Isolated large fourth ventricle in early pregnancy--a possible benign transient phenomenon.

Twenty-one fetuses with an enlarged fourth ventricle were detected by ultrasound at 14-16 weeks' gestation. No other central nervous system anomalies were observed and a normal size fourth ventricle was noted in all cases on follow-up scans at 22-23 weeks' gestation. Five fetuses had associated structural anomalies: a single umbilical artery in two cases, non-septated cystic hygroma in two cases and ventricular septal defect in one fetus. All fetuses had a normal brain sonogram after delivery. Nineteen newborns who were followed up to the age of one year had no developmental problems. It is concluded that an isolated enlarged fourth ventricle might be a physiological variant in early fetal life.