The Gastrointestinal Tract and Intraabdominal Organs

The stomach forms at about 4 weeks after conception by the development of a spindle-shaped dilatation of the foregut caudal to the esophagus. The stomach descends into the abdomen at about 6 to 7 weeks after conception, and by 11 weeks the muscles in its wall are developed. Visualization with ultrasound is possible as early as the 9th week of menstrual age. Different parts of the gastric anatomy (greater curvature, fundus, body, and pylorus) can be seen by the 14th week. Peristalsis is rarely visible before the 16th week. Table 7-7 (see p. 254) displays data on fetal stomach dimensions. Observations of this organ over a 3-hour period of time have demonstrated that there are no dramatic changes in size and therefore these dimensions should be useful in evaluating stomach size. On occasion, an echogenic mass (fetal gastric pseudomass) is seen within the fetal stomach. The origin of this monographic image is not clear. If it disappears on subsequent examinations, it has no proven pathologic significance. The small and large bowels can be distinguished from each other. The small bowel is centrally located, and it changes its position and appearance with peristalsis, which is visible as early as the 18th week of menstrual age. Early in gestation, peristaltic waves are characterized by vigorous and fleeting movements with a duration of less than 3 seconds. Later in gestation, peristaltic waves are more vigorous, ubiquitous, and of longer duration. A small bowel loop of more than 7 mm in internal diameter should raise the index of suspicion for bowel obstruction. Generally, segments of small bowel measure less than 15 mm in length. Meconium formation begins at 16 to 20 weeks of menstrual age. Sonographically, meconium appears hypoechogenic in comparison to the bowel wall. The large bowel appears as a large tubular structure in the periphery of the fetal abdomen. Haustral clefts are sonographically apparent at 30 weeks in 87 percent of the fetuses and in all cases at 31 weeks of menstrual age. Table 7-8 (see p. 254) shows the correlation between transverse colonic diameter and gestational age. The echogenicity of the colon is assessed and graded in comparison with bladder and liver echogenicity. Grade 0 means the abdomen is uniform in appearance and the colon is not identified. Grade 1, the colonic appearance is hypoechogenic and essentially identical to the stomach and bladder. Haustra may be identified. Grade 2, the echogenicity of the colon is greater than the bladder, but less than the liver. Grade 3, colonic