Duodenal atresia is found in approximately 1 in 10,000 births (Best, 2012; Dolk, 2010). It is characterized by the sonographic double-bubble sin, which represents distention of the stomach and the irst part of the duodenum (Fig. 10-38). This finding is usually not present before 22 to 24 weeks' gestation. Demonstrating continuity between the stomach and proximal duodenum confirms that the second "bubble" is the proximal duodenum. Approximately 30 percent of afected fetuses have an associated chromosomal abnormality or genetic syndrome, particularly trisomy 21. Of cases without a genetic abnormality, a third have associated anomalies, most commonly cardiac defects and other gastrointestinal abnormalities (Best, 2012). Obstructions in the more distal small bowel usually result in multiple dilated loops that may have enhanced peristaltic activity.
Large-bowel obstructions and anal atresia are less readily diagnosed by sonography, because hydramnios is not a typical feature and the bowel may not be signiicantly dilated. A transverse view through the pelvis may show an enlarged rectum as an anechoic structure between the bladder and the sacrum.
The fetal kidneys are visible adjacent to the spine, frequently in the first trimester and routinely by 18 weeks' gestation (Fig. 10-39). he length of the kidney approximates 20 mm at 20 weeks and grows by about 1.1 mm each week thereafter (Chitty, 2003). With advancing gestation, the kidneys become relatively less echogenic, and a rim of perinephric fat aids visualization of their margins.
he fetal bladder is readily visible in the second trimester as a round, anechoic structure in the anterior midline of the
FIGURE 10-39 Normal fetal kidneys. The kidneys are visible adjacent to the fetal spine in this 29-week fetus. With advancing gestation, a rim of perinephric fat aids visualization of the kidney margins. A physiological amount of urine is visible in the renal pelves and is marked in one kidney by an arrow.
pelvis. With application of Doppler, the bladder is outlined by the two superior vesical arteries as they become the umbilical arteries of the umbilical cord (Fig. 10-40 and Chap. 6, p. 1i17). The fetal ureters and urethra are not visible sonographically unless abnormally dilated.
The placenta and membranes are the major sources of amnionic fluid early in pregnancy. However, after 18 weeks' gestation, most of the luid is produced by the kidneys (Chap. 11, p. 225). Fetal urine production rises from 5 mLlhr at 20 weeks to approximately 50 mLlhr at term (Rabinowitz, 1989). Normal amnionic luid volume in the second half of pregnancy suggests urinary tract patency with at least one functioning kidney. But, unexplained oligohydramnios suggests a urinary tract defect or placental perfusion abnormality.
FIGURE 10-40 Normal fetal bladder. The normal fetal bladder is readily visible as a round, fluid-filled structure in the anterior pelvis, outlined by the two superior vesical arteries as they become the umbilical arteries of the umbilical cord.