menarche is the beginning of menstruation
Menarche (say MEN-ar-kee) is a girl's first menstrual period. A first period usually happens after breasts, pubic hair, and underarm hair have begun to grow. Menarche is a sign of growing up and becoming a woman. It can happen as early as about age 9 or up to age 15. The first few periods are usually light and irregular. About 2 out of 3 girls have a regular pattern of menstrual periods within 2 years of menarche. 1 During the teen years, periods may become longer and heavier. For more information, see Menarche.