The human female menstrual cycle is divided into several phases, which vary in length among women and among cycles; average times are indicated. The first phase of the menstrual cycle is the follicular phase, and it begins the day that menstrual bleeding starts. A decrease in the levels of estrogen and progesterone triggers the top layers of the thickened endometrium to break down and be shed, resulting in bleeding. Concomitant with this, levels of FSH increase very slightly, stimulating the development of several oocyte-containing follicles. FSH levels subsequently decrease and only one or two follicles continue to develop. The developing follicles release estrogen, and this initiates thickening of the endometrium, something that continues throughout the rest of the menstrual cycle. The second phase of the menstrual cycle is the ovulatory phase. It begins at approximately day 13, when levels of LH and FSH increase dramatically; levels of estrogen also peak at this time, and levels of progesterone begin to increase. The high levels of LH stimulate ovulation. The final phase of the menstrual cycle is the luteal phase. During this phase, levels of LH and FSH decrease and the ruptured follicle forms the corpus luteum, which produces large amounts of progesterone. Progesterone modifies the endometrium so that it is receptive to implantation of an embryo if fertilization has occurred. In the absence of fertilization, the corpus luteum degenerates and the loss of progesterone production, combined with decreased levels of estrogens, initiates a new menstrual cycle.