Endometriosis is a painful—and often debilitating—disorder in which tissue similar to the tissue that normally lines the uterus (the endometrium) grows outside of the uterus. Endometriosis most commonly involves the ovaries, fallopian tubes, and the tissue lining the pelvis. One of the main symptoms of endometriosis, chronic pain, tends to be especially strong during ovulation and menstruation. The symptoms of …