Menstruation (also called menses) is the monthly bleeding that occurs in women of reproductive age. It marks the beginning of the menstrual cycle — which is the cycle that serves to prepare the uterus for the implantation of a fertilized egg. If no implantation occurs, the lining of the uterus is shed as menstrual blood, which flows through the cervix and out the vagina; menstruation is the name for this process of shedding the uterine lining. Most menstrual periods last from three to seven days, with an average length of about five days.