Estriol - the main steroid hormone, synthesized by the placenta. In the first stage of the synthesis, which takes place in the embryo, the cholesterol produced de novo or coming from the pregnant woman's blood turns into pregnenolone, which is sulphated by the adrenal cortex of the fetus in DHEA, then fetal liver turns into α-hydroxy-DHEAS, and then into estriol in placenta.