Left-right asymmetries of the temporal speech areas of the human fetus.

Left-right asymmetries of the transverse temporal (Heschl) gyri and the temporal plane become recognizable by 31 weeks' gestation. The transverse temporal gyri are larger in number and extent on the right side in 54% of 207 serially sectioned fetal brains ranging in gestational age from 10 to 44 weeks, and the temporal plane is larger on the left side in those brains. There are two transverse temporal gyri on the left and a single right transverse temporal gyrus on the right in 18% of the brains. No asymmetry of number of transverse temporal gyri or extent of the temporal plane is apparent in 28%. These findings, which confirm those in adult brains, suggest that anatomical asymmetries for left hemispheral speech and language dominance may be established during the last trimester of fetal life.