http://www.womenshealth.gov/faq/menstru.htm
Menstruation is part of the menstrual cycle (or "period"), which helps a woman's body prepare for the possibility of pregnancy each month. This page provides answers to commonly asked questions about menstruation, including the onset, length of cycle, possible problems (such as amenorrhea, dysmenorrhea, and menorrhagia) and when to see a health care professional.