hCG stands for Human Chorionic Gonadotropin. It's the hormone that is produced by the placenta right when implantation first takes place - which makes it a wonderful indicator of a pregnancy. However, the point of hCG, its "manufacture" by your baby, is not just to let you know he or she is there! The biological function of hCG is to "tell" the corpus luteum to maintain the production of progesterone. Progesterone warms the body and at elevated levels will prevent you from having a period. In effect, the presence of hCG ensures the further production of progesterone, and therefore the integrity of the endometrial-uterine lining is preserved (along with your pregnancy). Menstruation will not occur thanks to the dynamic interaction between hCG and progesterone.