Midbrain and falx in fetuses with absent corpus callosum at 11-13 weeks.

OBJECTIVE To describe the first trimester diagnosis of agenesis of the corpus callosum (ACC). METHODS The midbrain and falx cerebri were examined in stored images of the midsagittal view of the fetal brain at 11(+0)-13(+6) weeks' gestation from 15 fetuses with ACC and 500 normal controls. The midbrain diameter and falx diameter were measured and their ratio was calculated. The values in fetuses with ACC and normal controls were compared. RESULTS In the control group, the midbrain and falx diameters increased significantly with crown-rump length (CRL) from respective mean values of 5.1 and 6.9 mm at CRL of 45-6.9 mm and 12.1 mm at CRL of 84 mm. In the ACC group the midbrain diameter was above the 95th percentile of the control group in 8 (53.3%) cases, the falx diameter was below the 5th percentile in 6 (40.0%) cases and the midbrain diameter-to-falx diameter ratio was above the 95th percentile in 13 (86.7%) cases. CONCLUSIONS In the midsagittal view of the fetal brain at 11-13 weeks, the majority of fetuses with ACC have measurable abnormalities in the midbrain and falx area of the brain.