In the beginning of the female 28 day cycle GnRH from the hypothalamus triggers the Anterior pituitary lgand to release FSH. FSH causes the ovarian follicule to mature which releases estrogen. As estrogen builds and peaks around day 13, this stimulates the anterior pituitary to release LH. LH causes the mature follicule to rupture from the ovary. This is known as ovulation which produces a mature ovum ready for fertilization. The follicle left behind is called the corpus luteum. This luteum will produce progesterone and with estrogen build the lining of the uterus in preparation for fertilization and implantation. After about 7 days if fertilization doesn't occur, the corpus luteum degenerates and estrogen and progesterone levels rapidly decrease causes ischemia to the uteran lining. It begins to shed and the period begins. These low levels of hormone trigger the process all over again beginning at the hypothalamus and the anterior pituitary gland. Hopefully this puts this complicated process in perspective.