What causes endometriosis?
It is not known exactly what causes endometriosis. It may occur as a result of retrograde menstruation. This occurs when blood and tissue that is shed during a woman's period flows backwards into the fallopian tubes and pelvic cavity, instead of out of the vagina. It is thought that during this process, endometrial cells stick to these structures, which may lead to endometriosis. Another possibility is that endometriosis occurs as a result of the immune system not working properly, which means that your body doesn't recognise and destroy any endometrial tissue growing in the wrong place.