OBJECTIVE To evaluate the progression in utero of mild isolated fetal ventriculomegaly (defined as a transverse diameter of the atrium of the lateral ventricle measuring between 10 and 15 mm), and to estimate the proportion of fetuses that normalize (diameter decreasing to less than 10mm), stabilize (remaining between 10 and 15 mm), or progress to more severe ventriculomegaly (becoming greater than 15 mm). METHODS The obstetric databases of 3 institutions were queried for any studies mentioning ventriculomegaly or hydrocephalus. Reports and original images were reviewed to verify cases of isolated mild ventriculomegaly, with no other anomalies on comprehensive ultrasonographic examination. Fetuses that had 2 or more evaluations more than 3 weeks apart were included. RESULTS A total of 63 fetuses met the criteria for isolated mild ventriculomegaly. The mean gestational age and ventricular measurements were 24.7+/-3.7 weeks and 11.8+/-1.1mm, respectively, at the initial scan and 34+/-2.9 weeks and 12.1 +/-3.8mm, respectively, at the final scan. The mean number of scans was 3.75 per fetus (range, 2-6). Amniocentesis revealed the deletion of 5p, which causes the cri du chat, in 1 of 21 fetuses; 26 fetuses (41%) showed normalization of the lateral ventricles; 10 fetuses (16%) showed progression; and 27 (43%) appeared stable. shows the statistics of the individual groups. Three of the fetuses that "stabilized" improved from 15 mm to 11, 11.5, and 11.7 mm, respectively. Two worsened from 10.2 to 14 mm and from 11.4 to 13 mm. CONCLUSIONS More than 40% of the cases of mild isolated fetal ventriculomegaly resolved in utero. The significant overlap in measurements for the different groups precludes prediction in individual cases. However, of the 13 cases where the transverse diameter measured 13 mm or more, only 1 normalized, while 9 of the remaining 12 cases stabilized and 3 progressed.