# Irregular cycles: what is going on? > When your period keeps you guessing. Category: Cycle basics · 4 min read · Written by the Splash team · Sources checked October 2026 URL: https://splash.care/learn/cycle-basics/irregular-cycles ## Quick answer Irregular periods are cycles that change a lot in length from month to month, or are often shorter than 21 or longer than 35 days. Common reasons include puberty, perimenopause, stress, weight change, hard exercise, hormonal contraception, PCOS and thyroid problems. See a GP if they change suddenly, or if you miss three in a row. - Irregular means gaps under 21 or over 35 days, or a pattern that keeps changing. - Common causes include puberty, stress, weight or exercise changes, contraception, PMOS/PCOS and thyroid issues. - Irregular cycles make predictions less reliable, so keep tracking. - See a GP if your pattern changes, periods last over 7 days, or you have extra symptoms. The NHS says periods are irregular if the gap between them is shorter than 21 days or longer than 35 days. It is also worth a closer look if your usual pattern keeps changing. It is really common and often has a simple explanation. ## What causes irregular periods? - Puberty and the first few years of periods. - Nearing [menopause](https://splash.care/learn/glossary/menopause), usually between 45 and 55. - [Stress](https://splash.care/learn/cycle-basics/stress-travel-illness) and [anxiety](https://splash.care/learn/mind-mood/luteal-anxiety), big changes in weight (losing or gaining a lot), or too much [exercise](https://splash.care/learn/period-care/exercise-on-period). - Hormonal [contraception](https://splash.care/learn/contraception/methods-overview): the [progestogen](https://splash.care/learn/glossary/progestogen)-only pill, injection and hormonal coil can all change your pattern. - [Pregnancy](https://splash.care/learn/fertility-ttc/early-pregnancy-signs) and breastfeeding. - Conditions such as [PCOS](https://splash.care/learn/glossary/pcos) (now also called [PMOS](https://splash.care/learn/glossary/pmos)) or thyroid problems. ## Why do irregular periods matter? Irregular cycles often mean [ovulation](https://splash.care/learn/glossary/ovulation) is happening at different times, or sometimes not at all ([Anovulation](https://splash.care/learn/glossary/anovulation)). That makes fertile-window predictions less reliable and can make it harder to get pregnant. Long gaps between periods can also let the womb lining build up, which is one reason a GP check is worth having. ## What a GP might do Treatment is not always needed. A GP will ask about your cycle and health and may arrange blood tests. If they think a condition such as PMOS is behind it, the combined pill can help make periods more regular. Thyroid problems can be treated with medicine. Your tracking notes make this conversation much quicker. **Myth:** Irregular periods mean you cannot get pregnant. **Fact:** Many people with irregular cycles still ovulate sometimes and can get pregnant. If you do not want to be pregnant, you need contraception, not cycle tracking. > **Splash says:** keep logging even when it feels chaotic. Dates, flow and symptoms are gold when you talk to a doctor. If a cycle is unusual for a reason (illness, travel, new medicine), add a note so it does not distort your averages. ### When to see a doctor The NHS says to see a GP if your periods are irregular or your usual pattern changes, last longer than 7 days, or come with bleeding between periods. Also go if irregular periods come with weight gain, tiredness, thick hair on your face, or dry or oily skin, or if you are struggling to get pregnant. ## Questions people ask ### Can a period tracker work with irregular periods? Yes, but predictions will be less certain. A good tracker shows a wider range when your cycles vary, rather than one confident date. Your logs are also useful evidence to show a GP. ### Are irregular periods a sign of PCOS? They can be. Irregular or infrequent periods are one of the common signs of PCOS (now being renamed PMOS), alongside excess hair growth and acne. Only a doctor can diagnose it. ## Sources 1. [NHS: Irregular periods](https://www.nhs.uk/symptoms/irregular-periods/) 2. [NHS: Missed or late periods](https://www.nhs.uk/symptoms/missed-or-late-periods/) 3. [NHS: Polyendocrine metabolic ovarian syndrome (PMOS, formerly PCOS)](https://www.nhs.uk/conditions/polyendocrine-metabolic-ovarian-syndrome-pmos/) 4. [ACOG: Abnormal uterine bleeding](https://www.acog.org/womens-health/faqs/abnormal-uterine-bleeding) 5. [Jean Hailes for Women's Health: Periods](https://www.jeanhailes.org.au/health-topics/periods/) This is general information, not medical advice. Splash can’t diagnose. If you’re worried, talk to a GP, pharmacist or sexual health clinic. In an emergency call 999 (UK) or 911 (US).