What counts as an irregular period?
A cycle is clinically considered irregular when it varies by more than 7–9 days from month to month, or when cycles fall consistently outside the 21–35 day normal range. Having a 26-day cycle one month and a 28-day cycle the next is completely normal variation. Having a 22-day cycle one month and a 35-day cycle the next warrants monitoring.
Specific medical terms for different types of irregular cycles:
- Oligomenorrhoea — infrequent periods (fewer than 9 per year, or cycle longer than 35 days)
- Amenorrhoea — absent periods. Primary: never having had a period by age 15 (with secondary sex characteristics). Secondary: periods stopping for 3+ consecutive months in someone who previously had them.
- Polymenorrhoea — frequent periods (cycle shorter than 21 days)
- Metrorrhagia — irregular bleeding between periods
Common causes of irregular periods
Stress: High cortisol levels interfere with the hypothalamic-pituitary-ovarian (HPO) axis, which regulates the hormones that drive ovulation. Periods commonly skip or delay during periods of significant stress, illness, or major life changes.
PCOS (Polycystic Ovary Syndrome): The most common hormonal disorder in people of reproductive age, affecting approximately 10% of the population. PCOS causes elevated androgens (male hormones) and disrupted ovulation, resulting in irregular or absent cycles. It's often associated with weight changes, excess body hair, and acne. Diagnosis requires at least 2 of 3 criteria: irregular cycles, elevated androgens, and polycystic ovaries on ultrasound.
Thyroid disorders: Both hypothyroidism (underactive) and hyperthyroidism (overactive) can disrupt cycle regularity. The thyroid hormones T3 and T4 interact with the HPO axis. A simple blood test (TSH) can rule this out.
Significant weight changes: Both rapid weight loss and significant weight gain can disrupt ovulation. Very low body weight or body fat can suppress GnRH (gonadotropin-releasing hormone), shutting down the reproductive axis — a condition called hypothalamic amenorrhoea, common in people with eating disorders or extreme athletic training.
Perimenopause: The hormonal transition before menopause (typically beginning in the mid-to-late 40s) commonly causes irregular, heavier, or lighter cycles before periods eventually stop.
Stopping hormonal contraception: After stopping the pill, hormonal IUD, implant, or injection, cycles may take several months to return to their natural pattern.
Less common causes
- Uterine polyps or fibroids (usually cause heavier, more frequent periods)
- Premature ovarian insufficiency (POI) — ovaries stop functioning before age 40
- Prolactinoma — a benign pituitary tumour that raises prolactin and suppresses ovulation
- Endometriosis — tissue that lines the uterus grows elsewhere, causing pain and irregular bleeding
- Certain medications (antipsychotics, antidepressants, steroids)
When to see a doctor
- Periods have been irregular for 3 or more consecutive cycles
- You miss 3 or more periods in a row and aren't pregnant
- Cycles are consistently shorter than 21 days or longer than 35 days
- Bleeding is very heavy (soaking through protection hourly) — menorrhagia
- You're trying to conceive and cycles are irregular
- Periods are accompanied by severe pain that prevents normal activities