Almost nobody has a 28-day cycle every month. Cycle length varies with sleep, stress, travel, training and illness, and a few days of movement in either direction is ordinary. The useful question is not "is my cycle exactly regular" but "has something changed, and has it stayed changed".
What counts as within range
| Measure | Typical adult range |
|---|---|
| Cycle length | 21–35 days |
| Month-to-month variation | Up to about 7–9 days |
| Bleeding duration | 2–7 days |
Variation is naturally wider in the first years after periods start and again as menopause approaches, when cycles often shorten before becoming unpredictable.
Common causes of irregularity
- Energy availability. Eating too little for your training load can delay or stop ovulation. This is common in athletes and in people dieting hard, and it is reversible when energy intake improves.
- Significant weight change in either direction.
- Thyroid conditions. Both under- and overactive thyroid commonly disturb cycles.
- Polycystic ovary syndrome (PCOS). A frequent cause of long or unpredictable cycles.
- Stress and disrupted sleep, including shift work.
- Medications and contraception, including recent changes to hormonal methods.
- Perimenopause, typically from the mid-forties onwards.
- Pregnancy, which remains a possibility any time a period is missed.
A pattern like this is variable but within range. What matters is that it is your pattern, which only becomes visible after three or four tracked cycles.
What to record before an appointment
Doctors work far faster with data, and this is where tracking genuinely earns its place:
- First day of bleeding for each cycle, and cycle lengths
- Bleeding duration and how heavy it was
- Bleeding between periods or after sex
- Pain, and whether it stops you functioning
- Other changes — weight, hair, skin, mood, energy, temperature tolerance
- Medications, contraception and recent changes to either
Kiva's note
Three tracked cycles turn "my periods are all over the place" into a page of dates. That page changes the appointment completely.
When to book an appointment
See a doctor if your cycles are consistently shorter than 21 days or longer than 35, if you miss three or more periods in a row, if bleeding soaks through protection hourly or lasts longer than seven days, if you bleed between periods or after sex, if periods stop and pregnancy is possible, or if pain prevents you working or sleeping. Irregular cycles are common and often treatable — and some causes matter for long-term health, which is why they are worth investigating rather than waiting out.
Frequently asked questions
How much cycle variation is normal?
In adults, cycles between 21 and 35 days are generally considered within range, and a variation of a few days from month to month is common. Variation is usually greater in the years after periods begin and in the years approaching menopause.
What causes irregular periods?
Common contributors include significant weight change, very high training loads, low energy availability, stress, thyroid problems, polycystic ovary syndrome, some medications, and perimenopause. Pregnancy is also a possibility whenever a period is missed.
When should I see a doctor about irregular periods?
If cycles are consistently shorter than 21 or longer than 35 days, if you miss three or more in a row, if bleeding is very heavy or lasts more than seven days, if you bleed between periods or after sex, or if irregularity is new and persistent.
Keep reading
The four phases of your cycle, explained simply
What actually changes across follicular, ovulation, luteal and menstruation — and how to plan food, training and rest around it.
PMS symptoms: what actually helps
Which premenstrual symptoms are typical, what has evidence behind it, and when to see a doctor.