Menstruation, also known as a period, is a normal vaginal bleeding that occurs as part of a womans monthly cycle. It is a natural process controlled by female hormones that cause regular bleeding. Every month, the uterus prepares for pregnancy, and if no pregnancy occurs, the uterus sheds its lining, which is partly blood and partly tissue from inside the uterus. The menstrual blood passes out of the body through the vagina. Menstruation usually starts between the ages of 11 and 14 and continues until menopause at about age 51. The menstrual cycle is the monthly hormonal cycle a female’s body goes through to prepare for pregnancy. The average menstrual cycle lasts 28 days, but it can range from 21 to 35 days and still be normal. The menstrual cycle has four phases: menstruation, the follicular phase, ovulation, and the luteal phase. Menstrual cycles often change as a woman gets older. Some common symptoms of menstruation include abdominal or pelvic cramping pain, lower back pain, bloating, sore breasts, food cravings, and mood changes.