What Is PCOS? A Clear Explanation (It's Not Really About Cysts)

Polycystic ovary syndrome (PCOS) is a hormone condition that affects how the ovaries work. It’s one of the most common hormone conditions in women of reproductive age, affecting an estimated 10–13% of them worldwide.

Despite the name, PCOS is not mainly about cysts. The small fluid-filled sacs seen on an ultrasound are follicles, the structures that each hold an egg, that paused partway through developing. Many people with PCOS have none at all.

What’s actually going on

In a typical cycle, one follicle matures each month and releases an egg (ovulation). About two weeks later, if there’s no pregnancy, a period starts.

In PCOS, two things tend to interfere with that rhythm:

• Higher androgens. The ovaries (and sometimes the adrenal glands) make more androgens, hormones like testosterone, than usual. Androgens can cause acne and excess hair, and they disrupt follicle development.
• Insulin resistance. Many people with PCOS need more insulin to keep blood sugar steady. High insulin levels push the ovaries to make even more androgens, which feeds the cycle.

When ovulation is delayed or skipped, periods become irregular or stop. Several follicles may sit partly developed, which is what produces the “polycystic” look on a scan.

What causes PCOS?

There’s no single cause. PCOS runs strongly in families, so genes play a major role. Insulin resistance, inflammation and factors before birth are also thought to contribute. Weight can make symptoms more pronounced, but it doesn’t cause PCOS on its own. PCOS appears in people of every body size.

How PCOS is diagnosed

Most clinicians use the Rotterdam criteria, confirmed in the 2023 international guideline. You need two of three:

1. Irregular or absent ovulation, usually shown as irregular cycles
2. Signs of higher androgens, either on a blood test or as acne, excess hair or scalp hair loss
3. Polycystic ovaries on ultrasound, or a raised anti-Müllerian hormone (AMH) level in adults

If you have both irregular cycles and signs of higher androgens, an ultrasound isn’t needed to diagnose PCOS. Ultrasound also isn’t recommended for diagnosis within eight years of a first period, because many follicles are normal in teenagers.

Before confirming PCOS, a doctor should rule out other conditions that cause similar symptoms, such as thyroid disease, high prolactin and non-classic congenital adrenal hyperplasia.

Why it matters beyond periods

PCOS is more than a reproductive condition. Compared with people without PCOS, people with PCOS have a higher risk of:

• Type 2 diabetes and gestational diabetes
• High cholesterol and blood pressure
• Endometrial problems when periods are very infrequent, because the uterine lining isn’t shed regularly
• Anxiety, depression and eating difficulties
• Sleep apnoea

That sounds like a lot, but it also means there’s a clear path: regular checks of blood sugar, blood pressure and cholesterol, protecting the uterine lining when periods are rare, and support for mental health.

Living well with PCOS

Management depends on what matters most to you: regular cycles, fertility, skin and hair, weight, energy or mood. Common building blocks include:

• Everyday habits that improve insulin sensitivity, such as regular movement, balanced meals and enough sleep
• Medication where helpful, for example combined birth control pills, metformin or anti-androgens
• Fertility treatments such as letrozole when trying to conceive

Tracking makes these conversations easier. When you know your real cycle lengths and which symptoms bother you most, you and your clinician can choose a plan and see whether it’s working.

Sources

• World Health Organization, Polycystic ovary syndrome fact sheet
• 2023 International Evidence-based Guideline for the Assessment and Management of PCOS (Teede et al., J Clin Endocrinol Metab, 2023)
• ACOG, Polycystic ovary syndrome (PCOS) FAQ

This article is general education, not medical advice. Check with your doctor before making medical decisions.