Maternal endocrine adaptations to placental hormones in humans.

The remarkable endocrine alterations that are characteristic of human pregnancy are attributable to the placenta. In this tissue, steroid and peptide hormones are produced in extraordinary amounts. In addition, the haemomonochorioendothelial placentation of human pregnancy contributes to the unique distribution of products formed in trophoblasts into maternal and fetal compartments. In this review, the partial control exerted by the trophoblast on maternal metabolism is illustrated by the replacement in the maternal compartment of pituitary growth hormone (GH) with the trophoblast's own product, human placental GH. Placental GH differs from pituitary GH by 13 amino acids, has high somatogenic and low lactogenic activities and is secreted by the syncytiotrophoblast in a non-pulsatile manner. This continuous secretion appears to have important implications for the control of maternal levels of insulin-like growth factor I. Placental GH secretion is inhibited by glucose in vitro and in vivo, and is significantly decreased in the maternal circulation in cases of pregnancies with intrauterine growth retardation.