Late, early, missing, or unpredictable — here's what irregular periods actually mean.
An irregular period is one that comes earlier than 21 days after the last one, later than 35 days, or that varies by more than 7–9 days from cycle to cycle. About 14–25% of women have irregular cycles — it's very common. But "common" doesn't mean it should be ignored.
Most irregular periods come back to hormones. Estrogen and progesterone orchestrate your cycle — if either is out of balance, your cycle timing shifts. This includes both high and low estrogen states, and progesterone deficiency (common in luteal phase defects and perimenopause).
PCOS is the single most common cause of irregular periods in reproductive-age women. It involves elevated androgens and disrupted follicle development, leading to cycles of 35–90+ days, or no periods at all. It affects 1 in 10 women and is often undiagnosed for years.
Both hypothyroidism (underactive thyroid) and hyperthyroidism (overactive) affect menstrual regularity. Hypothyroidism often causes heavy, more frequent periods. Hyperthyroidism tends to cause light, infrequent periods. Thyroid function is one of the first things a doctor should check for irregular periods.
Cortisol (the stress hormone) directly interferes with GnRH, the hormone that triggers your cycle. Significant stress — whether physical (intense exercise, undereating) or psychological — can delay or stop ovulation, causing late or missed periods. This can happen even with one or two unusually stressful weeks.
Both substantial weight gain and weight loss can disrupt the hypothalamic-pituitary-ovarian axis. Extremely low body fat (as in eating disorders or extreme athletic training) causes a condition called hypothalamic amenorrhea where periods stop entirely.
The hormonal transition toward menopause (typically starting in the mid-40s, sometimes earlier) causes cycles to become increasingly irregular before stopping. If you're over 40 and your periods are changing, perimenopause is worth considering alongside other causes.
The pill, the implant, the hormonal IUD, and the injection all affect cycle regularity — both while using them and for several months after stopping. Post-pill amenorrhea (absence of periods after stopping the pill) is common and usually resolves within 3–6 months.
| Cause | Common Pattern | Who It Affects |
|---|---|---|
| PCOS | 35–90+ day cycles, or no period | 1 in 10 reproductive-age women |
| Thyroid disorder | Too frequent or infrequent | Women of all ages |
| Stress | Late period, then normal | Anyone under significant stress |
| Weight change | Missed periods, lighter periods | Anyone with significant weight change |
| Perimenopause | Increasingly irregular | Women 40+ |
| Post-pill | 3–6 month cycle reset | Anyone stopping hormonal contraception |
The most useful thing you can do before a doctor's appointment is bring 3–6 months of documented cycle data. A period tracking app gives your doctor concrete information — cycle lengths, symptom patterns, bleeding amount — rather than estimates. WomensPal lets you log all of this and export a report.
Document your cycle patterns, symptoms, and flow. Get insights and a doctor-ready export. Free forever.
Start free →A period is irregular if it arrives fewer than 21 or more than 35 days after the previous one, or if your cycle length varies by more than 7–9 days from cycle to cycle.
The most common causes are PCOS, thyroid disorders, stress, significant weight change, perimenopause, and recently starting or stopping hormonal contraception.
See a doctor if you've missed 3+ periods (and aren't pregnant), if your cycles are consistently outside the 21–35 day range, or if irregularity comes with symptoms like excessive hair growth or unexplained weight gain.
Yes — cortisol from stress disrupts the hormones that trigger ovulation, which can cause late or missed periods. Even one or two very stressful weeks can delay a period.