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Animation of the uterus and ovaries showing the changes that occur over the 28 days of the menstrual cycle. The levels of four hormones are shown at bottom left: follicle stimulating hormone (FSH, light blue), luteinising hormone (LH, pink), oestrogen (O, purple) and progesterone (P, yellow). The cycle begins with menstruation (a period), during which the thick lining of the uterus is shed through the cervix and vagina. After this, rising levels of oestrogen promote redevelopment of the lining. In the ovary, fast-rising levels of LH stimulate ovulation, the release of an ovum (pink), at day 14. This travels down the fallopian tube to the uterus. If it is not fertilised it degenerates within a day or so. The remaining part of the ovum's follicle in the ovary develops into a large yellow corpus luteum, which produces large amounts of progesterone. This in turn promotes new production of oestrogen, and inhibits LH and FSH production. Without LH and FSH the corpus luteum degenrates, and falling levels of progesterone and oestrogen lead to the spiral arteries in the endometrium atrophying, and the lining is shed at the start of the next cycle