What Is an Irregular Period?

The 28-Day Myth and When Not to Worry

a woman's hands adjusting wooden clock blocks
Westend61 / Getty Images

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. It can also 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. It's normal to have anywhere between 21 and 35 days between periods.

The day menstruation begins is considered day one. If it's very light spotting and not a true flow, then this may not be your first day. Talk to your doctor if you're unsure. When your next period begins, that is the next day one. Your menstrual cycle length is calculated by counting the days that occur between day one of one cycle and the next. A longer or shorter span of menstrual bleeding is not included in the calculation because it is based on when your period begins (day one) rather than when it ends.

Your period is irregular if:

  • It comes more frequently than 21 days.
  • You go 35 days or more between periods.
  • The length of your cycles varies greatly (even if they are typically within the typical range of 21 to 35 days).

For example, if one cycle is 25 days, but another is 33 days, that would be considered irregular. This would be irregular even though a 25- or 33-day cycle is normal otherwise.

It is normal if your cycles vary by just a few days from month to month. For example, if one month it's 33 days, and another it's 35 days, that's okay.

The 28-Day Myth

You may have heard that a 28-day cycle is normal. If your cycle is shorter or longer than 28 days, you might worry your periods are irregular. This is not accurate.

A 28-day cycle may be the average cycle length. But you shouldn't think it's the ideal. Your period can be longer or shorter than this, and you can still be very fertile. As well, you can have a textbook 28-day cycle and have fertility problems.

While irregular cycles can signal a fertility problem, regular cycles don't guarantee your fertility is perfect. There are many causes of female and male fertility, and only some will impact menstruation.

Occasional Irregular Periods

If your periods are frequently irregular, this may signal a problem. But an occasional irregular period can be normal.

Healthy individuals may have a missed or irregular period if they:

  • Experience illness, including the flu or bad cold
  • Are under a lot of stress
  • Are traveling, especially if their sleep patterns have been thrown off
  • Are breastfeeding, which in the early days may cause lactation amenorrhea (a total lack of periods)
  • Are pregnant

Excessive exercise can lead to irregular or even absent periods. This is common in athletes. 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. Slow and steady weight change is the healthiest way to get there.

Irregularity Beyond 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:

If you're concerned about any aspect of your period being irregular, speak to your doctor.

A Word From Verywell

Your period is one of the most easily observable aspects of your fertility. If you're trying to conceive, it's a good idea to keep a fertility calendar. This way you can see if your periods are regular or if there is a reason to be concerned. Irregular cycles indicate there may be a problem with ovulation. The good news is that many women can be easily treated for ovulatory infertility.

If your periods are regular but you still haven't conceived, doctors recommend you seek help for fertility if you don't get pregnant after one year of trying (or after six months, if you're age 35 or older). You can have clockwork periods and still have a fertility problem.

Whether your periods are normal or not, if you're worried about your reproductive health, talk 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.

Was this page helpful?
View Article Sources
  • Normal Menstruation. ClevelandClinic.com. https://my.clevelandclinic.org/health/articles/10132-normal-menstruation.
  • Period Problems. WomensHealth.gov. https://www.womenshealth.gov/menstrual-cycle/period-problems/#2.
  • Vaginal or Uterine Bleeding. MedlinePlus. https://medlineplus.gov/ency/article/007496.htm.