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Mild endometriosis, illustration. Endometriosis occurs when parts of the uterine lining (endometrium, red lining, centre) enter the abdominal cavity instead of exiting the uterus through the vagina (lower centre) as during a normal menstrual cycle. The endometrial tissue (irregular red areas) attaches to other abdominal organs (such as the ovaries at right and left) and forms cysts that bleed during menstruation and can cause pain, inflammation and scarring. (KEYSTONE/SCIENCE PHOTO LIBRARY/GUNILLA ELAM/SCIENCE PHOTO LIBRARY)