While the exact cause of endometriosis is still unclear, theres no evidence that period sex increases your risk. (In fact, one study linked sex during menses to a decreased risk.) The myth may stem from traditional Chinese medicine theories that having sex during your period disturbs the natural downward flow of menstrual blood, pushing it back into the uterus.
Western medicine has a term for a real phenomenon that sounds similar but isnt caused by sex, nor is it the likely trigger of endometriosis: retrograde menstruation. Many experts think that all women experience some menstrual backup from time to time, usually without ill effects, but that women who develop endometriosis may have a hormonal or immune system problem that allows the tissue to become implanted.