Polycystic ovary syndrome (PCOS) doesn’t have a single cause. It develops from a combination of genetic predisposition, hormonal imbalances, and environmental factors that reinforce each other in a cycle that can be difficult to untangle. PCOS affects an estimated 10 to 13% of women of reproductive age worldwide, and up to 70% of those cases go undiagnosed.
Genetics Play the Largest Role
Twin studies estimate that PCOS is roughly 70% heritable, making genetics the strongest known contributor. If your mother or sister has PCOS, your risk is significantly higher. Researchers have identified at least 19 genetic regions linked to PCOS risk across Korean, Han Chinese, and European populations. These genes influence a range of functions: how your body responds to insulin, how your ovaries process hormones, and how your brain signals your reproductive system.
Some of the most consistently identified genes involve insulin receptor signaling, the receptor for follicle-stimulating hormone (FSH), and a gene called DENND1A that’s involved in androgen production. That said, all known genetic variants together account for less than 10% of the condition’s heritability. The remaining genetic influence likely comes from many small-effect genes working together, plus epigenetic changes, which are modifications to how genes are expressed without altering the DNA itself.
Insulin Resistance Drives Androgen Overproduction
Insulin resistance is one of the central engines of PCOS. When your cells don’t respond well to insulin, your body compensates by producing more of it. Those elevated insulin levels don’t just affect blood sugar. They directly stimulate cells in your ovaries called theca cells to produce excess testosterone. At the same time, high insulin suppresses an enzyme that normally converts testosterone into estrogen, so testosterone accumulates further.
This creates a feedback loop. Excess androgens promote fat storage around the abdomen, which worsens insulin resistance, which drives more androgen production. Not every woman with PCOS is overweight, but even lean women with the condition often show measurable insulin resistance. The cycle can begin at any point, which is part of why the “root cause” is so hard to pin down for any individual person.
How Excess Androgens Stop Ovulation
Your ovaries need small amounts of androgens (like testosterone) to support early follicle growth. But when levels climb too high, the process stalls. Excess androgens suppress the genes that follicles need to mature and release an egg. Instead of one dominant follicle developing each cycle, many small follicles (typically 2 to 9 mm) begin growing but get stuck at an early stage. This is what creates the “polycystic” appearance on ultrasound, a ring of small, arrested follicles.
High androgen levels also disrupt the hormonal surge of luteinizing hormone (LH) that normally triggers ovulation. In healthy women, the ratio of LH to FSH is typically between 1 and 2. In many women with PCOS, that ratio flips to 2 or 3, reflecting an imbalance in brain signaling to the ovaries. The result is irregular or absent periods, because without ovulation, the normal menstrual cycle can’t complete.
Chronic Inflammation Adds Fuel
Women with PCOS consistently show signs of low-grade chronic inflammation. Their white blood cell counts and C-reactive protein (CRP) levels, a common marker of inflammation, tend to be elevated compared to women without the condition. Levels of specific inflammatory molecules, including interleukin-6, interleukin-18, and tumor necrosis factor alpha, also run higher.
The relationship between inflammation and PCOS runs in multiple directions. Body fat, particularly abdominal fat, generates inflammatory signals that worsen insulin resistance. But inflammation in PCOS isn’t purely a weight issue. Research shows that androgen levels themselves correlate with white blood cell counts, suggesting that the hormonal imbalance independently promotes an inflammatory state. Insulin resistance and inflammation then reinforce each other: CRP levels correlate directly with insulin levels, and both BMI and insulin resistance are the strongest predictors of elevated CRP in women with PCOS.
Fetal Programming Before Birth
One of the more surprising contributors to PCOS may be what happens in the womb. The fetal programming hypothesis suggests that exposure to excess androgens during a critical window of development can permanently alter how a baby’s hormonal systems are wired. Animal studies in monkeys, sheep, and rats have shown that exposing female fetuses to elevated testosterone produces offspring that develop polycystic ovaries, irregular cycles, and metabolic problems in adulthood.
During normal fetal development, there’s a sensitive period when the brain’s hormonal control center is being organized. If androgen levels are too high during this window, the system that regulates the pulsing release of reproductive hormones gets reprogrammed. The result is an increased frequency and amplitude of hormonal signals that elevate LH levels and impair normal follicle development, essentially setting the stage for PCOS years before any symptoms appear. This may help explain why PCOS runs so strongly in families: a mother with PCOS and elevated androgens during pregnancy could pass the condition to her daughter through both genetics and the hormonal environment of the womb.
Environmental and Lifestyle Triggers
Even with a strong genetic predisposition, environmental factors can determine whether PCOS fully manifests and how severe it becomes. Endocrine-disrupting chemicals are one area of growing concern. Bisphenol A (BPA), found in plastics and food container linings, can bind to androgen and estrogen receptors and interfere with normal hormone signaling. Parabens, common in personal care products, mimic estrogen by binding to estrogen receptors. Triclosan, an antibacterial agent found in some soaps and toothpastes, similarly amplifies estrogenic activity in the body.
Lifestyle factors activate the condition through epigenetic pathways. A poor-quality diet, physical inactivity, chronic emotional stress, and disrupted sleep patterns can all switch on gene expression patterns that promote insulin resistance and androgen overproduction. These aren’t just risk factors in the traditional sense. They appear to activate specific epigenetically programmed pathways in women who are already genetically susceptible. This is why lifestyle changes like improving diet quality and increasing physical activity can meaningfully reduce PCOS symptoms, even though they don’t address the underlying genetic predisposition.
How PCOS Is Diagnosed
Because PCOS has no single cause, diagnosis relies on identifying a pattern of features rather than a single test. Under the most current international guidelines (updated in 2023), a diagnosis in adults requires two of the following three criteria: irregular or absent periods, signs of excess androgens (either visible symptoms like acne and excess hair growth, or elevated levels on a blood test), and either polycystic ovary appearance on ultrasound (more than 20 small follicles in at least one ovary, or ovarian volume greater than 10 mL) or elevated anti-Müllerian hormone levels.
For adolescents, the criteria are stricter. Both irregular periods and evidence of excess androgens must be present, because polycystic-appearing ovaries are common in teenagers and don’t necessarily indicate the condition. Notably, ovarian appearance on ultrasound shouldn’t be used as a diagnostic feature until at least eight years after a girl’s first period, since the ovaries are still maturing during that window.
Why It’s Called a “Vicious Cycle”
What makes PCOS so persistent is that its causes aren’t independent. Insulin resistance increases androgen production. Excess androgens promote abdominal fat storage. Abdominal fat worsens insulin resistance and inflammation. Inflammation further impairs insulin signaling. Each factor amplifies the others, which is why researchers describe the condition as a self-reinforcing vicious cycle. Breaking into this cycle at any point, whether through reducing insulin resistance, lowering inflammation, or decreasing androgen levels, can improve symptoms across the board. But the genetic and epigenetic foundation means the tendency toward PCOS doesn’t disappear; it requires ongoing management.