Irregular periods: causes and when to see a doctor
Irregular periods are periods that come less than 21 or more than 35 days apart, or whose timing changes a lot from one cycle to the next. They are common, especially in the first years after periods start and in the years before menopause, and the cause decides whether anything needs to be done.
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Medically reviewed by Dr Mary Davidson, Reproductive endocrinologist, checked on
Irregular menstruation is common. NICHD estimates that 14 to 25 in 100 women of childbearing age have irregular menstrual cycles (NICHD, 2026), and in a study of women with regular cycles, 42.5 in 100 had cycles that varied by more than 7 days (Fehring et al., 2006). An irregular period now and then is normal; a pattern that changes, or comes with other symptoms, is worth checking.
What counts as an irregular period?
What is an irregular period, and what is considered an irregular period by doctors? It depends on who is defining it. US and UK sources share a core range of 21 to 35 days but differ at the edges:
| Source | Counted as irregular or abnormal |
|---|---|
| NHS (UK) | A gap between periods of less than 21 days or more than 35 days |
| ACOG (US) | Cycles shorter than 21 or longer than 35 days; cycle length that varies by more than 7 to 9 days; no period for 3 to 6 months |
| US Office on Women's Health | Outside 24 to 38 days, the range it calls regular |
| NICHD (US) | 21 to 35 days is normal for most women; 21 to 45 days in teenagers |
| 2023 international PCOS guideline | From 3 years after the first period: under 21 or over 35 days, or fewer than 8 cycles a year; 1 to 3 years after the first period: under 21 or over 45 days |
The NHS defines periods as irregular when the gap between them is less than 21 days or more than 35 days (NHS, 2026). ACOG uses the same lengths and adds cycles that vary by more than 7 to 9 days and going 3 to 6 months without a period (ACOG, 2021). The US Office on Women's Health calls periods regular if they usually come every 24 to 38 days (Office on Women's Health, 2025), so a 22-day cycle is normal by the NHS and ACOG ranges and outside the Office on Women's Health range, while a 37-day cycle is the other way round. NICHD gives 21 to 35 days for most women and 21 to 45 days in teenagers (NICHD, 2026). The 2023 international guideline on polycystic ovary syndrome treats cycles as normal in the first year after the first period, and also counts any single cycle longer than 90 days after that first year (Teede et al., 2023).
Are irregular periods normal?
Is it normal to have irregular periods? At some ages, yes. The NHS says irregular periods are more likely when periods first start during puberty and when you are nearing menopause (NHS, 2026). ACOG gives the ages: periods may not come regularly when they first start, around age 9 to 14, and the number of days between periods may change in perimenopause, around age 50 (ACOG, 2021). The US Office on Women's Health says girls usually get more regular cycles within three years of starting their periods (Office on Women's Health, 2025).
The reason cycles vary is mostly the follicular phase, the days before ovulation: in a study of 1,060 cycles it contributed most of the variation in cycle length (Fehring et al., 2006). The menstrual cycle phases guide explains how the phases fit together.
What causes irregular periods?
The NHS lists these common causes of irregular periods (NHS, 2026):
- puberty, when periods start
- the start of menopause, usually between the ages of 45 and 55
- pregnancy, since a missed period is often an early sign
- hormonal contraception such as the progestogen-only pill, the contraceptive injection and the intrauterine system (hormonal IUD)
- losing or gaining a lot of weight
- stress and anxiety
- exercising too much
It adds that conditions such as polyendocrine metabolic ovarian syndrome (PMOS), which used to be called polycystic ovary syndrome (PCOS), and thyroid problems can cause irregular periods (NHS, 2026). ACOG's list of causes of abnormal bleeding adds problems with ovulation, fibroids and polyps, adenomyosis, bleeding disorders, some medicines, and, less often, cancer of the lining of the womb, which mostly occurs after menopause (ACOG, 2021). ACOG explains that a lack of ovulation can cause irregular, sometimes heavy, bleeding (ACOG, 2021). Sometimes there is more than one cause.
Can stress cause irregular periods?
Yes. The NHS lists stress and anxiety among the common causes, alongside large changes in weight and too much exercise (NHS, 2026). These causes can overlap.
Irregular periods in perimenopause
Irregular periods in perimenopause are expected. The NHS gives the usual age for the start of menopause as 45 to 55 (NHS, 2026). The US Office on Women's Health says that in your 40s periods might stop for a month or a few months and then start again, and might be shorter, longer, lighter or heavier than usual (Office on Women's Health, 2025). ACOG notes that skipping periods and changes in flow are common at this time, and still advises talking to an ob-gyn about any abnormal bleeding (ACOG, 2021).
Irregular periods and PCOS
PCOS, which the NHS now calls PMOS, is one of the conditions behind irregular periods (NHS, 2026), and ACOG names it among the causes of irregular bleeding from a lack of ovulation (ACOG, 2021). The 2023 international guideline says that when cycles are irregular, PCOS should be considered and assessed (Teede et al., 2023). The guide to PCOS and pregnancy covers what it means for getting pregnant.
When to see a doctor about irregular periods
The NHS advises seeing a GP if (NHS, 2026):
- your periods are irregular
- your periods last longer than 7 days
- you have irregular periods and other symptoms such as weight gain, tiredness, hair growth on your face, or dry or oily skin
- you have bleeding between periods
- your periods are irregular and you are struggling to get pregnant
ACOG also counts cycle length that varies by more than 7 to 9 days, no period for 3 to 6 months, and bleeding after sex as abnormal (ACOG, 2021). The US Office on Women's Health advises talking to a doctor or nurse if you have not had a period for 90 days and are not pregnant or breastfeeding (Office on Women's Health, 2025).
Some bleeding needs emergency care. ACOG says that if you are changing pads or tampons every hour for more than 2 hours in a row and also have chest pain, shortness of breath, or feel lightheaded or dizzy, you should seek emergency medical care right away (ACOG, 2021).
Irregular periods treatment
Treatment is not always needed for irregular periods, and when it is, it depends on the cause (NHS, 2026). A GP may refer you to a specialist for tests to find the cause. If PMOS is the cause, the combined pill may help make periods more regular, and if a thyroid problem is the cause, medicines can treat it (NHS, 2026). ACOG says a doctor will ask about your health history, medicines and bleeding pattern, and suggests keeping track of the dates, length and type of bleeding before the visit (ACOG, 2021). No home remedy is shown in the sources cited here to regulate periods.
Irregular periods and pregnancy
Can you get pregnant with irregular periods? Yes, in any cycle in which you ovulate, but it can be harder. The NHS explains that getting pregnant can be more difficult with irregular periods because you might not release an egg regularly, and suggests having sex every 2 or 3 days throughout your cycle without contraception (NHS, 2026). If you are struggling to get pregnant, you may be referred for tests, and hormone medicine or fertility treatment may be recommended (NHS, 2026).
ACOG's patient guidance says that with highly irregular periods, trying to predict ovulation at home is not worth the time or money, that such periods may be a sign you are not ovulating, and that an ob-gyn or a reproductive endocrinologist is the person to see (ACOG, 2023). Any ovulation calculator for irregular periods has the same limit: its estimate assumes a predictable cycle. The ovulation calculator on this site flags cycle lengths outside 21 to 35 days for this reason. The guide to ovulation and irregular periods covers how to get pregnant with irregular periods in detail, and when to see a fertility doctor gives the timetable. If you need to know whether you are pregnant, the NHS says you can do a test 21 days after you last had unprotected sex when you do not know when your period is due (NHS, "Doing a pregnancy test", 2022); the pregnancy test calculator works out the date.
Tracking your periods
The NHS suggests using an app or a diary to see whether your periods are irregular, counting day 1 as the first day of a period (NHS, 2026). The US Office on Women's Health suggests marking the first day of each period on a calendar, which after a few months shows whether cycles are regular, and recording how long and how heavy each period is (Office on Women's Health, 2025). A record like this is also what a doctor will ask for (ACOG, 2021). The period calculator predicts the next period from your usual cycle length.
This guide is information, not a diagnosis or a recommendation of treatment, and it is no substitute for your own clinician. See the medical disclaimer.
Sources
- NHS. Irregular periods. nhs.uk, 2026.
- American College of Obstetricians and Gynecologists. Abnormal Uterine Bleeding (FAQ095). ACOG, 2021.
- Office on Women's Health, US Department of Health and Human Services. Your menstrual cycle. womenshealth.gov, 2025.
- Eunice Kennedy Shriver National Institute of Child Health and Human Development. Menstruation and Menstrual Problems. NICHD, 2026.
- Teede HJ, Tay CT, Laven JJE, Dokras A, Moran LJ, Piltonen TT, et al. Recommendations From the 2023 International Evidence-based Guideline for the Assessment and Management of Polycystic Ovary Syndrome. Journal of Clinical Endocrinology and Metabolism 108(10): 2447-2469, 2023.
- Fehring RJ, Schneider M, Raviele K. Variability in the phases of the menstrual cycle. Journal of Obstetric, Gynecologic and Neonatal Nursing 35(3): 376-384, 2006.
- American College of Obstetricians and Gynecologists. Trying to Get Pregnant? Here's When to Have Sex. ACOG, 2023.
- NHS. Doing a pregnancy test. nhs.uk, 2022.
Frequently asked questions
Why is my period irregular?
The NHS lists puberty, the approach of menopause, pregnancy, some hormonal contraception, losing or gaining a lot of weight, stress and anxiety, and too much exercise as common causes, and conditions such as PMOS (formerly PCOS) and thyroid problems as others (NHS, 2026). A doctor can look for the cause in your case.
How do you fix irregular periods?
It depends on the cause, and treatment is not always needed (NHS, 2026). When PMOS is the cause, the combined pill may make periods more regular, and thyroid problems are treated with medicine. No home remedy is shown in the sources cited here to regulate periods.
Is a cycle that changes by a few days irregular?
Not by ACOG's definition, which counts cycles as irregular when their length varies by more than 7 to 9 days (ACOG, 2021). In a study of women with regular cycles, 42.5 in 100 had cycles that varied by more than 7 days (Fehring et al., 2006).
What are the signs of pregnancy when you have irregular periods?
No symptom can tell you; a pregnancy test can, and with irregular periods there is no expected period date to wait for. The NHS says that if you do not know when your period is due, you can do a test 21 days after you last had unprotected sex (NHS, "Doing a pregnancy test", 2022).
Can you get pregnant with irregular periods?
Yes, in any cycle in which you ovulate. The NHS says it can be harder because you may not release an egg regularly, and suggests sex every 2 or 3 days throughout the cycle (NHS, 2026).