A number of single-gene disorders are detectable early in pregnancy through DNA analysis on chorionic villi. In some genetic disorders that are not yet amenable with this technique, prenatal diagnosis is based on the sonographic detection of neurodevelopmental abnormalities. In nine fetuses at risk for the Meckel-Gruber, Walker-Warburg, and Joubert syndromes, sonographic scanning was accomplished in the first and early second trimesters of pregnancy. In five out of nine fetuses, structural anomalies were detected. In two out of five affected fetuses, sonographic detection of a cephalocele was predated by the visualization of an enlarged rhombencephalon cavity diameter.