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Irregular Periods: What Counts as Irregular and What Causes It

What counts as an irregular period?

A cycle is measured from the first day of one period to the first day of the next. The Office on Women's Health considers it irregular when:

How to Understand Your Cycle Patterns
  • It is shorter than 24 days
  • It is longer than 38 days
  • Its length varies by more than 20 days from month to month — for example, a 25-day cycle followed by a 46-day cycle

A difference of a few days between cycles is normal.

What causes irregular periods?

Polycystic ovary syndrome (PCOS)

PCOS is a hormonal condition and one of the most common causes of irregular or missed periods. In a CDC survey from 2022–2023, 8.7% of U.S. women aged 20 to 49 had ever been diagnosed with PCOS.

Thyroid problems

Both an overactive and an underactive thyroid can cause irregular periods (NICHD).

Stress, weight and exercise

Chronic stress, eating disorders, low body weight, obesity and intense exercise can all disrupt ovulation.

Perimenopause

In your 40s, cycles often become irregular as you approach menopause. See perimenopause.

Other causes

High prolactin, certain medicines, primary ovarian insufficiency, pelvic inflammatory disease and uncontrolled diabetes (womenshealth.gov). Hormonal birth control and breastfeeding can also change your bleeding pattern.

Can you get pregnant with irregular periods?

Yes. Many people with irregular cycles ovulate, just on unpredictable days. Some cycles may have no ovulation, which can make getting pregnant take longer. A doctor can check whether you ovulate and treat the cause. See when do you ovulate?

When to see a doctor about irregular periods

  • Your periods become irregular after being regular
  • Your cycles are often shorter than 24 days or longer than 38 days
  • You have had no period for 90 days and you are not pregnant or breastfeeding
  • You have heavy bleeding, severe pain, unwanted hair growth or acne along with irregular cycles

A doctor will usually ask about your cycle history, so dates of your last several periods are the most useful thing to bring.

How tracking helps with irregular periods

Irregular cycles are hard to describe from memory. In Oona — Period Tracker & Calendar, log the first day of each period; the app shows the shortest, longest and average cycle length and how much they vary. Predictions are less precise when cycles vary a lot, and the app shows your history so you and your doctor can see the full pattern.

Oona is a tracking app. It is not a contraceptive method and not a diagnostic tool.

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