To evaluate the efficacy of examining the lateral ventricular atrium, cisterna magna, and cavum septi pellucidi as a means of ascertaining that the development of the fetal central nervous system (CNS) is normal, a retrospective evaluation of the sonograms of 112 fetuses (15-39 weeks gestational age) with sonographically diagnosed CNS anomalies was performed. Malformations included in the study were diverse. The lateral ventricular atrium was enlarged (greater than 10 mm) in 99 (88%) fetuses. Of the remaining 13 fetuses, seven had an abnormal-sized cisterna magna (less than 2 mm or greater than 11 mm). These two measurements alone could be used to identify the presence of a CNS abnormality in 95%. Three of the six remaining fetuses exhibited gross abnormalities easily seen on the standard axial images obtained for biparietal diameter measurement. Although the cavum septi pellucidi was absent in a number of cases, its absence did not enhance sensitivity in the cohort examined. Prospective examination of 130 consecutive normal fetuses (15-40 weeks gestational age) was also performed. When specifically sought, the ventricular atrium was identifiable and measurable 99% of the time; the cisterna magna, 90% of the time; and the cavum septi pellucidi, 95% of the time. Because major CNS anomalies are uncommon and these measurements afford high sensitivity, an extremely low probability (0.005%) of abnormal brain or spinal cord development can be predicted if a normal-sized lateral ventricular atrium and cisterna magna are present. These results should not be construed as a license to underexamine fetuses for malformations. Rather, these measurements should serve as simple positive steps to assist in a difficult task.