Menstruation (mammal)
Menstruation in mammals is the shedding of the uterine lining (endometrium). It occurs on a regular basis in uninseminated sexually reproductive-age females of certain mammal species.
Mammalian reproduction
Viviparous mammals are in the subclass Theria; those living today are in the Marsupialia and Placentalia infraclasses. A marsupial has a short gestation period, typically shorter than its estrous cycle, and gives birth to an underdeveloped (altricial) newborn that then undergoes further development; in many species, this takes place within a pouch-like sac, the marsupium, located in the front of the mother's abdomen. Some placentals, e.g. guinea pig, give birth to fully developed (precocial) young, usually after long gestation periods, while some others, e.g. mouse, give birth to underdeveloped young.