Endometriosis is an often painful disorder in which tissue that normally lines the inside of your uterus — the endometrium — grows outside your uterus. Endometriosis most commonly involves the ovaries, bowel or the tissue lining the pelvis. Endometrial tissue may spread beyond the pelvic region into the abdominal cavity. In endometriosis, displaced endometrial tissue continues to act as it normally would — it thickens, breaks down and bleeds with each menstrual cycle. Because this displaced tissue has no way to exit the body, it becomes trapped. Surrounding tissue can become irritated, eventually developing scar tissue and adhesions — abnormal tissue that binds organs together. Endometriosis can cause pain, that for some women can be severe, especially during their period. Fertility problems also may develop.