Irregular Periods and Fertility: Causes, What They Mean and What to Do

Irregular periods and fertility

What counts as irregular

Normal menstrual frequency is 24 to 38 days, measured from the first day of one period to the first day of the next. This is the FIGO definition, and it replaced the older 21 to 35 day range still widely quoted.

Cycles shorter than 24 days are termed frequent menstruation; longer than 38 days, infrequent.

Some variation is normal — cycles are still regular if the gap between your shortest and longest stays within about 7 to 9 days.

Why it matters for conception

Irregular cycles usually mean irregular or absent ovulation. That reduces the number of opportunities to conceive and makes the fertile window hard to identify.

The encouraging part: this is among the more treatable causes of difficulty conceiving. Restore ovulation and natural conception often follows.

The causes worth distinguishing

PCOS — the commonest. Diagnosed on the Rotterdam criteria, needing two of three: irregular ovulation, signs of raised androgens, and polycystic ovaries on ultrasound. Note that current guidance accepts a raised AMH as an alternative to ultrasound in adults, and that ultrasound should not be used diagnostically within about eight years of a girl's first period.

Terminology note: PCOS was renamed polyendocrine metabolic ovarian syndrome (PMOS) by global consensus in May 2026. Clinical content is unchanged.

Thyroid disorders — among the most common and most easily missed. An underactive thyroid typically causes heavier or more frequent bleeding but can also lengthen cycles. Correcting it usually restores normal cycles without treatment aimed at the periods themselves.

Raised prolactin — suppresses ovulation. Note that a single mildly raised result should be repeated under calm conditions, and macroprolactin excluded, before investigation escalates.

Functional hypothalamic amenorrhoea — cycles stop because energy availability is too low, from weight loss, low body weight or heavy exercise. Frequently mistaken for PCOS, and the management is close to opposite. FSH is typically low or normal rather than raised.

Premature ovarian insufficiency — diagnosis now requires only one raised FSH above 25 IU, changed from two measurements, specifically to enable prompt treatment.

What to do

Keep a cycle diary for three months. That pattern is often more informative than a single round of tests.

See a gynaecologist if cycles have been irregular for more than three months, or sooner if you are trying to conceive.

Expect day 2 or 3 FSH and LH, AMH, prolactin, thyroid function and testosterone, plus a pelvic ultrasound. Timing in the cycle matters for several of these — which is a common reason results come back falsely normal.

One caution

Polycystic-looking ovaries on a scan alone do not diagnose PCOS. A substantial proportion of women with no hormonal problem have that appearance.