Irregular periods: what's normal, what causes them, and when to worry

What counts as an irregular period?

Your cycle is counted from the first day of one period to the first day of the next. For most adults it's somewhere between 21 and 35 days, and it's completely normal for it to shift by a few days each month.

Your periods are usually described as irregular if:

• the length of your cycle keeps changing a lot from month to month
• your cycle is often shorter than 21 days or longer than 35 days
• you sometimes skip periods altogether
• the amount you bleed changes dramatically from one period to the next

If you've only just started your periods, or you're in your 40s, some irregularity is expected. In the first few years after periods begin, cycles often take time to settle, and in the years before menopause they tend to become less predictable again.

Common causes of irregular periods

Your cycle is controlled by a delicate balance of hormones, so lots of everyday things can nudge it off course:

• Pregnancy — always worth ruling out first if there's any chance
• Stress — emotional or physical stress can delay ovulation
• Weight changes — losing or gaining a lot of weight, or having a very low body weight
• Intense exercise — especially combined with not eating enough
• Hormonal contraception — the pill, implant, injection and hormonal coil can all change your bleeding pattern, and it can take a while for cycles to return after stopping
• Breastfeeding — periods may be absent or irregular
• Puberty and perimenopause — natural times of hormonal change

Medical conditions can also cause irregular periods, including polycystic ovary syndrome (PCOS), thyroid problems, raised levels of the hormone prolactin, and conditions such as fibroids or polyps that can cause unexpected bleeding. Some medicines can affect your cycle too.

Track your cycle so you can spot patterns

A simple record makes it much easier to see what's going on and to explain it to a doctor. For a few months, note:

• the first and last day of each period
• how heavy the bleeding is (for example, how often you change products)
• any spotting between periods or after sex
• pain, mood changes, acne or other symptoms
• big life events such as illness, travel, stress or changes in exercise

You can use a paper diary or an app. If privacy matters to you, check what any app does with your data, or keep notes offline.

What can you do about irregular periods?

Treatment depends on the cause, which is why getting checked matters if things don't settle. If stress, low weight or very intense training is behind it, addressing those can help your cycle return. If the cause is a condition like PCOS or a thyroid problem, treating it often helps.

Hormonal contraception is sometimes offered to make bleeding more predictable, but it manages the pattern rather than fixing the cause. If you're trying to get pregnant, irregular cycles can make ovulation harder to predict, so mention this to your doctor early.

When to see a doctor

Book an appointment with your GP or doctor if:

• your periods have stopped for three months or more and you're not pregnant
• your cycle is regularly shorter than 21 days or longer than 35 days
• there's a big difference (around 20 days or more) between your shortest and longest cycle
• you bleed between periods or after sex
• your periods are very heavy or very painful
• you have any bleeding after menopause
• you've had irregular periods and you're struggling to get pregnant
• you also have symptoms like excess hair growth, hair thinning, unexplained weight change or feeling constantly tired

A doctor will usually ask about your cycle and health, may suggest a pregnancy test, and might arrange blood tests or a scan.

Get urgent help if you have these symptoms

Seek urgent medical care if you have:

• sudden, severe pain in your tummy or pelvis, especially with a positive pregnancy test or a missed period
• bleeding so heavy you soak through a pad or tampon every hour for several hours
• dizziness, fainting, a racing heart or feeling very unwell with bleeding

These can be signs of something like an ectopic pregnancy or significant blood loss, and they need to be checked straight away.