The menstrual cycle is a recurring process which takes around 28 days. During the process, the lining of the uterus is prepared for pregnancy. If implantation of the fertilised egg into the uterus lining does not happen, the lining is then shed. This is known as menstruation or having a period.
Several hormones control this cycle – for example, they are involved in controlling the release of an egg each month from an ovary and changing the thickness of the uterus lining.