Question: How Often Do Girls Have Periods?
A menstrual cycle is the term used to describe a complete "monthy" cycle, which begins and ends with the first day of bleeding. An entire cycle typically lasts 28 days, so a new cycle starts about every 4 weeks.
As explained by Robin Elise Weiss, LCC, on About.com's Pregnancy & Childbirth guidesite
, your menstrual cycle is the basics of your reproductive health. Every month your body goes through two main phases to complete the cycle and begin again. These are the Follicular Phase and the Luteal Phase.
The length of the menstrual cycle is measured from the first day of bleeding or the first day of your period through the last day before your next period starts. Day 1 of the menstrual cycle is the first day you experience any amount of bleeding. The last day before your next period would be Day 28, if you have the average 28-day menstrual cycle. Normal menstrual cycle length, in adolescent girls, is 21 to 45 days. Normal menstrual cycle length in adult women is from 21 to 35 days.
See also: Your Menstrual Cycle Calendar
During the first few years of menstruation, periods are often irregular. Many girls skip several months between their first period, or menarche, and the next time they menstruate. Or a girl may have two periods that are 28 days apart and then skip a period, or have spotting every few weeks.
All About Menstruation Teen Health, Nemours Foundation; http://www.kidshealth.org/teen/sexual_health/girls/menstruation.html; accessed 08/29/07.