Hormones Maintain Pregnancy.
The trophoblast secretes human chorionic gonadotropin (hCG). hCG is a long-range autocrine signal and maintains the corpus luteum and stimulates it to secrete estrogen and progesterone hormones. Progesterone is what suppresses menstruation.
The hCG level continues to rise till the end of the second month as the trophoblast develops into a chorion membrane, which takes over the role of the corpus luteum thus hCG is not needed any longer. The levels of estrogen and progesterone continue to rise as the chorion takes over in secreting them.
hCG operates as an immunosuppressive mediator and has growth-promoting activity to promote blastocyst and placental development. hCG also has protease activity and levels are high when the trophoblast meets the endometrium so it may also have a role in implantation.
3D polypeptide structure of hCG courtesy of Wikipedia Commons under public domain: commons.wikimedia.org/wiki/File:PBB_Protein_CGB_image.jpg