For most girls, their first menstrual period, or menarche (say: MEH - nar - kee), begins about 2 to 2 1/2 years after breast development starts. (kidshealth.org)
It is possible to get pregnant before your first menstrual period. (cindyandjana.com)
Usually about 2 years after your breasts begin to form, you will have your first menstrual period. (caringforkids.cps.ca)