We don’t know exactly what causes endometriosis but there are a few theories. The best theory is called “sampson’s theory of retrograde menstruation”. When women have a period most of the blood comes out through the vagina. But some of the blood goes backwards out the fallopian tubes and lands in the abdominal/pelvic cavity. This blood contains endometrial tissue and in some women it implants into the skin of the abdomen and starts growing –forming endometriosis. There is a genetic basis for endometriosis. For example, in identical twin girls if one of the twins has endometriosis there is a 50% chance her twin sister will too. If endometriosis was all due to genetics there would be a 100% chance that identical twin sisters would both have the disease. As there is only a 50% chance there must be some environmental factors as well as genetic. Perhaps these environmental factors are diet or medication related. We also know that if your mother or sister has endometriosis you are more likely to have the disease.