menarche

menarke The onset of menstruation; the beginning of the menstrual period. A girl's first period.
1973, Patrick O'Brian, HMS Surprise:

How old is she? Nine? Ten? The menarche is not far off – a hint of a bosom, poor child.

The most potent and indeed dangerous of all menstrual blood was the menarche, the first day's flow of the first menstruation of a virgin girl.