The short answer: your periods are irregular if the gap between them is shorter than 21 days or longer than 35 days, or if it changes a lot from month to month. Stress, weight changes, PCOS, thyroid problems, hormonal contraception and life stage are the most common reasons. Irregular periods aren't always a problem, but they're worth checking with a doctor, especially if they come with other changes in your body.
What counts as irregular?
Count from the first day of one period to the first day of the next. That's your cycle length.
- Regular: your cycle is between 21 and 35 days, and it varies by only a few days each month.
- Irregular: your cycle is often shorter than 21 days or longer than 35, you skip periods, or the gap is very different each time (for example 24 days, then 45, then 30).
Periods lasting more than 7 days, or bleeding between periods, are also worth checking, even if your cycle length is normal.
Common causes
| Cause | What to look out for |
|---|---|
| Puberty | Irregular cycles are common in the first few years after periods start |
| Approaching menopause | From the mid-40s, cycles often become shorter, longer or unpredictable |
| Stress | Exams, work pressure or big life changes can delay ovulation |
| Weight loss or gain | Sudden changes in either direction can upset your hormones |
| Too much exercise | Especially when you're not eating enough to fuel it |
| PCOS | Long gaps between periods, often with acne, extra hair growth or weight gain |
| Thyroid problems | Tiredness, feeling too hot or too cold, weight changes |
| Hormonal contraception | The progestogen-only pill, injection and hormonal IUD can make bleeding irregular |
| Pregnancy or breastfeeding | A missed period is often the first sign of pregnancy; breastfeeding can delay periods for months |
| Night shifts and travel | Disrupted sleep patterns can affect your cycle |
What tests a doctor may do
Your doctor will ask about your cycle, so bring a record of your dates if you can. They may suggest:
- A pregnancy test
- Blood tests to check thyroid and hormone levels, and sometimes blood sugar
- A pelvic ultrasound to look at your womb and ovaries (usually only for adults)
How irregular periods are treated
Treatment depends on the cause, and sometimes none is needed.
- PCOS: certain contraceptive pills can make periods more regular, alongside lifestyle changes
- Thyroid problems: treating the thyroid usually settles the cycle
- Weight or exercise-related: reaching a healthy weight and fuelling your training properly often brings periods back
- Contraception side effects: your doctor may suggest a different method
What you can do
- Track your cycle every month, including the start date, how long you bleed and any symptoms.
- Eat regular, balanced meals. Skipping meals and crash diets can both upset your cycle.
- Keep active, without overdoing it.
- Protect your sleep, especially if you work shifts.
- Find ways to manage stress that work for you.
Be ready for surprises
The hardest part of irregular periods is not knowing when the next one will come. A few habits help:
- Wear a panty liner on days you think your period might start, especially if you get spotting first.
- Switch to a period panty when you feel early signs like cramps or bloating, so an unexpected start doesn't catch you out.
- Keep a spare period panty or a pad in your bag.
Irregular periods and getting pregnant
Irregular periods can make it harder to get pregnant, because you may not be ovulating every month. If you're trying for a baby, having sex every two or three days through your cycle can help. If you've been trying for a while without success, speak to a doctor, as there are treatments that can help you ovulate.
See a doctor if
- Your periods are irregular, or have recently become irregular
- Your periods last longer than 7 days
- You bleed between periods
- Irregular periods come with weight gain, tiredness, facial hair growth, or very dry or oily skin
- You're struggling to get pregnant
Frequently asked questions
Is it normal for teenagers to have irregular periods? Yes. It can take a few years after periods start for the cycle to settle. See a doctor if periods are very heavy, very painful, or stop for several months.
Can irregular periods go back to normal on their own? Often, yes, especially if the cause was stress, illness or a short-term change. If they stay irregular for several months, get checked.
Do irregular periods mean I have PCOS? Not necessarily. PCOS is one common cause, but there are many others. A doctor can tell you which applies to you.
If your period is simply late this month, see Period Late but Not Pregnant? 10 Common Reasons.
Related reading
- Period Late but Not Pregnant? 10 Common Reasons
- PCOS Explained: Symptoms, Diagnosis and Treatment
- PCOS vs PCOD: What's the Difference?
This article is for general information and doesn't replace advice from your doctor.
Sources: NHS: Irregular periods, NHS: Missed or late periods