Irregular Periods: Common Causes
A period that arrives a few days late is common and rarely a concern, but a cycle that is consistently unpredictable can be frustrating and worth understanding. The menstrual cycle is controlled by a chain of hormones, and any link in that chain can shift. Knowing the common causes helps you decide what needs a GP and what can be managed with lifestyle.
What counts as irregular
A typical cycle is between 21 and 35 days, but everyone has their own rhythm. Irregularity means the length varies widely from month to month, you skip periods, or bleeding occurs between periods. A single odd cycle is usually nothing to worry about; a pattern that persists for several months is worth attention.
The common causes
- Stress, which can delay ovulation through its effect on the hormones involved.
- Major weight change, up or down, which affects oestrogen production.
- Very heavy exercise combined with low energy intake.
- Thyroid problems, which disturb the whole cycle.
- Polycystic ovary syndrome, the most common hormonal cause.
- Perimenopause, when cycles lengthen and become unpredictable.
PCOS as the main culprit
PCOS is responsible for a large share of irregular periods, and it typically brings extra signs such as acne, unwanted hair growth and weight gain. The diagnosis involves a combination of symptoms and tests, and it is manageable, so an accurate picture is worth pursuing. Our PCOS overview walks through the symptoms and support options.
Thyroid and weight links
An underactive or overactive thyroid can both disturb periods, usually by affecting the hormones that drive ovulation. Because thyroid problems also affect weight and energy, a woman presenting with irregular cycles and tiredness is often worth a thyroid blood test. Your GP can arrange this simply.
Common patterns and next steps
| Cause | Typical pattern | Next step |
|---|---|---|
| Stress | Missed or late ovulation | Rest, routine, review in a few months |
| Low body weight | Periods stop entirely | Speak to a GP about energy intake |
| PCOS | Cycles long and erratic | Book a GP assessment |
| Thyroid problem | Irregular with fatigue | Ask for a thyroid blood test |
| Perimenopause | Lengthening, variable cycles | Symptom support and HRT options |
When to see a GP
See a GP if you have had no period for three months, if cycles are consistently under 21 or over 35 days, or if bleeding is very heavy or happens between periods. Sudden changes, bleeding after sex and pain are also reasons to seek advice promptly rather than waiting to see what happens.
Managing what you can
Tracking your cycle with an app or a calendar for a few months gives you real data instead of guesswork. Sleep, steady eating and moderate exercise all support a regular cycle, and for those in their forties the pattern may simply reflect perimenopause, which has its own support options. If period pain accompanies the irregularity, mention it, because the two together can point to conditions like endometriosis.
An irregular cycle is rarely an emergency, but it is a useful signal from your body. Track it, address the obvious lifestyle levers, and book a GP appointment if the pattern persists. Getting the cause identified is usually the first step to a steadier cycle.