The scale won’t budge. You’re sprouting zits on your chin and hair on your chest. It’s anyone’s guess when you’ll get your next period — you’ve given up keeping track. Even one of these problems would be crazy making, but that’s not the worst of it. These are some of the hallmarks of the most common hormone disorder among young women — polycystic ovary syndrome, or PCOS. More than 5 million women have PCOS, but fewer than half have been diagnosed, according to the nonprofit PCOS Foundation. It’s the leading cause of female infertility, so every woman of childbearing age should know about the condition.

Here are the facts.

The most likely cause insulin resistance. This disruption in the body’s ability to absorb blood sugar can drive up levels of androgens ("male" hormones), such as testosterone, according to experts at the Multidisciplinary PCOS Clinic at the University of California, San Francisco. That explains the acne, the extra body hair and the menstrual irregularity. The hormonal havoc can also interfere with ovulation and the ability to get pregnant.

Even worse, PCOS increases the risk of a generous handful of health problems, including Type 2 diabetes, high blood pressure, high cholesterol, heart disease and sleep apnea. So even for a woman who doesn’t plan to have children any time soon, early treatment of PCOS is vital.

For such a common disease, it can be tough to diagnose. PCOS affects each woman differently. Besides weight gain (that can result in obesity), acne, increased hair growth (on the face as well as the body) and irregular periods, symptoms include male-pattern baldness or thinning hair, dark patches on skin, skin tags, depression, anxiety and sleep disturbances.

Because there’s no simple test for PCOS, doctors rely on the presence of symptoms, plus blood tests to check for elevated levels of androgens and blood sugar and a pelvic exam and/or vaginal ultrasound to check for cysts on ovaries.

PCOS can be a downer. It increases the risk of low self-esteem as well as depression, anxiety and eating disorders, according to PCOS experts from Australia’s Monash University. 

Treatment begins with diet and exercise. If you’re diagnosed with PCOS, losing just five to 10 percent of your weight may be enough to get your periods on track and help your body absorb blood sugar more easily, according to the Academy of Nutrition and Dietetics. In a British study of 143 women with PCOS, those who lost weight through diet and exercise saw more menstrual-cycle improvements than those who took the blood-sugar drug metformin. Although weight loss can be tremendously helpful, medication may still be necessary to control blood sugar, blood pressure and/or cholesterol levels if they’re already high.

You can get pregnant if you have PCOS. In fact, simply dropping extra pounds could dramatically improve your odds of conception. If not, ovulation-triggering medications such clomiphene or letrozole or other fertility treatments can make it possible to start a family when you’re ready. 

Sari Harrar is an award-winning health, medicine and science journalist whose work appears in Dr. Oz The Good Life magazine, Good Housekeeping, O--Oprah Magazine, Organic Gardening and other publications.