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).