Sex differences in the fetal human corpus callosum.

Midsagittal sections of fetal cerebra from the Yakovlev collection ranging from 26-41 weeks gestational age were photographed. The photographs were used to obtain areal measurements of the cross-sectional surface of the corpora callosa; and linear measurements of the widths of genu, body and splenium. A significant sex difference, favoring females, was found in the splenial width of the corpus callosum by 26 weeks gestational age. Although other variables, including the overall cross-sectional area of the corpus callosum, were larger in females both absolutely and relative to brain weight, the differences were not statistically significant. These results suggest that the gonadal steroids and/or genetic sex have an important role in utero in the differentiation of neural structures not associated with reproductive functions. Elaboration of sex differences, however, may occur postnatally.