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