PCOS doesn’t have a single cause. It develops from a combination of genetic predisposition, hormonal imbalances, and metabolic factors that reinforce each other in a cycle. An estimated 10 to 13% of women of reproductive age have the condition, though up to 70% go undiagnosed. Understanding what drives it can help you make sense of the symptoms and the treatment options available.
Genetics Play the Largest Role
PCOS runs strongly in families. Twin studies estimate that about 79% of the variation in PCOS risk comes from genetics, based on comparisons between identical and fraternal twins. If your mother or sister has PCOS, your own risk is significantly higher.
There’s no single “PCOS gene.” Instead, it’s a polygenic condition, meaning many small genetic variations across different chromosomes add up to increase risk. Large-scale genetic studies have identified at least 19 regions of the genome linked to PCOS in women of Han Chinese and European descent. These regions include genes involved in hormone signaling (receptors for LH, FSH, and insulin), ovarian follicle development, and androgen production. The genetic architecture resembles that of type 2 diabetes: lots of small contributions from many places, rather than one clear inherited mutation.
Insulin Resistance Fuels Androgen Overproduction
Insulin resistance is one of the most important drivers of PCOS, and it occurs in women with the condition regardless of their weight. When your body doesn’t respond well to insulin, the pancreas compensates by producing more of it. That excess insulin doesn’t just affect blood sugar. It directly acts on the ovaries.
Specifically, insulin binds to receptors on theca cells, the cells that surround developing egg follicles. These cells normally produce small amounts of androgens (often called “male hormones,” though all women produce them). When excess insulin floods these cells, they ramp up androgen production well beyond normal levels. This is the central mechanism behind many PCOS symptoms: excess facial and body hair, acne, thinning scalp hair, and disrupted ovulation.
Insulin resistance also reduces the liver’s production of a protein that binds to androgens in the bloodstream, leaving more of those hormones free and active in the body. The result is a double hit: more androgens are produced, and more of them circulate in their active form.
A Hormonal Signal Gets Scrambled
In a typical menstrual cycle, the brain’s pituitary gland releases two key hormones: luteinizing hormone (LH) and follicle-stimulating hormone (FSH). In healthy women, the ratio of LH to FSH generally falls between 1 and 2. In women with PCOS, this ratio is often reversed, reaching 2 or 3, sometimes higher.
This imbalance starts with a disruption in the signaling pattern of the hormone that controls both LH and FSH (released from the hypothalamus). When LH stays disproportionately high, it further stimulates the ovaries to produce androgens. Meanwhile, relatively low FSH means follicles don’t mature properly. Instead of one follicle developing fully and releasing an egg each month, multiple small follicles stall partway through development. These stalled follicles are what appear as the “cysts” on an ultrasound, giving the condition its name.
Chronic Low-Grade Inflammation
Women with PCOS consistently show elevated markers of inflammation in their blood, including C-reactive protein (CRP) and higher white blood cell counts. This holds true for women with PCOS across the weight spectrum, not just those who are overweight. The inflammation isn’t the kind you’d notice, like a sore throat or swollen joint. It’s a subtle, system-wide process that further stimulates the ovaries to produce androgens and worsens insulin resistance.
These inflammatory markers correlate with higher testosterone levels, higher insulin, and less favorable cholesterol profiles. Inflammation appears to be both a consequence of the hormonal and metabolic disruptions in PCOS and something that actively perpetuates them.
What Happens Before Birth May Matter
One of the more striking findings in PCOS research involves what happens in the womb. Daughters born to mothers with PCOS have roughly five times the risk of developing the condition themselves. Part of that is genetics, but part of it may be the hormonal environment during pregnancy.
Studies of amniotic fluid from pregnancies of women with PCOS show elevated testosterone during the second trimester, and cord blood from their newborns also contains higher androgen levels. Animal research in mice, rats, sheep, and monkeys has consistently shown that exposing a female fetus to excess androgens during development produces PCOS-like symptoms in adulthood: irregular cycles, polycystic ovaries, and metabolic disruption. This prenatal “programming” may alter how the ovaries, adrenal glands, and metabolic systems function for life, and some evidence suggests these changes can even carry into future generations.
The Adrenal Glands Contribute Too
The ovaries aren’t the only source of excess androgens in PCOS. The adrenal glands, which sit on top of the kidneys and produce stress hormones along with androgens, are a significant contributor in roughly 25 to 60% of women with the condition. In these women, the adrenals overproduce androgens like DHEA and its related compounds, adding to the hormonal excess coming from the ovaries. This dual source of androgens helps explain why PCOS symptoms can be difficult to manage and why they vary so much from person to person.
Environmental Chemicals as Possible Triggers
A growing body of research points to endocrine-disrupting chemicals as potential contributors to PCOS. Bisphenol A (BPA), a compound found in certain plastics and food container linings, has received the most attention. Cross-sectional studies find higher BPA concentrations in women with PCOS compared to women without the condition. In lab settings, BPA stimulates ovarian theca cells to produce androgens by affecting a key enzyme in steroid production.
Animal studies paint a broader picture. When pregnant rats were exposed to a mixture of BPA and phthalates (chemicals common in plastics and personal care products), their female offspring developed PCOS-like symptoms. Exposure to other industrial chemicals, including dioxins and certain pesticides, produced polycystic ovaries and reduced healthy egg reserves in both the first generation and the third generation of offspring, suggesting these effects can persist long after the initial exposure. The research in humans is still largely observational, so it’s not yet clear whether these chemicals cause PCOS or simply worsen existing vulnerabilities.
How Weight Fits Into the Picture
Obesity doesn’t cause PCOS on its own. Plenty of women with PCOS are lean, and plenty of women with obesity never develop the condition. But weight gain significantly worsens the metabolic and hormonal features of PCOS in women who are predisposed to it.
Excess body fat, particularly visceral fat stored around the organs, increases insulin resistance through the release of inflammatory signaling molecules and fatty acids. This deepens the insulin resistance that’s already intrinsic to PCOS, driving more androgen production and more disrupted ovulation. It’s a feedback loop: PCOS makes it easier to gain weight (partly through insulin resistance and hormonal shifts), and gaining weight makes PCOS worse. This is why even modest weight loss of 5 to 10% of body weight can meaningfully improve symptoms in women who are overweight, not because weight was the root cause, but because it was amplifying every other driver.
Why No Two Cases Look the Same
PCOS is diagnosed when a woman has at least two of three features: irregular or absent ovulation, elevated androgens (detected through blood tests or symptoms like excess hair growth and acne), and polycystic-appearing ovaries on ultrasound. This means the condition can present in several different combinations. One woman might have high androgens and irregular periods but normal-looking ovaries. Another might ovulate occasionally, have polycystic ovaries, and show no visible signs of androgen excess.
This variability reflects the fact that PCOS isn’t one disease with one cause. It’s the end result of multiple overlapping pathways: genetic susceptibility, insulin dynamics, hormonal signaling, inflammation, prenatal programming, and environmental exposures, each contributing to a different degree in different women. That’s also why treatment is individualized. What matters most is identifying which drivers are most active in your particular case.