With endometriosis, endometrial cells can become attached to ovaries, fallopian tubes, and the lining of the pelvis (see picture). During the regular menstrual cycle, the tissue thickens, swells, and sheds, but the ectopic (normal cells in an abnormal location) endometrial tissue does not get shed during menstruation as it normally would as a part of the inner lining of the uterus.