Endometriosis
Although the exact cause of endometriosis isn't known, female hormones, especially estrogen has an important role to play. Back flow of menstrual debris, an overactive immune system, a family history of endometriosis, and a history of abdominal or pelvic surgery are other possible causes. Women who menstruate before 11 or have a cycle shorter than 27 days and heavy, long periods are at risk.