Follicle-stimulating hormones (FSH) trigger the development of many follicles in the ovary, known as the follicular phase. Usually, only one follicle reaches maturity. As the follicle develops, it secretes estrogen, which stimulates the thickening of the endometrium during the menstrual cycle, as well as small amounts of progesterone. As the follicle matures, it migrates to the surface of the ovary. The fimbriae of the fallopian tubes extend across the top of the ovary to capture the emerging oocyte. The remnants of the follicle remain on the ovary and form the corpus luteum, which marks the beginning of the luteal phase. Say no to plagiarism. Get a tailor-made essay on "Why Violent Video Games Shouldn't Be Banned"? Get an original essay The corpus luteum secretes large amounts of progesterone and small amounts of estrogen. Progesterone causes the endometrium to continue to thicken and become more vascular, preparing it for pregnancy. High levels of progesterone and estrogen also inhibit the pituitary gland from producing FSH and LH, so no more follicles develop. If fertilization does not occur, the corpus luteum degenerates into inactive scar tissue known as the corpus albicans. Estrogen and progesterone levels decrease, causing the endometrium to shed, resulting in menstruation. When there is a decline in ovarian hormones, the pituitary gland is no longer inhibited and thus follicle stimulating hormone (FSH) levels begin to increase and the menstrual cycle begins. The first phase of the menstrual cycle is the menstrual phase which is the first day of noticeable vaginal bleeding that can last three to five days. Menstruation is the result of the endometrium losing its functional layer, otherwise known as the functional layer. The proliferative phase is when menstruation stops and occurs around the fifth day of the cycle, where only the base layer otherwise known as the basal layer remains in the uterus. Around day six, increased levels of estrogen secreted by the ovaries stimulate repair of the basal layer and growth of blood vessels. During this phase, the endometrium thickens to two or three mm. Please note: this is just a sample. Get a custom paper from our expert writers now. Get a Custom Essay In the secretory phase, after ovulation, which is around day 14, progesterone increases from the corpus luteum causes further thickening of the functional layer, this time due to fluid secretion and accumulation. During this stage, the endometrium develops into a bed about 5-6 mm thick, which is the right size for a fertilized egg. During the premenstrual phase, if fertilization does not occur, the corpus luteum atrophies and progesterone levels decrease. The blood vessels that nourish the endometrium narrow, cutting off blood flow. The endometrium becomes ischemic and necrotic, causing it to detach from the uterine wall. This forms the menstrual flow.