I’m 18 years old and have been getting my period since I was 14 but it still doesn’t come every month. Some years, it comes as infrequently as once every three months and, sometimes, as frequently as once every three weeks. Why is this happening?

This is a great question! What you’re experiencing is called an irregular period. Irregular periods are pretty common in teens, but it sounds like in this case there might be something else going on. Since you’ve been having irregular periods for four years, it’s time to talk to your healthcare provider. They can help you figure out what’s going on and what (if anything) to do about it. Below, we go over some period basics and talk about some common causes of irregular periods you should be aware of.

Your period is caused by changes in hormone levels during your menstrual cycle. During your menstrual cycle, the lining of your uterus thickens. The uterine lining is there to accept and nourish an egg if it becomes fertilized. If a fertilized egg doesn’t implant in the uterine lining, you shed your uterine lining through your vagina. This is your period. If a fertilized egg does implant, this means a pregnancy has started, and you won’t get your period.

A period is considered irregular if:

  • It comes more frequently than every 21 days or less frequently than every 35 days
  • It lasts longer than 8 days.
  • The time between your periods varies a ton each cycle. For example, if one cycle lasts 22 days and the next lasts 33.

There are lots of reasons your period could be irregular.

It is common for people going through puberty to have irregular periods because of the changing amounts of the hormones estrogen and progesterone in their bodies. Most people have irregular periods when they first start getting them, but they become more regular over two years or so.

Stress, exercising way more than usual, gaining or losing a lot of weight and other factors can cause irregular periods. Hormonal birth control can also disrupt the body’s normal production of hormones and cause irregular periods. Usually, this happens when you start birth control (or a new method of birth control) and stabilizes over time. If you think your irregular period is related to your birth control, talk to your healthcare provider. A different birth control method might not mess with your period in quite the same way.

Sometimes, an irregular period can be a sign of a health issue like Polycystic Ovary Syndrome (PCOS) or an over or underactive thyroid. These can mess with your hormones, and therefore your period.

Like we said though, the best thing to do is talk to your healthcare provider. They can help you figure out what’s going on. If you live near NYC, you can make an appointment at the Mount Sinai Adolescent Health Center for completely free, confidential, comprehensive healthcare.