In the absence of the T.D.F. gene, differentiation of the genital tract follows the female development with Wolffian ducts regression, elaboration of Müllerian ducts, and homologous uro-genital sinus organization. Up to week 32, the fetal ovary secretes no organizing substance and no estrogen. Even if the maternal body is heavily impregnated by estrogens from the fetal-placental unit, the estrogenic and progestogenic activity of any steroid hormones which cross the placental barrier and enter the fetal body is reduced as a result of metabolic degradation within the fetal liver. It is only from week 32 that in normal pregnancies, phenomena of estrogenic action on the uterine cervix of the fetus are seen. This consists of hyperplasia of the endo-cervical epithelium and its emergence towards the exo-cervix. The slightest addition of non-steroid synthetic estrogens administered to the mother may interfere with this hormonal environment of the fetus and result in abnormal development of Müller's tract, particularly in the cervico-vaginal zone.