BACKGROUND—Slight morphological asymmetry of the cerebral hemispheres has been observed in fetal and newborn brains. In adults, sex differences in hemispheric asymmetry have also been reported. OBJECTIVE—To establish whether cerebral hemisphere asymmetry correlates with sex in fetuses. METHODS—Left-right cerebral hemisphere asymmetry, and the correlation with sex, were studied in 51 male and 51 female fetuses of 20-22 weeks gestation, using diagnostic ultrasound scanning. RESULTS—A total of 102 fetuses were examined. The diameter of the left hemisphere was larger than that of the right, in both female and male fetuses. The mean (SEM) diameter of the left hemisphere was 2.804 (0.174) cm in female fetuses and 2.781 (0.287) cm in male fetuses; the corresponding values for the right hemisphere were 2.627 (0.192) cm and 2.681 (0.267) cm. There was no sex related difference between hemispheric diameters. The interhemispheric difference was significant for both sexes: male fetuses, p = 0.017; female fetuses, p = 0.016. CONCLUSIONS—Left-right fetal brain asymmetry, as measured by in utero ultrasound examination, is apparent at 20-22 weeks gestation regardless of sex.
