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Menarche
Menarche, or first period, just means when menstruation starts. It usually occurs between 12 and 15 years old in girls, although as young as 8 would still be normal. The menstrual fluid that is discharged in the first menstruation may be brown and muddy, as opposed to the blood red of later menstruation. Pregnancy is possible up to 2 weeks before menarche.
Some people say that a girl who has had menarche has "become a woman". This is a bit silly, but harmless if we remember just because you're getting your period you are in no way ready to have sex yet.
See also
This article duplicates from the menstruation article, but I can't think of a better solution for now.