menstrual cycle

Biological cycle occurring in female mammals of reproductive age that prepares the body for pregnancy. At the beginning of the cycle, a Graafian (egg) follicle develops in the ovary, and the inner wall of the uterus forms a soft spongy lining. The egg (ovum) is released from the ovary, and the uterus lining (endometrium) becomes vascularized (filled with blood vessels). At this stage fertilization can take place. If fertilization does not occur, the corpus luteum (remains of the Graafian follicle) degenerates, and the uterine lining breaks down, and is shed. This is what causes the loss of blood that marks menstruation. The cycle then begins again. Human menstruation takes place from puberty to menopause, except during pregnancy, occurring about every 28 days.

The monthly release of an egg from a woman's ovaries and the changes in the thickness of the lining of her womb (uterus) are controlled by hormones produced by the pituitary gland and by the ovaries. If fertilization occurs, the corpus luteum persists and goes on producing progesterone.