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What causes irregular periods?
Irregular periods explained: what counts as irregular, common causes from stress to PCOS and thyroid problems, and when to see a doctor.
Updated
In short
What causes irregular periods?
Irregular periods come less than 21 days or more than 35 days apart, or change a lot in timing from month to month. They're common in the first years after periods start and in the years before menopause. Other causes include pregnancy, stress, big weight changes, hard exercise, hormonal birth control, PCOS and thyroid problems. See a doctor if your usual pattern changes.
Written by the Gyne team · Medical review: pending
Key takeaways
- For most adults, a cycle anywhere from 21 to 35 days long is normal.
- Cycles are often irregular for a few years after periods start and again before menopause.
- Stress, weight changes, hard exercise and hormonal birth control can all change your cycle.
- PCOS and thyroid problems are common medical causes that a doctor can check for.
- Tracking your periods gives your doctor the clearest picture of what is going on.
What counts as an irregular period?
Your cycle is counted from the first day of one period to the first day of the next. The average is about 28 days, but anywhere from 21 to 35 days is normal for adults.
Your periods are irregular if the gap is shorter than 21 days, longer than 35 days, or changes a lot from one month to the next. A typical period lasts 2 to 7 days.
A cycle that shifts by a few days now and then is normal. One off month is common and usually nothing to worry about.
What are the most common causes of irregular periods?
Most causes affect ovulation, the release of an egg. When ovulation comes late or doesn't happen, your period comes late or is skipped. Common causes include:
- Puberty, in the first few years after periods start.
- Perimenopause, the years before periods stop, usually in your 40s.
- Pregnancy or breastfeeding.
- Stress and anxiety.
- Losing or gaining a lot of weight, or eating too little.
- Exercising very hard, especially without eating enough.
- Hormonal birth control, like the progestin-only pill, the shot or a hormonal IUD, or stopping it.
- Polycystic ovary syndrome (PCOS).
- Thyroid problems, when the gland in your neck makes too much or too little hormone.
- Eating disorders and some bleeding disorders.
Are irregular periods normal in your teens and 40s?
Yes, often. In the first few years after periods start, cycles of 21 to 45 days are common, because the body is still learning to ovulate regularly.
In your 40s, perimenopause can make cycles shorter, longer or unpredictable. Periods may get lighter or heavier before they stop. Any bleeding after your periods have stopped for 12 months should be checked.
Can stress or exercise make your periods irregular?
Yes. The part of your brain that controls your cycle reacts to stress, illness and low energy. When it slows down, ovulation can be delayed or skipped.
This is more likely when hard training is combined with eating too little. Periods often settle again once stress eases or energy intake goes back up.
Could irregular periods be a sign of PCOS or a thyroid problem?
They can be. PCOS often causes long or missed cycles, along with acne, extra hair on the face or body, or weight gain.
Thyroid problems can make periods lighter, heavier or less regular. Tiredness, feeling cold or hot, hair loss and a racing heart can be other signs.
These patterns are clues, not answers. Only a doctor can diagnose this.
How do doctors find the cause of irregular periods?
Your doctor will ask about your periods, your health, medicines and family history. A record of your bleeding days and flow helps a lot.
Common next steps include:
- A pregnancy test.
- A pelvic exam and tests for sexually transmitted infections (STIs).
- Blood tests for hormones, including thyroid hormones.
- A pelvic ultrasound, a scan that looks at the uterus and ovaries.
Can you get pregnant with irregular periods?
Yes. You may ovulate less often or at less predictable times, which can make it harder to time sex, but pregnancy is still possible.
Irregular cycles are not a form of birth control. If you're trying to get pregnant and your periods are irregular, ask your doctor about checking how often you ovulate.
When to see a doctor
- Your periods were regular and then became irregular.
- Your cycles are often shorter than 21 days or longer than 35 days.
- You've missed three periods in a row and you're not pregnant.
- Your period lasts longer than 7 days, or you soak a pad or tampon every 1 to 2 hours.
- You bleed between periods, after sex or after menopause.
- You also have new acne, extra hair growth, hair loss, or feel very tired all the time.
- You have heavy bleeding with dizziness or fainting: call 911 or your local emergency number.
How Gyne helps
Gyne shows your next period as a range of days, so an irregular cycle doesn't set off false alarms. As you log, the range gets narrower, and when stress, illness or travel may have shifted your cycle, Gyne asks a short question and explains the change in one line.
PeriodsThis guide is general information, not medical advice. Only a doctor can diagnose a condition.