The normal tissue that lines the uterus and bleeds during the menstrual period is called the endometrium. Endometriosis is a non-cancerous disorder in which tissue that is similar to the endometrium develops outside of the uterus. Typically this occurs in the pelvis, but it may occur in virtually any part of the body.
The most common locations for endometriosis are: the outer surface of the ovaries, peritoneum (the tissue that lines the abdomen) and peritoneal structures (the area behind the uterus and the various ligaments that hold the uterus in place), uterus, fallopian tubes, bowel, and bladder. Most women have endometriosis in more than one location.