Timing of sexual differentiation in the human fetus.
Fetal age* (weeks)/Crown-rump length (mm)/Sex differentiating events
blastocyst Inactivation of one X chromosome
4 2-3 Development of wolffian ducts
5 7 Migration of primordial germ cells in the undifferentiated gonad
6 10-15 Development of müllerian ducts
7 13-20 Differentiation of seminiferous tubules
8 30 Regression of müllerian ducts in male fetus
8 32-35 Appearance of Leydig cells. First synthesis of testosterone
9 43 Total regression of müllerian ducts. Loss of sensitivity of müllerian ducts in the female fetus
9 43 First meiotic prophase in oogonia
10 43-45 Beginning of masculinization of external genitalia
10 50 Beginning of regression of wolffian ducts in the female fetus
12 70 Fetal testis is in the internal inguinal ring
12-14 70-90 Male penile urethra is completed
14 90 Appearance of first spermatogonia
16 100 Appearance of first ovarian follicles
17 120 Numerous Leydig cells. Peak of testosterone secretion
20 150 Regression of Leydig cells. Diminished testosterone secretion
24 200 First multilayered ovarian follicles. Canalisation of the vagina
28 230 Cessation of oogonia multiplication