Irregular Periods: Causes, Tracking and When to See a Doctor

Published November 8, 2026 · Updated November 8, 2026 · 7 min read · by the Sakura5 team

Irregular periods are cycles that come less than 21 days or more than 35 days apart, or whose length changes a lot from month to month, and the most common causes are puberty, approaching menopause, pregnancy, hormonal contraception, stress, big changes in weight, intense exercise, and conditions such as polycystic ovary syndrome (PCOS) or thyroid problems. Many of these are harmless and temporary. Some need a doctor, which is why tracking the pattern matters.

The hard part about irregular periods is not usually the definition. It is knowing whether your irregularity is normal for you, and what to tell a doctor if it is not. A few months of simple records answer both questions better than memory ever can.

This guide covers what counts as irregular according to the NHS and ACOG, the common causes, the signs that mean you should book an appointment, and how to track your cycle so the conversation with your doctor is useful.

What counts as an irregular period

A cycle is counted from the first day of one period to the day before the next one starts. The NHS describes the average cycle as around 28 days and says periods are considered irregular if the gap between them is less than 21 days or more than 35 days.

The American College of Obstetricians and Gynecologists (ACOG) uses a similar normal range of 21 to 35 days, with a period usually lasting up to 7 days. ACOG lists these as abnormal bleeding patterns:

• Cycles longer than 35 days or shorter than 21 days
• Cycle lengths that vary by more than 7 to 9 days from one to the next
• Not having a period for 3 to 6 months
• Bleeding that lasts more than 7 days
• Bleeding or spotting between periods or after sex
• Heavy bleeding, including soaking through one or more pads or tampons every hour
• Any bleeding after menopause

So a cycle that is 27 days one month and 31 the next is within normal variation. A cycle that is 24 days, then 40, then 29 is the kind of pattern worth paying attention to.

Common causes of irregular periods

The NHS and ACOG list a broad set of causes. They fall into a few groups.

Life stages

Periods are often irregular for the first few years after they start, and again during perimenopause, the years leading up to menopause. The NHS notes that menopause usually happens between 45 and 55. ACOG says skipped periods and lighter or heavier bleeding are common during perimenopause, while still recommending you mention any abnormal bleeding to a gynecologist.

Pregnancy and after birth

A missed period is often the first sign of pregnancy. Periods can also take time to settle after having a baby, especially while breastfeeding.

Contraception and medication

Hormonal contraception, including the progestogen-only pill, the injection, and the hormonal IUS, can change bleeding patterns or cause bleeding between periods. ACOG notes that some medications, such as blood thinners, can cause heavier bleeding, and that the copper IUD can make periods heavier, especially in the first year.

Lifestyle factors

The NHS lists stress, significant weight loss or gain, and excessive exercise as common causes. These can disrupt ovulation, and when ovulation is delayed or skipped, the period moves too.

Medical conditions

Polycystic ovary syndrome and thyroid problems are both well-known causes. ACOG explains that when ovulation does not happen for several cycles, bleeding can become irregular and sometimes heavy. Fibroids, polyps, adenomyosis, endometriosis, bleeding disorders, and, less commonly, conditions of the womb lining can also cause abnormal bleeding. That is why persistent changes deserve a proper assessment instead of guesswork.

When to see a doctor about irregular periods

The NHS recommends seeing a GP if:

• Your periods are irregular
• Your periods last longer than 7 days
• You have irregular periods together with other symptoms, such as weight gain, tiredness, extra facial hair, or very dry or oily skin
• You bleed between periods
• Your periods are irregular and you are having difficulty getting pregnant

ACOG adds an urgent warning: if you are soaking through 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, seek emergency care right away. Any bleeding after menopause should also be checked promptly.

If you have had a positive pregnancy test and develop bleeding or pain, contact a doctor, midwife, or urgent care service rather than waiting.

How to track irregular periods for your doctor

ACOG specifically suggests keeping track of your bleeding for several weeks before an appointment, noting the dates, how long it lasts, and the type (light, medium, heavy, or spotting), on a calendar or in a smartphone app designed for tracking cycles. The NHS also suggests an app or diary to see whether your periods are irregular. Here is what to record.

1. The first day of every period

This is the most important data point. Cycle length is calculated from it, and without it nothing else is reliable.

2. How many days you bleed, and how heavily

Note each day as spotting, light, medium, or heavy. If you are changing products very often, write down roughly how often.

3. Bleeding outside your period

Spotting between periods or after sex is easy to forget by the time of an appointment and is exactly what a doctor will ask about.

4. Other symptoms and changes

Pain, tiredness, skin or hair changes, a big change in stress, travel, illness, weight, or exercise. You are not diagnosing anything, just giving context.

5. Medications and contraception

Note any start, stop, or change, because it often explains a change in bleeding.

A tracker makes this far easier than a paper diary. Petala, a period and ovulation tracker for iPhone, now free to download on the App Store, is built for this kind of record: you log your periods and symptoms, and it calculates cycle lengths and predicts what comes next. Because irregular cycles are personal health information, Petala keeps everything on your iPhone, with no account, and you can lock it with Face ID or Touch ID. Predictions for very irregular cycles will naturally be less certain, which is itself useful information to bring to an appointment.

A worked example: six months of records

Mai, 29, feels her periods have become "all over the place" since a stressful job change, but she cannot remember the details. She starts logging and, six months later, her records show cycles of 33, 41, 30, 45, 38, and 36 days, with periods lasting 5 to 6 days and no bleeding in between. She has also noted more tiredness and a few months of oily skin and breakouts.

At her appointment she shows the list instead of saying "they are irregular." The doctor can see straight away that most of her cycles are longer than 35 days and vary by much more than 7 to 9 days, and that she has other symptoms the NHS lists as reasons to check further. The doctor arranges blood tests and an assessment, in part to look at causes such as thyroid problems or PCOS.

Whatever the outcome, the records saved time and made the conversation specific. Mai did not need to diagnose herself. She needed a clear history, which is exactly what tracking provides.

What you can do in the meantime

Treatment depends entirely on the cause, so there is no single fix for irregular periods. A few general steps are sensible while you wait for an appointment or if your doctor has found nothing concerning:

• Keep tracking. Patterns over months matter more than any single cycle, and a private tracker such as Petala makes the record easy to keep.
• Look after the basics. Regular meals, enough sleep, manageable exercise, and stress support all help keep cycles steady when lifestyle is the cause.
• Do not stop prescribed medication or contraception without speaking to the prescriber first.
• Use contraception if you do not want to get pregnant. Irregular periods do not mean you cannot conceive, and ovulation is harder to predict when cycles vary. A period tracker is not birth control.

If you are trying to conceive with irregular cycles, our guide to calculating your fertile window explains why body signs matter more than dates when cycles vary.

FAQ

Are irregular periods normal?

Some irregularity is normal, especially in the first few years after periods start and in the years before menopause. A few days' difference between cycles is common. Cycles shorter than 21 days, longer than 35 days, or varying by more than about a week deserve a conversation with a doctor.

Can stress cause irregular periods?

Yes. The NHS lists stress and anxiety as common causes, along with significant weight change and excessive exercise. These can delay or prevent ovulation, which shifts or skips the period. If stress seems to be the cause but irregularity continues, it is still worth seeing a doctor to rule out other causes.

Can I get pregnant with irregular periods?

Yes. Irregular periods can make ovulation harder to predict, but many people with irregular cycles still ovulate and can conceive. If you are trying for a baby and your periods are irregular, the NHS recommends seeing a GP. If you do not want to get pregnant, use reliable contraception.

Should I track my period if it is irregular?

Yes. ACOG recommends recording the dates, length, and type of bleeding for several weeks before an appointment, using a calendar or a cycle-tracking app. A clear record helps your doctor spot patterns and decide which tests, if any, you need.

This article is general information, not medical advice, and it cannot diagnose the cause of irregular periods. Petala's predictions are estimates and are not a method of contraception. If you are worried about your periods, talk to a doctor, nurse, or gynecologist.

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