Menarche is the medical term used to refer to a woman's first menstrual period. The average age at which menarche occurs is usually 12 years old, although it may vary from one woman to another.
From this time on, women will begin to have menstrual cycles of about 28 days. Most women experience irregular periods the first few times after menarche, but then the cycle will normalize. In addition, the intensity and quantity of menstruation may also vary until it stabilizes.