Abstract: Objective To detect structural changes in the brain in fetuses with agenesis of the corpus callosum (ACC) and
holoprosencephaly (HPE) in the first trimester. Methods The ultrasound data were analyzed retrospectively in 620 normal
singleton fetuses between 11 and 13 + 6 gestational weeks, 5 fetuses diagnosed to have ACC, and 13 fetuses with HPE. The
midbrain diameter (MD) and falx diameter (FD) were measured and their ratio (MD/FD) was calculated for comparative
analysis. Results No significant difference was found in the MD, FD, and MD/FD ratio between fetuses with ACC and HPE (P>
0.05). Compared to the normal fetuses, all the fetuses with ACC and HPE showed significantly increased mean MD and MD/
FD ratio (P<0.05); 4 (80%) fetuses with ACC and 11 (84.6%) with HPE had a reduced FD. All the fetuses with ACC and HPE
had MD/FD ratios greater than 1, which were below 1 in all the normal fetuses. Conclusion In the first trimester, fetuses with
ACC and HPE have measurable abnormalities in the midbrain and falx area of the brain, and these changes, represented by
abnormal midsagittal MD, FD and their ratio, can be of value in detecting ACC or HPE in fetuses in the first trimester.