How accurate is prenatal sonography for the diagnosis of imperforate anus and Hirschsprung's disease?

Imperforate anus (IA) and Hirschsprung's disease (HD) are the commonest causes of coloanal obstruction in the neonate. We prospectively studied 15,092 pregnancies over a 4-year period to determine the accuracy of prenatal sonography (US) in diagnosing these conditions. Based on location and appearance, 13 fetuses were thought to have colonic dilatation on prenatal US. Nine of these had dilated bowel located in the pelvis. Postnatally, 7 were normal and 2 had an IA. Four fetuses were thought to have dilated proximal colon based on the peripheral location of the bowel loops in the abdomen. All 4 had normal colons, but had small-bowel obstruction postnatally (3 meconium ileus, 1 malrotation). Neonatal or fetal pathologic follow-up was obtained for all pregnancies. Twenty-three fetuses without dilated colon on US had coloanal obstruction (20 IA, 3 HD). Other anomalies had been sonographically evident in 17 of these fetuses (all with IA).Overall sensitivity and positive predictive value for sonographically dilated colon in predicting IA or HD were very low in this series (8% and 15%, respectively). No cases of HD were diagnosed prenatally. The incidence of associated anomalies was high in fetuses with IA. Dilated bowel loops outside the pelvis represented small bowel rather than colon, even if they were peripheral in location. These data suggest that prenatal US is of limited value for either diagnosing or excluding IA and HD.