The hormone hCG is important for maintaining the corpus luteum, a cyst in the ovary that produced the egg. This area generates large amounts of the hormone progesterone at the beginning of pregnancy. At conception, corpus luteum produces progesterone for several weeks, preventing your period from coming as the egg attaches itself in the uterus. Though it's not fully understood, it seems that hCG continues to stimulate the placenta's own production of the hormones estrogen and progesterone.