Endometriosis is a problem affecting a woman’s uterus. Endometriosis occurs when the kind of tissue that normally lines the uterus grows somewhere else. It can grow on the ovaries, behind the uterus or on the bowels or bladder. Rarely, it grows in other parts of the body. This “misplaced” tissue can cause pain, infertility, and very heavy periods. The pain is usually in the abdomen, lower back or pelvic areas. Some women have no symptoms at all. Having trouble getting pregnant may be the first sign.