Contrary to the implication of “polycystic,” some women with the condition don’t have any cysts. A diagnosis requires only two of the following three criteria to be met: elevated levels of male sex hormones (which can cause excess hair growth, acne, and baldness), irregular or absent periods, and/or at least 12 follicular cysts on one or both ovaries.

“If a woman has fewer than eight menstrual periods a year on a chronic basis, she probably has a 50 to 80 percent chance of having polycystic ovary syndrome based on that single observation,” said John Nestler, the chair of the department of internal medicine at Virginia Commonwealth University. “But if she has infrequent menstruation and she has elevated levels of androgens such as testosterone in the blood, than she has a greater than 90 percent chance of having the condition.”

When the syndrome was first described in 1935 by American gynecologists Irving Stein, and Michael Leventhal, it was considered a rare disorder. Today as many as five million women in the United States may be affected, according to the Department of Health and Human Services, but researchers are still just beginning to uncover the disorder’s full impact.

“Classically, we thought of PCOS primarily as an infertility disorder or a cosmetic annoyance, but we now know that it’s also a metabolic disorder and a serious long-term health concern,” Nestler said.

According to a recent study published in the Endocrine Society’s March 2015 issue of Journal of Clinical Endocrinology & Metabolism, women diagnosed with PCOS are twice as likely to be hospitalized for heart disease, diabetes, mental-health conditions, reproductive disorders, and cancer of the uterine lining. The cost of evaluating and providing care to women with PCOS is approximately $4.36 billion per year.

The definitive cause of PCOS is unknown, but researchers have found a strong link to insulin resistance, a genetic condition often associated with diabetes, in which the muscle, fat, and liver cells do not respond properly to insulin and thus cannot easily absorb glucose (sugar) from the bloodstream. As a result, the body produces higher and higher levels of insulin to help glucose enter the cells.

“The excess insulin that's being produced stimulates the ovary to make testosterone, which can interfere with ovulation, rendering many women infertile,” said Nestler. PCOS is the most common cause of infertility in industrialized nations. “The exact cellular and molecular mechanisms are still being explored and are not completely understood.”

In one study, insulin resistance was found in 95 percent of overweight women with PCOS and 75 percent of lean women with PCOS. Perhaps relatedly, women with PCOS have a more than 50 percent risk of getting Type 2 diabetes or pre-diabetes before age 40.