PCOS and irregular periods: why it happens and how to regulate it.

A typical cycle is driven by ovulation: an egg is released, the leftover follicle produces progesterone, and when progesterone falls about two weeks later, you bleed. In PCOS, ovulation is often delayed or skipped. Without that progesterone rise and fall, the cycle loses its timing — and periods become late, unpredictable or absent.

What counts as “irregular” in PCOS?

The 2023 international guideline defines irregular cycles in adults (from three years after the first period) as:

• Cycles shorter than 21 days or longer than 35 days, or
• Fewer than eight cycles a year.

In the first years after periods start, wider ranges are normal; a single cycle over 90 days is considered irregular even in the first year. Note that regular cycles don’t guarantee ovulation — some people with PCOS have regular-looking cycles without ovulating.

Track your own pattern with the free cycle & ovulation calculator.

Common PCOS period patterns

• Oligomenorrhoea: infrequent periods, often 35–90+ days apart.
• Amenorrhoea: no period for three or more months (when not pregnant or on hormonal contraception).
• Heavy or prolonged bleeding when a period finally comes, because the lining has been building for longer. Very heavy bleeding can lead to iron deficiency — worth checking if you’re tired.
• Spotting between periods, sometimes from an unstable lining.

Why long gaps matter: the uterine lining

Estrogen continues to stimulate the uterine lining even when you don’t ovulate. Without progesterone to stabilise and shed it, the lining can keep thickening. Over years, this is associated with a higher risk of endometrial hyperplasia and endometrial cancer. A meta-analysis in Human Reproduction Update found the odds of endometrial cancer were roughly 2.8 times higher in women with PCOS — while emphasising that the absolute risk remains low.

The practical takeaway is reassuring: endometrial protection is easy. Many clinicians suggest inducing a bleed or providing progestin if you have fewer than about four periods a year. Tell your clinician about any bleeding that is unusually heavy, prolonged, or between periods.

How to regulate periods with PCOS

Lifestyle

Improving insulin sensitivity can restore ovulation in some people. In a randomised trial, more women following a low-glycaemic-index diet showed improved menstrual regularity than those on a conventional healthy diet. For people with excess weight, a 5–10% loss can restore cycles in many. See PCOS diet and exercise.

Combined oral contraceptive pill

The guideline recommends the combined pill as first-line pharmacological treatment for irregular cycles (and hirsutism) in adults with PCOS who aren’t trying to conceive. It produces predictable withdrawal bleeds and protects the lining. It doesn’t “treat” the underlying PCOS — symptoms return after stopping — but it’s an effective management tool.

Cyclic progestins or a hormonal IUD

For people who can’t or don’t want to take estrogen, a progestin taken for 10–14 days every one to three months, or a levonorgestrel IUD, protects the lining.

Metformin

Metformin can improve menstrual frequency; a Cochrane review found higher menstrual frequency and ovulation with metformin than placebo, with more gastrointestinal side effects.

If you want to conceive

Ovulation induction with letrozole is first line. See PCOS and fertility.

When to get checked promptly

• No period for three months and a negative pregnancy test.
• Bleeding that soaks a pad or tampon every hour for several hours, or lasts more than 7 days.
• Bleeding between periods or after sex.
• Symptoms of anaemia: unusual tiredness, breathlessness, dizziness.

Irregular cycles also have other causes — thyroid disease, high prolactin, under-eating or over-training — which is why the diagnostic work-up matters.

How to track your cycle usefully

Good records shorten diagnosis and make treatment decisions easier. Note:

• The first day of full bleeding (not spotting) for each period — cycle length is counted from one first day to the next.
• How heavy and how long each bleed is, including any flooding or clots larger than a coin.
• Spotting between periods.
• Ovulation signs if you’re tracking for fertility: fertile-quality cervical mucus, ovulation test results, or a basal temperature rise.
• Medication changes, since hormonal contraception, progestins and metformin all affect patterns.

The cycle calculator estimates your phase and next period from your recent cycles; with irregular cycles, treat its estimates as a range rather than a date.

Periods after stopping the pill

Many people discover irregular cycles only after stopping hormonal contraception. The pill doesn’t cause PCOS — it masks its cycle symptoms by producing regular withdrawal bleeds. After stopping, it can take a few months for a natural rhythm to establish. If periods haven’t returned within about three months, or are very irregular after six, ask about an evaluation for PCOS and other causes.

Irregular periods in teenagers

Irregular cycles are normal in the first one to two years after periods begin, which is why PCOS is diagnosed more cautiously in adolescents. The guideline still considers cycles longer than 90 days, or patterns that remain irregular several years after the first period, as worth assessing. Teens with irregular cycles and signs of high androgens may be described as “at risk” of PCOS and reassessed later.