In the female reprouctive system, the menstrual cycle is a recurring cycle of physiologic changes that occurs in reproductive-age females. Overt menstruation (where there is blood-flow from the vagina) occurs 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 completely reabsorbed by the animal (covert menstruation) at the end of its reproductive cycle. This article concentrates on the menstrual cycle as it occurs in human beings.