What it means
Irregular means cycles outside about 21-35 days, or cycles that vary a lot from one month to the next.
Two things make irregularity more common than people expect:
- The first few years after the first period are often irregular as the cycle matures.
- Cycle length naturally shifts with age, stress, illness, travel and changes in routine.
Irregularity is not the same as the same cycle every time. Someone whose cycle is reliably 40 days is not having “irregular” periods — it is a long but regular cycle, and that is worth discussing, but it is a different situation.
What can vary
Common and often treatable causes include:
- PCOS — the most common cause in younger women
- Thyroid problems
- Very low body fat, intense exercise, or restrictive eating — insufficient energy availability stops ovulation
- Significant weight change, either direction
- Stress, including long-term stress
- Irregular sleep and shift work
- Some medications
- Perimenopause, usually later in life
Possible explanations
A clinician considers the above, and usually starts with a pregnancy test, a hormone check and a review of weight, exercise and eating patterns. Ultrasound may be used.
Why it matters even if you do not want children: oestrogen protects bone density. Long periods without ovulation mean lower oestrogen, which affects bone strength. This is the most commonly overlooked reason to get irregular periods checked.
Warning signs
- Periods stopping for three months or more while not pregnant
- Periods stopping alongside hot flushes, night sweats, or vaginal dryness
- A history of an eating disorder, very low weight, or very intense training
- Sudden severe pelvic pain with absent periods — rarely, this can indicate an ectopic pregnancy, which is an emergency
- Very heavy, very infrequent bleeding
What to do next
Book an appointment and take a cycle history with you: how many days between the start of one period and the next, roughly how many days of bleeding, and how it has changed.
Do not wait to see whether it self-corrects if it has been going on for months. Irregularity that resolves is not something to assume will happen next time.
Self-care
If a clinician identifies an energy-availability or lifestyle cause, the treatment is usually nutritional: eating enough, reducing excessive training, and addressing any eating disorder with support.
For PCOS specifically, treatment focuses on the symptoms you have — cycle control, acne, fertility — and there are effective options for each.
Nothing you can do without investigation reliably regulates your cycle, and anyone claiming otherwise is selling something.
When to seek help
Routine: occasional irregularity, particularly in the first years after your first period.
Prompt: three months without a period; new irregularity in someone previously regular; irregularity with other symptoms.
Emergency: severe pelvic pain with absent periods.
How to talk about it
“My periods are about 40 days apart.”
“I haven’t had a period for four months and I’m not pregnant.”
“They used to be regular but now they’re all over the place.”
Related articles
Sources
Public sources are linked so you can read the detail rather than take this site’s word for it. None of this has yet been reviewed by a qualified clinician — see how we handle medical review.
References
Where this comes from
Linked so you can check it against a recognised public health source rather than take this site's word for it.
- Irregular periods — NHS NHS (opens in a new tab)
- Amenorrhea — ACOG American College of Obstetricians and Gynecologists (opens in a new tab)
These are public sources from recognised health organisations, linked so you can read the detail rather than take this site's word for it. Reading them does not tell you whether any of it applies to you, and none of these articles has yet been reviewed by a qualified clinician. How we handle sourcing and review.