How do you know if your period is regular or irregular? I can't figure mine out!
- Laura*

The fact that the word "cycle" is used to describe a menstrual period makes it seem as though it happens like clockwork every 4 weeks. But the truth is that there is a lot of normal variation in cycle length.

A girl's monthly cycle — the number of days from the start of her period to the start of the next one — is, on average, 28 days. But cycle lengths vary — some girls have a 24-day cycle, some have a 34-day cycle. And it's not unusual, especially in the first few years after beginning menstruation, to skip periods or to have an irregular menstrual cycle. Other things that affect cycles include diet, stress, exercise, and, occasionally, illness.

If your period isn't coming regularly, is especially uncomfortable, or has stopped (if you are sexually active this may be a sign of pregnancy), you should see a health professional. Most of the time, though, some variation is normal.

Reviewed by: Larissa Hirsch, MD
Date reviewed: August 2010

* Names have been changed to protect user privacy.