Changes in gender characteristics during fetal development The reproductive differences of the fetus are determined after the fertilized egg is formed, while gender characteristics are gradually formed during the development of the fetus. During the four months of fetal development, the gender characteristics of male and female fetuses change differently. The formation of gonads In the 4th week of embryonic development, the reproductive ectoderm begins to differentiate into the reproductive venation. In the 6th week, the reproductive venation further differentiates into gonad primordia. The gonad primordia of male and female fetuses are similar in morphology, but in the 7th week, the male fetuses