TABLE 1. 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
28	230	Descent of testis

* Fetal age in weeks after the last menstrual period.
Source: Sizonenko (50).