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.
Menarche
Menstruation is a cultural as well as scientific phenomenon as many societies have specific rituals and cultural norms associated with it. These rituals typically begin at menarche and some are enacted during each menstruation cycle. The rituals are important in determining a status change for girls. Upon menarche and completion of the ritual, they have become a woman as defined by their culture.