Irregular periods are most often caused by everyday factors such as stress, weight change, intense exercise, hormonal contraception, breastfeeding, or the natural changes of puberty and perimenopause, but they can also point to conditions such as PCOS or thyroid problems. For adults, a cycle is usually considered irregular if it is shorter than 21 days or longer than 35 days, or if it varies a lot from month to month. This guide explains what counts as irregular, the common causes, how a doctor finds the reason and the red flags that mean you should not wait.
Key takeaways
- The average menstrual cycle is about 28 days, and the NHS describes periods as irregular when the gap between them is less than 21 days or more than 35 days.
- Common causes include pregnancy, stress, significant weight change, excessive exercise, hormonal contraception, breastfeeding, puberty and perimenopause.
- Medical causes include PCOS, thyroid and pituitary problems, fibroids, polyps, endometriosis and some medicines.
- The NHS advises seeing a GP if you miss three periods in a row or if your periods have not started by age 15.
- Periods that stop or become irregular before 45 can be a sign of early menopause and should be checked by a doctor.
What counts as an irregular period?
A period is irregular when the gap between the start of one period and the start of the next is less than 21 days or more than 35 days, or when the length keeps changing. The average cycle is around 28 days, and it is normal for it to be a little shorter or longer1.
Cleveland Clinic notes that most periods last four to seven days and that normal cycles range from 21 to 35 days4. The 2023 international PCOS guideline gives more detailed definitions by life stage, which are useful because cycles naturally settle in the years after the first period5.
| Life stage | Cycle pattern considered irregular |
|---|---|
| Less than 1 year after the first period | Irregular cycles are normal while the body adjusts |
| 1 to 3 years after the first period | Cycles shorter than 21 days or longer than 45 days |
| More than 1 year after the first period | Any single cycle longer than 90 days |
| 3 years after the first period until perimenopause | Cycles shorter than 21 days or longer than 35 days, or fewer than 8 cycles a year |
| Perimenopause (often from around 45) | Cycles commonly become irregular as menopause approaches |
Irregular also covers other changes: periods that last longer than seven days, bleeding between periods, or a clear change from your usual pattern14. Tracking start dates, bleeding days and symptoms for three months gives you, and your doctor, a far clearer picture than memory alone.
What are the most common causes of irregular periods?
The most common causes are natural life stages, pregnancy and breastfeeding, hormonal contraception, stress, weight change and exercise. Medical conditions such as PCOS and thyroid disorders are also common, especially when irregular periods come with other symptoms12.
Life stages
Cycles are often irregular for the first few years after periods start, and again in perimenopause, the years before menopause that usually begin between 45 and 5512. Periods can also take time to settle after pregnancy, and breastfeeding commonly delays their return2.
Age matters here. The NHS describes early menopause as periods stopping before the age of 45, and premature menopause as this happening before 406. If your periods become irregular or stop in your late 30s or early 40s, especially with hot flushes or night sweats, a doctor can check whether this is the cause and discuss support, because an earlier menopause has implications for bone and heart health.
Pregnancy
A missed or late period is the classic early sign of pregnancy2. If there is any chance you could be pregnant, take a home pregnancy test before considering other causes, and before taking any herbal product.
Hormonal contraception
The progestogen-only pill, contraceptive injection and hormonal intrauterine system (IUS) can all make periods irregular, lighter or absent1. Cleveland Clinic notes that cycles can take up to six months to settle after stopping birth control pills4.
Stress and anxiety
Stress is a recognised cause of late, missed and irregular periods12. Exams, job changes, bereavement, relationship strain and illness can all shift a cycle for a month or two.
Weight change and exercise
Significant weight loss or gain can disturb the cycle, and so can very intense exercise, particularly when body fat becomes very low14. This is common in athletes, dancers and people who restrict food, and it deserves medical attention because it can affect bone health over time.
Polycystic ovary syndrome (PCOS)
PCOS, now also called PMOS, is one of the most common medical causes of irregular periods17. It occurs when ovulation happens less often, and it is often accompanied by acne, excess facial or body hair, scalp hair thinning or weight changes7. Our explainer on PCOD vs PCOS covers how it is diagnosed.
Thyroid and pituitary problems
Both an overactive and an underactive thyroid can make periods irregular or stop them12. Disorders of the pituitary gland, the small gland at the base of the brain that controls many hormones, can also affect the cycle4; one example doctors commonly check for is a raised level of the hormone prolactin.
Conditions of the womb and pelvis
Fibroids, polyps, endometriosis, adenomyosis and pelvic inflammatory disease (an infection usually caused by an untreated sexually transmitted infection) can cause irregular, heavy or painful bleeding34. Rarely, bleeding changes are a sign of womb or ovarian cancer, which is why persistent changes should always be checked34.
Medicines and other health conditions
Some medicines, including steroids and anticoagulants (blood thinners), can affect bleeding4. Long-term conditions such as diabetes and heart disease can also cause periods to stop2, and bleeding disorders such as von Willebrand disease can cause heavy periods3.
| Possible cause | Typical clues | What a doctor may check |
|---|---|---|
| Pregnancy | Missed period, breast tenderness, nausea | Pregnancy test |
| Stress, weight change or intense exercise | Recent life events, dieting, heavy training | History, weight trend, sometimes bone health |
| PCOS (PMOS) | Long gaps between periods, acne, excess hair growth | Hormone blood tests, blood sugar, ultrasound in adults |
| Thyroid problems | Tiredness, feeling cold or hot, weight change, hair or skin changes | Thyroid function blood tests |
| Raised prolactin or other pituitary problems | Milky nipple discharge, headaches, vision changes | Prolactin blood test, sometimes a scan |
| Fibroids, polyps or endometriosis | Heavy or painful periods, bleeding between periods | Pelvic examination, ultrasound |
| Perimenopause or early menopause | Hot flushes, night sweats, age over 40 | History, sometimes hormone blood tests |
Can stress really make your period late?
Yes. The NHS lists stress and anxiety among the common causes of irregular and missed periods12. Ovulation is controlled by signals from the brain, and these signals are sensitive to physical and emotional stress, illness and poor sleep.
A single late period during a difficult month is common and usually settles once life calms down. What matters is the pattern. If cycles stay irregular for several months, or if you miss three periods in a row, it is worth seeing a doctor rather than assuming stress is the whole story2. In the meantime, steady habits help: regular meals, daily movement, consistent sleep and simple relaxation practices. Our guide to an Ayurvedic daily routine for food, movement, sleep and stress works well for anyone with an unsettled cycle, not only those with PCOD.
Is an irregular period a sign of PCOS?
Sometimes, but not on its own. Irregular or absent periods are one of three features used to diagnose PCOS, and a diagnosis needs at least two of them after other causes have been excluded57.
The other two features are signs of higher androgens (such as excess facial hair, acne or raised testosterone on a blood test) and polycystic-looking ovaries on ultrasound or, in adults, a raised anti-Müllerian hormone (AMH) level5. The WHO notes that ovarian cysts are not required for a diagnosis7. Because thyroid problems, raised prolactin and other conditions can look similar, doctors check for them before diagnosing PCOS.
If irregular cycles come with acne, unwanted hair growth or weight changes, the NHS advises seeing a GP1. Getting a clear answer matters because PCOS is linked with long-term risks such as type 2 diabetes, and regular check-ups help manage those risks7.
Are irregular periods dangerous?
Usually not in the short term, but they are worth understanding because the cause can matter for long-term health. An occasional irregular cycle is common; a persistent pattern deserves a check-up.
Why long gaps matter
When ovulation happens rarely, the lining of the womb may be exposed to oestrogen for long stretches without being shed regularly. The 2023 PCOS guideline notes that premenopausal women with PCOS have a markedly higher risk of endometrial hyperplasia (thickening of the womb lining) and endometrial cancer5. Doctors can reduce this risk, which is one reason very infrequent periods should not simply be ignored.
Why heavy bleeding matters
Irregular cycles sometimes come with heavy bleeding. Losing a lot of blood month after month can lead to iron deficiency anaemia, which causes tiredness and breathlessness, and doctors may check for it with a blood test3.
Why the cause matters
Conditions behind irregular periods, such as PCOS and thyroid disorders, have effects beyond the cycle, including on blood sugar, energy, mood and fertility7. Finding the cause early gives you more options and fewer surprises later.
How will a doctor find the cause of irregular periods?
A doctor starts with your history and an examination, then uses targeted tests. The NHS explains that a GP will ask about your health, may do blood tests and may refer you to a gynaecologist or endocrinologist if more tests or scans are needed2.
- History: your cycle dates, how heavy the bleeding is, pain, contraception, medicines, weight changes, stress, exercise and family history.
- Pregnancy test: usually the first step if pregnancy is possible.
- Blood tests: commonly thyroid function, prolactin, reproductive hormones and a full blood count to look for anaemia; blood sugar may be checked if PCOS is suspected5.
- Examination: including a pelvic examination where appropriate.
- Ultrasound: a pelvic scan can show fibroids, polyps and the appearance of the ovaries; in adolescents, a scan is not recommended for diagnosing PCOS5.
Bring your cycle record to the appointment. Noting the start date, number of bleeding days, how many pads or tampons you use on heavy days, and any pain or spotting saves time and improves the conversation.
What can you do at home while you wait for answers?
Track your cycle, look after the basics of food, movement, sleep and stress, and avoid starting new supplements without telling your doctor. These steps support your overall health and make your medical appointment more productive.
- Track: use a period app or a simple diary and record bleeding, pain, mood and energy.
- Eat regularly: balanced meals with vegetables, pulses, whole grains and protein; our Indian PCOD diet chart is a useful template for anyone.
- Move most days: walking, yoga or strength exercises, without pushing into exhaustion.
- Protect sleep: keep regular bedtimes and wake times.
- Look after iron: if your periods are heavy, include iron-rich foods such as leafy greens, pulses, roasted chana and til (sesame), and ask your doctor about an iron test.
What to write in your cycle diary
- The first day of each period and how many days you bleed.
- Flow on the heaviest day, for example how many pads or tampons you use and whether you pass clots.
- Any spotting between periods or after sex.
- Pain, its timing and whether painkillers help.
- Changes in skin, hair, weight, mood, sleep or energy.
- New medicines, contraception changes, travel, illness or major stress.
In Ayurveda, herbs such as Ashoka (Saraca asoca) and Lodhra (Symplocos racemosa) have long been used in menstrual wellness. Our guide to Ashoka, Lodhra and Lajjalu explains their traditional use and the limits of the research.
Where Purely Yours Flo Veda fits
Flo Veda combines five Ayurvedic herbs: Lajjalu, Ashoka, Rasanjana, Lodhra and Amragandhi Haridra. It is traditionally used to support menstrual comfort, and should be taken as directed on the pack. Flo Veda is not a treatment for PCOS, thyroid problems or any other cause of irregular periods, and it is not a substitute for finding out why your cycle has changed. Please see a doctor first if you have any of the warning signs below. Rasanjana is a Berberis extract, and berberine is not advised in pregnancy or breastfeeding, so do not use Flo Veda if you are pregnant, could be pregnant, are trying to conceive or are breastfeeding. If you take diabetes medicines or any regular prescription, check with your doctor first. You can also explore Ova Calm herbal tea in our female wellness collection or book a free consultation.
When to see a doctor
- You have not had a period for 3 months or more, or you have missed 3 periods in a row, and you are not pregnant.
- Your periods have not started by the age of 15.
- Very heavy bleeding: changing a pad or tampon every 1 to 2 hours, passing clots larger than about 2.5 cm, bleeding through to clothes or bedding, or periods lasting more than 7 days.
- Bleeding between periods, after sex or after menopause.
- Severe pain during or between periods.
- Signs of anaemia, such as tiredness, breathlessness, dizziness or pale skin.
- You might be pregnant, or you are finding it difficult to conceive.
- Irregular periods together with excess facial or body hair, acne, weight change or tiredness.
- Your periods stop or become irregular before the age of 45.
- Seek urgent care for sudden severe pelvic pain, fainting or very heavy bleeding with light-headedness, especially if you could be pregnant.
The bottom line
Irregular periods are common and often linked to stress, weight change, contraception or life stage, but they can also be a sign of PCOS, thyroid problems or other conditions that are worth diagnosing early. Track your cycle, look after the basics, and see a doctor if periods stop for three months, become very heavy or come with the red flags above.
Frequently asked questions
How late can a period be before it is considered irregular?
For adults, cycles are usually considered irregular if they are shorter than 21 days or longer than 35 days, or if the length changes a lot from month to month. A single late period is common and often linked to stress or illness. If you could be pregnant, take a pregnancy test first, and see a doctor if periods stop for three months.
Can stress cause irregular periods?
Yes. Stress and anxiety are among the common causes of irregular or missed periods listed by the NHS. Stress can affect the hormones that control ovulation, so a period may come late or be skipped during a hard month. If irregular cycles continue after the stressful period passes, see a doctor to look for other causes.
Are irregular periods a sign of PCOS?
They can be, but not always. Irregular or absent periods are one of the three features used to diagnose PCOS, along with signs of higher androgens and polycystic-looking ovaries. Many other things also cause irregular cycles, including thyroid problems, weight change, contraception and approaching menopause, so a doctor needs to assess you before any diagnosis.
Is it normal for periods to be irregular in teenagers?
Some irregularity is normal in the first few years after periods start. The 2023 PCOS guideline describes cycles shorter than 21 or longer than 45 days as irregular between one and three years after the first period. See a doctor if periods have not started by 15, if any cycle lasts more than 90 days, or if bleeding is very heavy.
Which tests are done for irregular periods?
A doctor usually starts with questions, an examination and a pregnancy test. Blood tests may check thyroid hormones, prolactin, other reproductive hormones, blood count for anaemia and sometimes blood sugar. A pelvic ultrasound may be suggested in adults to look at the womb and ovaries. The exact tests depend on your age and symptoms.
Sources
- NHS: Irregular periods (2026)
- NHS: Stopped or missed periods (2026)
- NHS: Heavy periods (2024)
- Cleveland Clinic: Abnormal menstruation (periods)
- Human Reproduction: Teede HJ et al. Recommendations from the 2023 International Evidence-based Guideline for the Assessment and Management of Polycystic Ovary Syndrome (2023)
- NHS: Early menopause (2025)
- World Health Organization: Polycystic ovary syndrome, fact sheet (2026)
