June 4, 2026

The Rotterdam criteria, explained plainly

If you've read anything about how PCOS is diagnosed, you've probably bumped into the phrase "Rotterdam criteria". It sounds like jargon, but the idea behind it is genuinely simple — and understanding it explains a lot about why PCOS diagnosis works the way it does, and why two people with PCOS can look quite different.

Three features, in plain English

The Rotterdam approach defines PCOS using three possible features:

Infrequent or absent ovulation. In practice this usually shows up as irregular, infrequent or absent periods — cycles that are much longer than typical, or that vary widely.
Signs of higher androgen levels. Androgens are hormones every body makes. Higher levels can show up clinically — as excess hair growth, persistent acne or hair thinning — or biochemically, on a blood test. Either route counts.
Polycystic ovarian morphology on ultrasound. This means an ovary with a particular appearance on a scan. The name is misleading: these aren't cysts in the everyday sense, and "polycystic ovaries" on a scan don't automatically mean you have the syndrome.

That last point trips up a lot of people. The word "polycystic" describes one possible feature — not the diagnosis itself.

Why two of three is enough

Here's the part that surprises people: you don't need all three features. Under the Rotterdam approach, meeting two of the three is generally enough to support a PCOS diagnosis — once other conditions that could explain the same signs have been ruled out.

That threshold is deliberate. PCOS doesn't present the same way in everyone. One person might have irregular cycles and clear signs of raised androgens, with ovaries that look unremarkable on a scan. Another might have polycystic-appearing ovaries and androgen signs, but reasonably regular periods. Both can meet the criteria by different combinations. The "two of three" rule is what lets a single definition cover a genuinely varied condition.

"But my periods are regular"

This is one of the most common sources of confusion, and the criteria explain it neatly. Because you only need two of the three features, it's entirely possible to have regular periods and still meet the definition — for instance, through androgen signs plus polycystic-appearing ovaries. Regular cycles don't rule PCOS in or out on their own.

The flip side is just as true. Irregular periods alone don't confirm PCOS either, because plenty of other things can affect your cycle. That's exactly why no single feature is treated as proof — and why a clinician has to weigh the whole picture rather than any one finding.

The criteria are still evolving

It's worth being honest that this isn't frozen science. The international, evidence-based PCOS guideline — led by a Monash University team — was updated in 2023, and it refines how these features are assessed, including in adolescents, where the criteria are applied more cautiously. You can read about the work behind that guideline via Monash's PCOS research centre. One area drawing growing attention is the use of AMH (anti-Müllerian hormone) blood testing as a possible alternative to a scan for the ovarian-appearance feature — an approach that's being studied and adopted carefully rather than treated as settled.

The takeaway isn't that the ground is shifting under your feet. It's that the people who set these definitions keep checking their own work — which is reassuring, not unsettling.

Why this isn't a self-diagnosis tool

It's tempting to read the three features, count up your own, and reach a verdict. Please don't. The criteria look simple on the page, but applying them safely depends on the one step that's easy to skip: ruling out other conditions that can mimic the same signs. That exclusion is clinical work, and it's the reason a diagnosis has to come from a clinician rather than a checklist.

What the criteria are good for is orientation. Understanding them helps you see why your GP asks what they ask, why a scan might or might not be ordered, and why "two of three" means your particular mix of symptoms can still add up to PCOS. The diagnosis stays with your clinician — but you can walk into that conversation understanding the framework they're using. The NHS overview of PCOS is a sound place to read more.

PCOS Diary is a tracking tool, not medical advice — please talk to your GP about anything here.

PCOS Diary is coming soon to the App Store — it turns months of one-tap entries into a doctor-ready PDF.