The placenta secretes a hormone that paicipates in stimulation of ductal growth in the mammary gland during pregnancy. This hormone is:

All of these hormones are produced by the placenta, but hCG is the hormone that maintains the corpus luteum of pregnancy. The synthesis and secretion of hCG begins about day 6 of pregnancy, presumably from the trophoblast cells. Pregnancy tests utilize the measurement of hCG for detection of the embryo. Endothelial growth factor (EGF) is synthesized by the cytotrophoblast cells in the early placenta, then is synthesized by the syncytiotrophoblast cells later (6-8 week old placenta). EGF maintains the trophoblasts. Relaxin is synthesized by decidual cells at the time of paurition and acts to "soften" the cervix and pelvic ligaments. IGF act similarly to EGF by stimulating differentiation of the cytotrophoblast cells. HCS(Human chorionic somatotropin) is synthesized by syncytiotrophoblast cells and will promote general growth. It is essential in the stimulation of mammary duct proliferation in development of the breast during pregnancy. Ref: Cunningham F.G., Leveno K.J., Bloom S.L., Hauth J.C., Rouse D.J., Spong C.Y. (2010). Chapter 3. Implantation, Embryogenesis, and Placental Development. In F.G. Cunningham, K.J. Leveno, S.L. Bloom, J.C. Hauth, D.J. Rouse, C.Y. Spong (Eds), Williams Obstetrics, 23e.