What counts as an irregular period?
A 'normal' adult cycle is 21–35 days, with bleeding lasting 2–7 days. Irregularity is any cycle outside that range, any cycle that varies by more than 7–9 days month to month, missed periods for 3+ months (without pregnancy), or new heavy/clotting/breakthrough bleeding. Teenagers in the first 1–2 years and women in the year before menopause are exceptions where some irregularity is normal.
The four most common causes
1) PCOS — usually long or absent cycles, often with acne or weight changes. 2) Thyroid disease — both hypo- and hyperthyroidism shift cycle length and flow. 3) Perimenopause — typically shorter cycles in the late 30s, then longer gaps in the 40s. 4) Energy deficiency — under-eating, over-exercising or significant stress can suppress ovulation (functional hypothalamic amenorrhea).
What to test, and when
A baseline panel can be drawn any day if cycles are absent. If you're still cycling, day 2–5 (early follicular) gives the cleanest FSH/LH/estradiol picture; progesterone needs a separate luteal-phase draw about 7 days after suspected ovulation. We add TSH, free T4, prolactin, ferritin and (where indicated) AMH and androgens.
When irregularity is urgent
See a doctor within weeks (not months) if: you've missed 3+ periods and aren't pregnant, you're bleeding between periods or after sex, periods are soaking through a pad/tampon every hour, or you're under 40 with new cycle changes.
The most common hormonal condition in women — driven mostly by insulin resistance, not by 'cysts'.
Frequently asked
Anything over 35 days from day 1 to next day 1 is considered irregular. Missing 3 or more cycles without pregnancy warrants testing.
Yes — significant physical or emotional stress can suppress ovulation through the hypothalamic-pituitary-ovarian axis. It's usually reversible once the stressor lifts.
No. PCOS is the most common single cause but thyroid disease, perimenopause and under-fuelling are all frequent — and treated very differently.
The pill imposes a withdrawal bleed, but it doesn't restore your own cycle. Once you stop, the underlying irregularity is still there.
Yes, but it's harder to time. Confirming whether you're ovulating (and treating any underlying cause) usually restores fertility quickly.
PCOS typically shows up in your teens or 20s with long cycles, acne and weight changes. Perimenopause typically shows up after 38–40 with shortening then lengthening cycles. Bloods + age usually distinguish them.