Fetuses start swallowing amniotic fluid early in gestation, and the fluid-filled stomach is visible as early as 9 weeks of gestation as a C-shaped sonolucent structure in the upper left quadrant of the abdomen. The bowel has normally a uniform echogenic appearance until the third trimester of pregnancy when meconium-filled loops of large bowel are commonly seen. The liver is large prenatally and comprises most of the upper abdomen. The gallbladder is usually seen since midgestation as an ovoid cystic structure to the right and below the intrahepatic portion of the umbilical vein. The spleen may also be visualized posterior and to the left of the fetal stomach. The proximal and distal esophagus can be at times visualized, when the fetus is in a favorable position and particularly in the course of swallowing. However, it is impossible to visualize the entire length. The anal complex can also be seen although usually only in late gestation (Fig. 1.2).

As the esophagus is poorly and anyhow incompletely visualized with fetal sonography, most cases of atresia escape antenatal detection [4]. The majority of cases are associated with a tracheoesophageal fistula that allows distal transit of fluid and filling of the stomach. In late gestation, however, the size of the fistula does not allow adequate transit and as a consequence of this fluid accumulated into the amniotic cavity, and the stomach appears minimally distended.

The diagnosis of esophageal atresia is suspected when, in the presence of polyhydramnios (usually only in the third trimester), repeated ultrasonographic examinations demonstrate a small stomach bubble. In most cases the condition can only be suspected and the final diagnosis is only possible after birth. The only exception is in cases in which during swallowing the dilated proximal esophageal pouch is seen, as an elongated upper mediastinal and retrocardiac anechoic structure. This finding however is present only after 28 weeks and transiently [4].

The differential diagnosis for the combination of a small stomach bubble and polyhydramnios includes intrathoracic compression, by conditions such as diaphragmatic hernia, and muscular-skeletal anomalies causing inability of the fetus to swallow. Fetal magnetic resonance has also been reported to be of help in these cases [4].

In one of the largest available series, polyhydramnios was present in 50 % of cases, and the atresia was suspected or diagnosed antenatally in about one-third of cases, at a median gestational age of 31 weeks. As expected type 1 atresia was more frequently suspected (polyhydramnios in 100 % of cases, small gastric bubble in over 80 % of cases) than cases with a tracheal fistula (polyhydramnios 50 % of cases, small gastric bubble in 25 %) [4].

Esophageal atresia and tracheoesophageal fistula are often associated with other major defects, including chromosomal anomalies, malformations, and syndromic associations, that are not always obvious on prenatal examinations (Fig. 1.3).

Prenatal diagnosis is based on the demonstration of the characteristic “double bubble” appearance of the dilated stomach and proximal duodenum, commonly associated with polyhydramnios. Although the characteristic “double bubble” can be seen as early as 20 weeks, it is usually not diagnosed until after 25 weeks suggesting that the fetus is unable to swallow sufficient volume of amniotic fluid for bowel dilatation to occur before the end of the second trimester of pregnancy.

In a review of the literature, prenatal diagnosis was made in 77 % of cases. Other malformations were often present and trisomy 21 was found in about one-third of cases [5] (Fig. 1.4).

Few cases of pyloric atresia, often in association with other malformations, have been described. The typical finding includes polyhydramnios and a large stomach that usually appear only in the third trimester of gestation. The index of suspicion is increased when dilatation of the esophagus is also seen [5].