OBJECTIVE To evaluate with respect to gestational age (GA) the effect of magnetic resonance (MR) imaging on changes in diagnosis, patient counseling, and case management regarding fetuses suspected of having central nervous system (CNS) anomalies. METHODS The authors compared images from 242 ultrasonographic (US) studies and 242 MR imaging studies of the CNS in 214 fetuses. Reference standards of postnatal physical examination, imaging, surgical, and autopsy findings were available in 171 (79.9%) fetuses. Referring physicians were surveyed on how MR imaging findings changed patient counseling or case management. Outcomes were compared with respect to GA. Statistical tests used were the Fisher exact test, Student t test, and analysis of variance. RESULTS Confirmatory US findings were normal in 69 fetuses. MR imaging findings changed diagnosis in 46 of 145 (31.7%) fetuses with abnormal US findings. The mean GA of 72 of 145 fetuses with changes in maternal counseling (25.9 weeks) was significantly greater than that in 73 of 145 fetuses without changes in maternal counseling (22.6 weeks, P <.01). The mean GA of the 46 fetuses with changes in diagnosis (26.3 weeks) was significantly greater than that of the 99 fetuses with no major change in diagnosis (23.3 weeks, P <.01). There were 27 of 145 changes in case management, with no significant difference in mean GA of fetuses with and those without changes in case management. In fetuses with abnormal US findings, MR images were used to decide to terminate the pregnancy (n = 13; mean GA, 20.1 weeks), continue the pregnancy (n = 4; mean GA, 19.2 weeks), direct the mode and/or location of delivery (n = 5; mean GA, 30.5 weeks), and direct perinatal care (n = 5; mean GA, 30.2 weeks). CONCLUSIONS When a CNS anomaly is detected or suspected at US, MR imaging may demonstrate additional findings that can alter diagnosis and case management. Changes in management are GA dependent.