The purpose of a menstrual period is to get rid of the new uterine lining that grew in preparation for a fertilized egg during the reproductive process, according to WebMD. When no fertilized egg implants into this lining, it sheds off to allow for a new cycle to begin.
During a menstrual cycle, the female body increases levels of the hormones estrogen and progesterone to prepare the reproductive tract for pregnancy. Estrogen, released first, helps the uterus grow a new lining, and progesterone, released in the middle of a cycle, helps support this lining. When implantation does not occur, progesterone levels drop and cause the new lining to break down, which the body expels through the vaginal canal.
In the days leading up to a menstrual period, women may experience symptoms of premenstrual syndrome, such as abdominal cramping, bloating, mood swings, breast tenderness, acne and fatigue. According to Mayo Clinic, about 75 percent of women experience PMS symptoms, which are usually relieved once menstruation begins. According to the U.S Department of Health and Human Services, 3 to 8 percent of women suffer from a more severe form of PMS known as premenstrual dysphoric disorder, while other women experience no PMS symptoms at all.