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Your symptom: Your period pain is bordering on unbearable.

In the U.S., endometriosis affects more than 11 percent of women ages 15 to 44, and it's often diagnosed in a woman's 30s. Still, it can be hard to spot—even for your ob-gyn. "It's a complicated story, since the symptoms overlap with normal menstrual cramps" and PMS, says Maura Quinlan, MD, MPH, an ob-gyn in Chicago and an assistant professor of obstetrics and gynecology at Northwestern University. The common hallmarks are painful periods, pain during sex and with bowel movements and urination, and excessive bleeding. "Typically, if the pain is hard to control with NSAIDS [non-steroidal anti-inflammatory drugs] like Motrin, the woman needs to take off work for painful periods and sex is painful nearly all of the time regardless of position changes, then it is worth considering endometriosis," she says.

Experts aren't sure what causes endometriosis, but they know what happens: Tissue that normally lines the uterus's interior (the endometrium) grows outside, usually around the ovaries, fallopian tubes and tissue lining the pelvis. This out-of-place endometrial tissue doesn't know it's not inside the uterus where it belongs, and it continues to go through the monthly cycle of thickening, breaking down and bleeding, but it can't be flushed out with your period like normal endometrial tissue. The trapped tissue can irritate the area and even eventually lead to scar tissue there, and down the line, it can lead to fertility problems. That's why clueing in your doctor is so important—the sooner you catch it, the better. "Endometriosis is typically first treated with birth control pills," Quinlan says, since pills can essentially stall that monthly cycle. Sometimes, surgery is needed to confirm the diagnosis and remove extra tissue.