What Causes an Irregular Period?

On average, a menstrual cycle lasts around 28 days. However, there is a wide range of what is considered to be "normal" when talking about the menstrual cycle. So, it's nothing to worry about if yours regularly lasts a few days longer or if your cycle changes from 28 to 31 days.

That said, if your cycle routinely comes and goes at unexpected times—also known as irregular periods—it can be a signal that something in your body is not quite right.

Knowing how to tell if your periods are irregular will help you understand your body better. Often, the term "irregular" may refer to a change in what's normal for you. Learn more about irregular periods below, including different types of irregularity, what causes them, and when to contact an OB-GYN or health care provider.

How Do I Know If My Periods Are Irregular?

You can find out whether your periods are regular or irregular by charting how long your menstrual cycles are. You can determine your menstrual cycle length by counting from day 1 of your period to day 1 of the next period. Day 1 is traditionally the first day of actual flow. It's normal to have cycles between 21 and 35 days.

According to the American College of Obstetricians and Gynecologists (ACOG), your period is irregular if it tends to come more frequently than every 21 days or less often than every 35 days. Additionally, if your cycle length falls in the normal range, but varies by seven to nine days from cycle to cycle, that's a sign of an irregular period, too.

For example, if one cycle is 25 days and the next is 33 days, your cycles would be considered irregular (even though a 25- or 33-day cycle is otherwise normal). If your cycle is always between 25 and 35 days and fluctuates by just a few days—maybe 33 days one month and 35 days the next—that's not typically cause for concern.

Types of Irregular Periods

Timing and length of your cycles are not the only red flags that your period is irregular. It's important to pay attention to these other signs that something may be off with your health or your hormones.

Too-light bleeding

Bleeding very little or not bleeding at all (known as amenorrhea) with each menstrual cycle is atypical. Light or absent bleeding can result from being very underweight or very overweight.

If you are overweight, losing weight might help make your periods more regular. If you are underweight, gaining weight can help restore and regulate your menstrual cycle. (Slow and steady weight change is the healthiest way to get there.)

Mid-cycle spotting

Hormones are often the cause of spotting between periods. Your hormones could be affected by your birth control medication, fertility drugs, or a thyroid disorder. Fibroids and polyps in your reproductive tract can also lead to regular spotting.

Too-heavy or extended bleeding

Bleeding that soaks through one or more tampon or pad every hour for several hours in a row OR contains clots that are as big as a quarter or larger is abnormal, according to ACOG. So is a period that lasts more than seven days.

Excessive bleeding, or menorrhagia, is a common concern, affecting up to one-third of people who menstruate. Though common, you should see an OB-GYN or health care provider to discuss these issues. Heavy periods can be a sign of a fertility issue or a health problem, including fibroids, polyps, or endometriosis.

Severe cramps

You should never ignore extreme period pain, also known as dysmenorrhea. Conditions that could affect your fertility and overall health could be to blame, including fibroids, endometriosis, or pelvic inflammatory disease (an infection in your reproductive organs).

What Are the Causes of Irregular Periods?

There are many factors that can influence your menstrual cycle and cause you to occasionally experience an irregular period, such as your cycle changing from 28 to 31 days. An extremely stressful event, illness, intensive exercise, or even a big trip—especially if your sleep pattern is disrupted—can alter your cycle or cause spotting. However, if your cycle goes back to normal the next month, occasional irregularity is unlikely to be a cause for concern.

You can also have an abnormal cycle or bleeding if you are breastfeeding, pregnant, or having a miscarriage.

A dramatic change in your activity patterns or weight can affect your menstrual cycle, too. Excessive exercise and rapid weight loss can lead to irregular or even absent periods. This is common in athletes, but should not be ignored, especially if you are hoping to conceive in the near future.