(somatomammotropin) Secretion begins around the 5th week of pregnancy; probably promotes growth of some fetal tissues as well as aiding in the development of the mother's breasts; a general metabolic hormone for both mother and fetus.

A polypeptide hormone of approximately 25 kDa that is produced by the SYNCYTIOTROPHOBLASTS of the PLACENTA, also known as chorionic somatomammotropin. It has both GROWTH HORMONE and PROLACTIN activities on growth, lactation, and luteal steroid production. In women, placental lactogen secretion begins soon after implantation and increases to 1 g or more a day in late pregnancy. Placental lactogen is also an insulin antagonist.