If fertilisation does not occur after ovulation, the corpus luteum will degenerate and production of progesterone will decrease, so the stimulus for maintaining the thick endometrium will disappear. The reduction in progesterone causes the shedding of the thick endometrial lining. The muscular wall of the uterus (the myometrium) contracts to help cut off the blood supply to the endometrium, causing it to break away from the uterus. The endometrium was richly supplied with blood vessels to nourish the fetus if a pregnancy occurred, so when it disintegrates and passes down the vagina, some blood is mixed in with it. The mixture of tissue and blood passes out through the vagina as the menstrual fluid (or menstrual flow), usually for a period of about three to five days. Other common names for menstruation are 'monthly bleeding' or 'menstrual period'.
Menstruation usually occurs at monthly intervals throughout the reproductive years, except during pregnancy when it is completely suppressed and the woman cannot get pregnant again until after the baby is born. Breastfeeding a baby also suppresses menstruation, but there is a risk that ovulation and pregnancy may still occur.