Understanding Endometriosis Endometriosis is a condition in which the tissue that normally lines the inside of the uterus (called the endometrium) grows outside it—on the ovaries, fallopian tubes, or pelvic lining. This misplaced tissue behaves like normal endometrial tissue: it thickens, breaks down, and bleeds during each menstrual cycle. However, since it has no way to exit the body,...