As women, we’ve almost all experienced cramping around our periods at some point or another. But according to the Office on Women’s Health, for more than 11 percent of women ages 15 to 44 (aka reproductive ages), there’s one big culprit behind those pelvic aches: endometriosis.

“Endometriosis is a condition where the inner tissue lining of the uterus grows outside of the uterus and causes an inflammatory response,” says Jennifer Conti, MD, clinical assistant professor of obstetrics and gynecology at Stanford University.

Read more at Prevention.