Estriol is the main steroid hormone synthesized by the placenta. In the first stage of synthesis, which occurs in the embryo, cholesterol, formed de novo or coming from the blood of the pregnant woman, is converted to pregnenolone, which is sulfated by the adrenal cortex of the fetus into DHEAS, then converted in the liver of the fetus into α-hydroxy-DHEAS, and then into estriol in the placenta.