Pelvic endometriosis. Light micrograph of a section through tissue from the abdomen, in a case of pelvic endometriosis. Endometriosis is where parts of the uterine lining (endometrium) enter the abdominal cavity instead of exiting the uterus through the vagina as during a normal menstrual cycle. The endometrial tissue attaches to other abdominal organs and forms cysts that bleed during menstruation and can cause pain, inflammation and scarring. Pelvic endometriosis refers to endometrial tissue found in the pelvic cavity.