Endometriosis. Illustration of the organs inside a woman's abdomen, showing sites where endometriosis can develop. Endometriosis occurs when fragments of the lining of the uterus are displaced to other parts of the body and continue to grow. Possible sites are shown in blue. From top: abdominal wall, intestines, fallopian tubes, ovary, appendix, uterus, peritoneum, cervix, rectum, bladder, vulva. The displaced fragments continue to respond to the menstrual cycle, so each month they bleed. Because the blood cannot escape it builds up into painful cysts that can grow as big as pineapples. Treatment includes drugs to prevent menstruation & sometimes surgery to remove cysts or the uterus.