stomach such as an enteric duplication cyst, choledochal cyst, liver cyst, ovarian cyst, and splenic cyst ( Fig. 17-11 ). In addition, a pseudo–double bubble in which the scan plane traverses the gastric fundus and antrum should not be mistaken for a dilated duodenum and stomach ( Fig. 17-12 ). Both fluid-filled structures are of gastric origin in the setting of a pseudo–double bubble, and so they are typically located to the left of the midline, whereas a dilated duodenal bulb is more often found on the right. Adjusting the scan plane to connect the structures of a pseudo–double bubble shows the typical configuration of a curved stomach, rather than the stomach connected to a duodenal bulb by a more narrow pyloric channel. Approximately one third of fetuses with duodenal atresia have trisomy 21, and therefore a detailed scan should be performed when duodenal obstruction is suspected.

Rarely, esophageal and duodenal atresias occur together in the same fetus. This results in a closed C-shaped loop of dilated fluid-filled bowel in the abdomen and chest, comprising the esophagus distal to the atretic esophageal segment, the stomach, and the duodenum proximal to the atretic duodenal segment ( Fig. 17-13 ).

Jejunum and Ileum

Loops of small bowel containing minimal fluid can often be seen peristalsing in the mid abdomen of a normal fetus. Normal small bowel loops typically measure less than 7 mm in diameter and 15 mm in length ( Fig. 17-14 ; Video 17-3 ). Dilated loops of small bowel are most commonly due to jejunal or ileal atresia, both of which occur secondary to a vascular event. Malrotation, volvulus, and meconium ileus also cause intestinal obstruction. Ultrasound findings of jejunal and ileal obstruction include dilated loops of bowel, sometimes with increased peristalsis ( Fig. 17-15 ; Video 17-4 ). Occasionally only a single dilated loop of bowel is seen immediately proximal to the site of obstruction, although multiple dilated loops are more often identified. Polyhydramnios is seen in some but not all fetuses with small bowel atresia and is more likely to be present the higher the level of obstruction. The stomach may be dilated or normal in size. Ultrasound evidence of jejunal or ileal obstruction is usually not seen until near the end of the second trimester or later. Jejunal atresia is more common, more likely to exhibit multiple atretic sites, and more frequently associated with abnormalities outside the gastrointestinal tract than ileal atresia. The ileum is less compliant than the jejunum, and therefore jejunal atresia is more likely to be associated with markedly dilated bowel loops and ileal atresia is more likely to result in bowel perforation.