---
title: "Irregular Periods: What Counts, Causes and What to Do"
description: "Irregular periods: the NHS counts a gap under 21 or over 35 days. Causes, what to track and what to tell your doctor, from NHS, ACOG and WHO."
url: https://periodcalc.app/irregular-periods/
lang: en
---
# What are irregular periods?

Irregular periods are not usually a sign of a problem ([NHS](https://www.nhs.uk/symptoms/irregular-periods/)). Here is what counts as irregular, the usual reasons, what to write down and what to tell your doctor.

The NHS counts your periods as irregular when the gap from one to the next is under 21 days or over 35 days ([NHS](https://www.nhs.uk/symptoms/irregular-periods/)). ACOG’s list of abnormal bleeding also includes a cycle length that varies by more than 7 to 9 days ([ACOG](https://www.acog.org/womens-health/faqs/abnormal-uterine-bleeding)).

By [Hossam Hamouda](https://periodcalc.app/about/), developer of Period Calc · Last updated: 25 October 2026

Predictions are estimates, not medical advice or a diagnosis, and not a method of contraception.

## What counts as irregular

Count each cycle from the first day of one period to the first day of the next ([ACOG](https://www.acog.org/womens-health/faqs/amenorrhea-absence-of-periods)). For an example, see [how long a period and a cycle last](https://periodcalc.app/how-long-does-a-period-last/).

ACOG’s list of abnormal uterine bleeding has the same limits as the NHS (under 21 or over 35 days). That term means bleeding from the womb that differs from what is normal when you are not pregnant. The list also includes not having a period for 3 to 6 months ([ACOG](https://www.acog.org/womens-health/faqs/abnormal-uterine-bleeding)).

## What causes irregular periods?

The NHS lists these causes, and ACOG lists more.

- **Puberty and the years before menopause.** Irregular periods are more likely then ([NHS](https://www.nhs.uk/symptoms/irregular-periods/)). For teens, ACOG says most cycles last between 21 and 45 days ([ACOG](https://www.acog.org/womens-health/faqs/heavy-and-abnormal-periods)).
- **Pregnancy and hormonal contraception.** A missed period is often an early sign of pregnancy. Contraception that can change periods includes the progestogen-only pill, the contraceptive injection and the intrauterine system (IUS) ([NHS](https://www.nhs.uk/symptoms/irregular-periods/)). If you may be pregnant, read about a [late period](https://periodcalc.app/late-period/) and use the [pregnancy test date calculator](https://periodcalc.app/pregnancy-test-date/).
- **Weight, stress and exercise:** a big weight loss or gain, stress and anxiety, and exercising too much ([NHS](https://www.nhs.uk/symptoms/irregular-periods/)).
- **Polycystic ovary syndrome (PCOS).** The WHO calls it a leading cause of irregular periods ([WHO](https://www.who.int/news-room/fact-sheets/detail/polycystic-ovary-syndrome)). The NHS now calls it polyendocrine metabolic ovarian syndrome (PMOS), a hormone condition that can change periods, fertility, hair growth and mood ([NHS](https://www.nhs.uk/conditions/polyendocrine-metabolic-ovarian-syndrome-pmos/)). ACOG still says PCOS ([ACOG](https://www.acog.org/womens-health/faqs/polycystic-ovary-syndrome-pcos)).
- **Thyroid problems** ([NHS](https://www.nhs.uk/symptoms/irregular-periods/)), **a lack of ovulation** and **fibroids and polyps**. ACOG says that not ovulating can cause irregular, sometimes heavy, bleeding. It says fibroids and polyps, which are noncancerous growths, can cause irregular or heavy bleeding ([ACOG](https://www.acog.org/womens-health/faqs/abnormal-uterine-bleeding)).
- **Other causes of abnormal uterine bleeding.** ACOG lists bleeding disorders and problems related to pregnancy, such as ectopic pregnancy or miscarriage. It also lists three conditions of the womb and pelvis. Endometriosis is womb-lining tissue found outside the womb. Adenomyosis is womb-lining tissue growing into the womb wall. Pelvic inflammatory disease (PID) is an infection in the upper part of the female genital tract. ACOG says abnormal uterine bleeding can also be an early sign of cancer of the womb lining ([ACOG](https://www.acog.org/womens-health/faqs/abnormal-uterine-bleeding)).

## What to track

An app or a diary can help you see whether your periods are irregular ([NHS](https://www.nhs.uk/symptoms/irregular-periods/)). ACOG says a doctor will want detailed information about several cycles ([ACOG](https://www.acog.org/womens-health/faqs/heavy-menstrual-bleeding)). It includes the dates your period started, how many days the bleeding lasted and how heavy the flow was (light, medium, heavy or spotting) ([ACOG](https://www.acog.org/womens-health/faqs/heavy-menstrual-bleeding)).

If your last four cycles were 26, 31, 29 and 36 days, the longest is 10 days longer than the shortest. That is above ACOG’s 7 to 9 days, but ACOG does not say which cycles to compare, so this is only a rough check. The 36-day cycle is longer than 35 days ([NHS](https://www.nhs.uk/symptoms/irregular-periods/)). This sum is our own arithmetic, not a diagnosis.

In Period Calc you can log your periods, flow and symptoms in one tap. The app shows a note when your cycles vary a lot, because its predictions may then be less exact. This is its own rule, much looser than the NHS and ACOG limits. Not seeing the note does not mean your periods are regular.

## What to tell your doctor

ACOG says a doctor should ask about your health history and your family’s, your pregnancy history, your medicines and your birth control method. It also helps to know when the bleeding happens and how much ([ACOG](https://www.acog.org/womens-health/faqs/abnormal-uterine-bleeding)).

Tests may include a physical and pelvic exam, a blood count, a pregnancy test and tests for sexually transmitted infections ([ACOG](https://www.acog.org/womens-health/faqs/abnormal-uterine-bleeding)). Treatment is not always needed, and when it is, it depends on the cause ([NHS](https://www.nhs.uk/symptoms/irregular-periods/)).

Period Calc Premium, an optional subscription, can make a PDF report of your cycles, periods and symptoms to share with your doctor. It has your average cycle, your shortest and longest cycle, your average period and a table of your cycles (last 6, last 12 or all).

## When to see a doctor

The NHS says to speak to a doctor if your periods are irregular or your normal pattern changes ([NHS](https://www.nhs.uk/symptoms/irregular-periods/)). ACOG says to talk to your ob-gyn, a doctor with special training in women’s health, about any abnormal uterine bleeding ([ACOG](https://www.acog.org/womens-health/faqs/abnormal-uterine-bleeding)). Make an appointment sooner if:

- your periods last longer than 7 days, or you bleed between periods ([NHS](https://www.nhs.uk/symptoms/irregular-periods/));
- irregular periods come with weight gain, tiredness, hair growth on your face or dry or oily skin, or you find it hard to get pregnant ([NHS](https://www.nhs.uk/symptoms/irregular-periods/));
- you have missed 3 periods in a row ([NHS](https://www.nhs.uk/symptoms/missed-or-late-periods/)), or a period has stopped for about 3 months without explanation ([ACOG](https://www.acog.org/womens-health/faqs/amenorrhea-absence-of-periods)).

ACOG says to seek emergency medical care right away if:

- you are changing pads or tampons every hour for more than 2 hours in a row, and
- you also have chest pain, have shortness of breath, and are lightheaded or dizzy ([ACOG](https://www.acog.org/womens-health/faqs/abnormal-uterine-bleeding)).

The NHS asks for urgent medical advice if:

- you have recently missed a period,
- have unusual vaginal bleeding, and
- have pain anywhere in your tummy, including your lower tummy (pelvic pain) ([NHS](https://www.nhs.uk/symptoms/vaginal-bleeding-between-periods-or-after-sex/)).

This could be a sign of an ectopic pregnancy ([NHS](https://www.nhs.uk/symptoms/vaginal-bleeding-between-periods-or-after-sex/)). Our own plain advice: if you have any of these signs, get medical help. Use the emergency or urgent medical service of your country.

## Questions about irregular periods

### Are irregular periods normal?

Irregular periods are not usually a sign of a problem, and they are more likely in puberty and when you are nearing menopause ([NHS](https://www.nhs.uk/symptoms/irregular-periods/)). The NHS still says to speak to a doctor if your periods are irregular or your normal pattern changes. The WHO says concerns about menstruation deserve to be heard and evaluated by health and care workers ([WHO](https://www.who.int/news-room/fact-sheets/detail/menstrual-health)).

### How much can my cycle change before it counts as irregular?

ACOG’s list of abnormal bleeding includes a cycle length that varies by more than 7 to 9 days ([ACOG](https://www.acog.org/womens-health/faqs/abnormal-uterine-bleeding)). The NHS counts a gap of less than 21 days or more than 35 days ([NHS](https://www.nhs.uk/symptoms/irregular-periods/)). The NHS also counts a wide variation in the time between periods, the amount of blood you lose or the number of days a period lasts ([NHS](https://www.nhs.uk/conditions/periods/period-problems/)). The descriptions overlap but are not the same.

### Can stress make my periods irregular?

Yes, stress can make periods irregular. The NHS lists stress and anxiety among the common causes, with a big weight loss or gain, exercising too much and hormonal contraception ([NHS](https://www.nhs.uk/symptoms/irregular-periods/)). The sources do not say by how many days stress can change a cycle.

### Can I get pregnant if my periods are irregular?

Yes, you can still get pregnant with irregular periods, but it can be harder, because you might not release an egg regularly ([NHS](https://www.nhs.uk/symptoms/irregular-periods/)). The NHS says that if you are trying to get pregnant, having sex every 2 or 3 days throughout your cycle can help. It also says to speak to a doctor if you find it hard to get pregnant. This page is not about avoiding pregnancy. See also the [signs of ovulation](https://periodcalc.app/signs-of-ovulation/).

### Can a calculator or an app predict irregular periods?

A calculator can only predict irregular periods roughly, because it works from averages. Enter your average cycle length in the [period calculator](https://periodcalc.app/period-calculator/), and see [what to track](https://periodcalc.app/irregular-periods/#what-to-track) for the app’s note on cycles that vary.

## How this page was made

Hossam Hamouda, the developer of Period Calc, wrote this page from the guidance of the organisations listed under [Sources](https://periodcalc.app/irregular-periods/#sources). They are the NHS (the UK’s National Health Service), ACOG (the American College of Obstetricians and Gynecologists) and the WHO (the World Health Organization). These organisations have not reviewed the page, and neither has a doctor. The NHS and ACOG describe irregular periods differently, so the page names the source of each number. It cannot diagnose anything. If something worries you, see [when to see a doctor](https://periodcalc.app/irregular-periods/#see-a-doctor).

## Sources

1. [Irregular periods](https://www.nhs.uk/symptoms/irregular-periods/) (NHS)
2. [Missed or late periods](https://www.nhs.uk/symptoms/missed-or-late-periods/) (NHS)
3. [Period problems](https://www.nhs.uk/conditions/periods/period-problems/) (NHS)
4. [Vaginal bleeding between periods or after sex](https://www.nhs.uk/symptoms/vaginal-bleeding-between-periods-or-after-sex/) (NHS)
5. [Polyendocrine metabolic ovarian syndrome (PMOS)](https://www.nhs.uk/conditions/polyendocrine-metabolic-ovarian-syndrome-pmos/) (NHS)
6. [Abnormal Uterine Bleeding](https://www.acog.org/womens-health/faqs/abnormal-uterine-bleeding) (ACOG)
7. [Heavy and Abnormal Periods](https://www.acog.org/womens-health/faqs/heavy-and-abnormal-periods) (ACOG)
8. [Amenorrhea: Absence of Periods](https://www.acog.org/womens-health/faqs/amenorrhea-absence-of-periods) (ACOG)
9. [Heavy Menstrual Bleeding](https://www.acog.org/womens-health/faqs/heavy-menstrual-bleeding) (ACOG)
10. [Polycystic Ovary Syndrome (PCOS)](https://www.acog.org/womens-health/faqs/polycystic-ovary-syndrome-pcos) (ACOG)
11. [Menstrual health](https://www.who.int/news-room/fact-sheets/detail/menstrual-health) (WHO)
12. [Polycystic ovary syndrome](https://www.who.int/news-room/fact-sheets/detail/polycystic-ovary-syndrome) (WHO)

## Get reminders before your next period

**Period Calc** for iPhone learns from your own cycles and reminds you two days before your period — discreetly, with no details unless you turn them on.

- The Hijri date on every calendar day
- Your fasting days, every month
- A PIN and Face ID lock, and no account

[Download on the App Store](https://apps.apple.com/app/id6814298894)

## Related guides

- [Late period](https://periodcalc.app/late-period/): Pregnancy and stress are common reasons for a late period, and most home tests work from the first day you miss it.
- [How long a period lasts](https://periodcalc.app/how-long-does-a-period-last/): A period usually lasts 2 to 7 days and a cycle 21 to 35 days.
- [Signs of ovulation](https://periodcalc.app/signs-of-ovulation/): Mucus, temperature, tests and pain can show ovulation, but a temperature rise only confirms it afterwards.

## Free calculators

- [Period calculator](https://periodcalc.app/period-calculator/): Your next six periods, fertile days and ovulation, in Gregorian and Hijri dates.
- [Ovulation calculator](https://periodcalc.app/ovulation-calculator/): Your fertile window and ovulation day for the next three cycles.
- [Due date calculator](https://periodcalc.app/due-date-calculator/): Your due date, how many weeks pregnant you are and your trimester.
- [Pregnancy week calculator](https://periodcalc.app/pregnancy-week-calculator/): How many weeks and days pregnant you are, your trimester and due date, from your last period or your due date.
- [Conception date calculator](https://periodcalc.app/conception-date-calculator/): Count back from your due date to the days you most likely conceived: an estimate, not an exact day.
- [Pregnancy test date calculator](https://periodcalc.app/pregnancy-test-date/): Shows the day your next period is due. The NHS says most tests can be done from the first day of a missed period.

Predictions are estimates, not medical advice or a diagnosis, and not a method of contraception.

The prayer & purity guide gives the general rules of the four schools. It is not a fatwa — for your own case, ask someone whose knowledge you trust.

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