Fig. 3.

Estrogen biosynthesis in the fetoplacental system. Cholesterol is taken by the placenta from LDL present in the maternal circulation, then trasformed by P450-Scc into pregnenolone, which is sulfated and taken up by fetal adrenal glands to produce dehydroepiandrosterone sulfate (S-DHEA), then hydroxylated to 16α-OH-S-DHEA in the fetal liver. 16α-OH-S-DHEA is desulfated in the placenta to produce 16α-OH-DHEA which then is taken up by the syncytiotrophoblasts for aromatization into E3. Desulfated DHEA may alternatively be converted into androstenedione which is aromatized into E1 or first transformed into testosterone then aromatized to E2. Alternatively, placental pregnenolone may be converted to progesterone which returns to the maternal circulation.