Irregular periods are one of the most common reasons people seek gynaecological advice — yet what counts as "irregular" is often misunderstood. A cycle that's different from your friend's cycle isn't necessarily abnormal. Here's a clear breakdown of what irregular actually means, what causes it, and when it's time to get checked out.
What Counts as Irregular?
A "regular" cycle is one that varies by 7 days or fewer between cycles. For example, if your cycles range from 27 to 33 days over several months, that's considered regular. Medically, the following are considered irregular:
- Polymenorrhoea: Cycles shorter than 21 days
- Oligomenorrhoea: Cycles longer than 35 days, or fewer than 8 periods per year
- Amenorrhoea: No periods for 3+ months (or 6+ months for previously irregular cycles) in someone who isn't pregnant or postmenopausal
- Metrorrhagia: Unpredictable or irregular bleeding between periods
- Cycle-to-cycle variation: Significant changes in cycle length from month to month (e.g. 22 days one cycle, 40 days the next)
Common Causes of Irregular Periods
Stress
Chronic psychological or physical stress elevates cortisol, which suppresses the hypothalamus — the brain region that initiates the hormonal cascade driving the menstrual cycle. The hypothalamus reduces its secretion of GnRH, which in turn reduces FSH and LH, and ovulation can be delayed or suppressed entirely. This is the body's evolutionary mechanism to prevent pregnancy during dangerous or resource-scarce periods.
Stress-related cycle disruption often presents as a longer-than-usual cycle or a late or missed period around stressful life events: exams, job changes, relationship crises, illness, or bereavement.
Significant Weight Changes
Body fat plays a role in oestrogen production. Both underweight and rapid weight loss can disrupt the hormonal signals needed for ovulation. Athletes with very low body fat and high training loads frequently experience amenorrhoea (known as hypothalamic amenorrhoea or, in the context of the female athlete triad, now called relative energy deficiency in sport — RED-S).
Rapid weight gain can also disrupt cycles, particularly if it involves increased insulin resistance — as in the early stages of PCOS.
Polycystic Ovary Syndrome (PCOS)
PCOS is one of the most common causes of irregular periods, affecting up to 10–13% of reproductive-age women. Elevated androgens and insulin resistance disrupt the normal hormonal cascade, leading to infrequent or absent ovulation and consequently irregular, infrequent, or absent periods. PCOS cycles are often 35+ days or unpredictable, and periods when they do occur may be heavy.
Thyroid Disorders
The thyroid gland regulates metabolism throughout the body — including the reproductive system. Both hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid) can cause menstrual irregularity:
- Hypothyroidism: Can cause heavier, more frequent periods or irregular cycles
- Hyperthyroidism: Can cause lighter, less frequent, or absent periods
Thyroid disorders are common, often under-diagnosed, and very treatable — a simple blood test (TSH, T3, T4) can identify them.
Hyperprolactinaemia
Prolactin is a hormone primarily associated with breastfeeding — it suppresses ovulation in lactating people. Elevated prolactin in non-breastfeeding people (hyperprolactinaemia) can cause irregular or absent periods and sometimes milky nipple discharge. Causes include prolactinoma (a benign pituitary tumour), certain medications (antidepressants, antipsychotics, blood pressure medications), hypothyroidism, and kidney disease.
Perimenopause
In the lead-up to menopause (perimenopause, typically the 40s to early 50s), cycles naturally become less predictable as ovarian hormone production declines. Cycles may shorten or lengthen, bleeding may become heavier or lighter, and skipped periods become more common. Any age-appropriate irregularity in the late 30s or 40s that isn't explained by another cause may be early perimenopause.
Hormonal Contraception — Starting or Stopping
Hormonal contraception (particularly the combined pill, progestogen-only pill, hormonal IUDs, injections, and implants) all affect the menstrual cycle. After stopping hormonal contraception, some people experience a delay in the return of regular cycles — sometimes for several months. This is particularly common after the contraceptive injection (Depo-Provera). This is generally temporary, but can cause anxiety for those trying to conceive.
Other Causes
- Endometriosis: Often associated with painful and/or heavy periods; can also cause irregular bleeding
- Uterine fibroids or polyps: Can cause heavy or irregular bleeding between periods
- Pelvic inflammatory disease (PID): Infection that can disrupt menstrual patterns
- Certain medications: Blood thinners, some antidepressants, antipsychotics, epilepsy medications
- Pregnancy or miscarriage: Always consider pregnancy if cycles are irregular — this includes very early pregnancy before a test would be expected to be positive
When Is Irregularity "Normal"?
Some irregularity is expected and doesn't require investigation:
- In the first few years after menarche (first period): It typically takes 2–3 years for cycles to regularise after puberty.
- While breastfeeding: Prolactin suppresses ovulation; periods may be absent or irregular for the duration of breastfeeding.
- Immediately after stopping hormonal contraception: Allow 3–6 months for cycles to regularise.
- During perimenopause: Irregular cycles are expected and normal as you approach menopause.
- During significant acute stress or illness: One or two missed or delayed periods in context of obvious life stress are usually not concerning.
When to See a Doctor — Red Flags
Seek medical evaluation for:
- No period for 3+ months when not pregnant and not breastfeeding
- Very heavy bleeding (soaking through a pad or tampon every hour for 2+ consecutive hours)
- Bleeding after sex
- Bleeding or spotting after menopause
- Sudden change in cycle pattern without obvious explanation
- Severe pelvic pain during periods or between them
- Signs of androgen excess (new facial hair growth, acne, scalp hair loss) alongside irregular cycles — possible PCOS
- Milky nipple discharge alongside irregular periods — possible hyperprolactinaemia
- Irregular cycles alongside fatigue, weight gain, cold intolerance, or hair loss — possible thyroid disorder
How Tracking Helps
Tracking your cycles — even when they're irregular — builds a valuable record. Knowing your cycle length history, the patterns in your bleeding, and your associated symptoms gives a doctor far more to work with than trying to recall from memory. Even noting "no period this month" is useful data.
Mozi's irregular cycle mode is designed for people who can't rely on regular predictions. Rather than generating misleading dates, it focuses on logging what's actually happening — a more honest and helpful approach for irregular cycles.