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What Is an Irregular Period?

The 28-Day Myth, Irregularity Beyond Menstruation, When Not to Worry


Updated August 12, 2014

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

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Irregular periods can be a signal that something isn't quite right with your reproductive system. In fact, for some women, irregular periods are an early infertility symptom.

That said, there is also a wide range of normal. Understanding what's considered irregular and what's not can help you understand your body and help you better explain to your doctor your current state of reproductive health.

What Is an Irregular Period?

When someone says they have an irregular period, they are typically referring to the number of days between cycles or the variation between periods.

Your period is considered irregular if the number of days between periods is either shorter or longer than the normal range. Anything shorter than 21 days or longer than 35 days is considered to be irregular.

It can also be considered irregular if your cycles vary by several days. For example, if your cycle is typically 33 days and it varies by a couple days on either side, that's normal. But if your cycles are unpredictable, with some cycles coming 25 days apart and others 33 days apart, that would be an abnormal variation, even though the number of days between periods is still in the "normal" range.

The 28-Day Myth

You may have heard that a 28-day cycle is normal and anything longer or shorter than 28 days is irregular. This is a common myth.

While a 28-day cycle may be the average, you shouldn't think that a 28-day cycle is the ideal. Your cycle can be longer or shorter than this and you can still have great fertility. By the same token, it's possible for someone with a 28-day cycle to have fertility problems.

The 28-day cycle is not the golden ticket to perfect fertility. You shouldn't be concerned if your cycle doesn't match this textbook model.

Occasional Irregular Period Can Be Normal

If your periods are frequently irregular, this may signal a problem. On the other hand, the occasional irregular period can be normal.

Normal reasons for a missed or irregular period include:

  • Illness, including a flu or bad cold
  • High stress
  • Travel, especially travel that messes with your sleep patterns
  • Breastfeeding, which in the early days may cause lactation amenorrhea (a total lack of periods)
  • Pregnancy

Exercise can lead to irregular or even absent periods, and this is common in athletes. If you're not an athlete and you're exercising to the point that your periods become irregular or stop, you should speak to your doctor.

Some athletes don't know that their fertility can be impacted by their exercise regimen. If you're an athlete and you want to get pregnant, you may need to cut back to restart your periods and ovulation.

Also, you may experience irregular periods if you lose or gain a significant amount of weight. This is a "normal" reaction, but this doesn't mean that extreme weight loss or gain is good for your health. For women who are overweight, losing weight may regulate the menstrual cycle; for women who are underweight, gaining some weight can help regulate things.

Irregularity Beyond Menstrual Cycle Length

Even though the phrase "irregular periods" refers to cycle length, you shouldn't think this is the only aspect of your period that can go awry.

You can have normal cycle lengths but experience abnormal spotting, too heavy or too light bleeding, severe cramps, extreme mood swings or other abnormal period symptoms.

If you're concerned about any aspect of your period being irregular, speak to your doctor. It's better to ask and receive reassurance that all is well than ignore a potential problem or fail to share a telling symptom that could help your doctor make a diagnosis.

More on getting pregnant with infertility:

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Menstruation and the Menstrual Cycle. WomensHealth.gov. Accessed June 2, 2011. http://www.womenshealth.gov/faq/menstruation.cfm

Menstrual periods - Heavy, Prolonged, or Irregular. MedlinePlus. Accessed June 2, 2011. http://www.nlm.nih.gov/medlineplus/ency/article/003263.htm. Link expired.

Normal Menstruation. ClevelandClinic.com. Accessed June 2, 2011. http://my.clevelandclinic.org/anatomy/female_reproductive_system/menstruation/hic_normal_menstruation.aspx

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