[Regulation of human placental progesterone synthesis]

The steroid hormone progesterone is necessary for the maintenance of human pregnancy. It protects the uterus against the labour-inducing effect of prostaglandins and oxytocine. Before the 8.-9th week of gestation progesterone is produced by the corpus luteum and thereafter by the human placenta. The progesterone production of the human placenta is widely independent of endocrine organs of the mother and the foetus and controlled by autoregulation. Progesterone is formed from cholesterol taken up from the maternal circulation in the form of lipoprotein complexes. After side-chain cleavage of cholesterol pregnenolone is formed, which is converted by a 3 beat-hydroxysteroid dehydrogenase, delta 4-5 isomerase to progesterone. Progesterone is released partly to the mother and via the umbilical cord to the foetus. In the foetus a high conversion to 20 alpha-dihydroprogesterone takes place and this compound is transported to the placenta and recycled to progesterone. The steroidogenic enzymes of the progesterone pathway are controlled by various feedback inhibitions; activators could not be isolated so far.