See also "Mensuration", a term sometimes used to describe Measurement. Menstruation is the shedding of the uterine lining (endometrium). It occurs on a regular basis in reproductive age females of certain mammal species. Overt menstruation (where there's bleeding from the vagina) is found primarily in humans and close evolutionary relatives such as chimpanzees. The females of other species of placental mammal have estrous cycles, in which the endometrium is reabsorbed by the animal (covert menst… (More on Menstruation)