The menstrual period or menstruation is vaginal bleeding that is a part of a woman’s monthly cycle. This occurs when the egg is not fertilized. The body sheds the endometrium (the lining of the uterus), causing bleeding. The bleeding lasts for three to five days. The menstrual cycle is a series of changes that occur in a woman’s body as part of the preparation for a possible pregnancy. It is a cycle that usually begins between 12 and 15 years and continues until menopause, which, on average, occurs at the age of 52.