In females, each month one egg is set free by either of the ovaries. The uterus forms a thickening to support a foetus to keep it ready in case of pregnancy. If fertilization of egg does not take place, then egg and thickening in uterus are removed in small parts. As a result, bleeding in the uterus takes place for some days. This bleeding through uterus at the end of menstrual cycle, is known as menstruation.
The sequence of events starting from the release of an egg and till egg gets removed is called menstrual cycle. A menstrual cycle normally comprise of 28 to 30 days.