Reproductive Health

The menstrual cycle is the process by which a woman's body gets ready for the chance of a pregnancy each month. The average menstrual cycle is 28 days from the start of one to the start of the next, but it can range from 21 days to 35 days.

Most menstrual periods last from three to five days. In the United States, most girls start menstruating at age 12, but girls can start menstruating between the ages of 8 and 16.

Menopause is the absence of menstrual periods for 12 months. The menopausal transition begins with varying menstrual cycle lengths and ends with the final menstruation.

Source: MedicineNet.com
http://www.medicinenet.com/reproductive_health/article.htm