Menstruation also known as "periods," is the shedding of the uterine lining (endometrium) when an egg is not fertilized at the end of the cycle for a potential pregnancy. It occurs on a regular basis in reproductive-age females of certain mammal species. Overt menstruation (bleeding from the uterus through the vagina) is found primarily in humans and close evolutionary relatives such as chimpanzees. Menstruation generally indicates that pregnancy is possible.
