Irregular periods: 8 common causes and what to do

Your cycle isn't supposed to run like clockwork. A little variation is normal — but if your cycles are noticeably shorter, longer, or unpredictable from one month to the next, your body is telling you something.

Here's what "irregular" actually means, the eight most common causes, and when it's worth seeing a doctor.

What counts as "irregular"?

Current international guidance (FIGO) considers cycles between 24 and 38 days normal for adults. It becomes "irregular" when:

• Cycle length varies by more than 7–9 days between consecutive cycles
• You skip periods (more than 60 days without one)
• Your cycles are consistently outside the 24–38 day range

The 8 most common causes

1. PCOS (polycystic ovary syndrome). The most common cause in women of reproductive age. Usually comes with acne, hair changes, or weight gain.
2. Thyroid problems. Both an under- and over-active thyroid mess with cycle timing.
3. Stress. Physical stress (illness, big life event) or chronic stress raises cortisol and can delay or skip cycles.
4. Weight changes. Sudden weight loss or gain, very low body fat (common in athletes), or being significantly overweight can all disrupt ovulation.
5. Perimenopause. The 4–10 years before menopause naturally bring shorter, longer, or unpredictable cycles.
6. Contraception changes. Starting, stopping, or switching hormonal contraception can take 3–6 months to settle.
7. Prolactin issues. A hormone called prolactin (which normally rises during breastfeeding) can go up for other reasons too, and it suppresses ovulation.
8. Uterine issues. Fibroids, polyps, or adenomyosis change bleeding patterns.

What your doctor may check

• Blood tests: thyroid (TSH), prolactin, FSH/LH, testosterone, AMH
• Blood sugar / HbA1c (especially if PCOS is suspected)
• Pelvic ultrasound

What you can do

• Track carefully. Bring 3+ months of cycle data to your visit — it's the single most useful thing you can hand a doctor.
• Stabilise weight and sleep. These are the biggest lifestyle levers for cycle regularity.
• Manage stress. Regular movement + 7–8 hours of sleep helps more than most supplements.
• Don't self-medicate with over-the-counter "period regulator" pills — they're usually just hormones and can mask a diagnosis.

When to see a doctor

• You go 3+ months without a period (and aren't pregnant)
• Cycles are consistently under 21 or over 35 days
• Cycle length varies by more than 9 days
• You're trying to conceive