Possible sex differences in the developing human fetal brain.

Left-right regional volumetric asymmetries in five telencephalic regions were studied in the developing human fetal brain. Complete series of coronal sections of 21 fetal brains were digitized and regional volumes were integrated. Five regional indices of asymmetry and two overall indices of asymmetry were calculated and compared across the fetal sample. The two most asymmetrical regions in the developing fetal brain were region 1, roughly equivalent to prefrontal cortex, and region 5, which includes striate and extrastriate cortices. Region 5 also manifested a statistically significant sex difference (p less than .02) in the degree of volumetric asymmetry. It appears that striate-extrastriate cortices are far more asymmetrical in male brains than in their female counterparts (M = 33%; F = 13%). Overall indices of asymmetry indicated that, on the average, volumetric asymmetries in the male brain favor the right hemisphere. In contrast, the human fetal female is likely to have two hemispheres of the same size or a left hemisphere that is slightly larger than its right counterpart. We believe that these results support the hypothesis that testosterone in utero may lead to a more rapid growth of the right hemisphere or, alternatively, retard the growth of the left hemisphere.