Irregular periods: definition and meaning
Irregular periods are menstrual cycles whose length changes a lot from one cycle to the next, usually by more than 7 to 9 days, or cycles that are regularly shorter than 21 days or longer than 35 days. Some variation is normal, especially in the first years after a first period and in perimenopause.
What counts as irregular
| Pattern | Typical range | Name |
|---|---|---|
| Normal adult cycle | 21 to 35 days, varying by a few days | Regular |
| Cycles under 21 days | Periods come very often | Polymenorrhea |
| Cycles over 35 days | Periods come rarely | Oligomenorrhea |
| No period for 3+ months | After having had periods | Secondary amenorrhea |
Common causes
- Puberty and perimenopause, when ovulation is less predictable
- Stress, illness, travel, big changes in weight or intense exercise
- PCOS and thyroid conditions
- Starting or stopping hormonal contraception
- Breastfeeding and the months after birth
Read the full guide to irregular periods for what helps and how to track them.
Irregular cycles and mood
When your period is unpredictable, PMS can feel like it comes out of nowhere. Logging mood alongside bleeding helps you notice the run-up to a period even when the date moves.
See a doctor if your periods suddenly become irregular, you go 3 months without one (and aren't pregnant), you bleed between periods or after sex, or bleeding is very heavy.
Frequently asked questions
Is it normal for my cycle to change by a few days?
Yes. A difference of a few days between cycles is common. Doctors usually look further if it varies by more than 7 to 9 days.
Can you get pregnant with irregular periods?
Yes. Ovulation may be harder to predict, so tracking signs like cervical mucus or ovulation tests can help.
Why do my periods feel more emotional when they are irregular?
Uncertainty itself is stressful, and when you cannot see a period coming, PMS mood changes can feel random. Tracking mood daily often shows they still cluster in the days before bleeding starts.
Sources
- NHS: Irregular periods
- ACOG: Abnormal Uterine Bleeding
- Bull JR et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. NPJ Digit Med (2019)
This page is for information only and isn鈥檛 medical advice. If something worries you, please talk to a doctor.
Keep reading
Irregular periods: what counts, common causes and what helpsIrregular periods explained: what counts as irregular, why cycles vary, common causes like PCOS, thyroid and perimenopause, and when to see a doctor.What is oligomenorrhea?Oligomenorrhea means infrequent periods, with cycles longer than 35 days or fewer than 9 periods a year. Common causes like PCOS, and when to see a doctor.What is PCOS (polycystic ovary syndrome)?PCOS (polycystic ovary syndrome) is a common hormone condition causing irregular periods, high androgens and ovulation problems. Signs, diagnosis and care.
Cycle length: definition and meaningCycle length is the number of days from the first day of one period to the day before the next. What's normal, how to measure it, and why it varies.Ovulation calculator for irregular periods: find your real windowOvulation calculator for irregular periods: enter your last period dates to see the full range of days you could ovulate, plus how to narrow it down. Free.