The menstrual cycle is a series of changes occurring in a woman’s body which prepares her for pregnancy. About once a month, the uterus develops a new lining (endometrium) to get ready for a fresh fertilized egg. When there is no fertilized egg, the uterus starts shedding its lining. This leads to monthly menstrual bleeding (menstrual period) that starts at the early teen years of women and continues till menopause, around age 50.