Intercourse during menstruation is prohibited for two main reasons: the first is to prevent the occurrence of pelvic inflammatory disease. The second point, to prevent the occurrence of pelvic endometriosis. Menstruation is a special physiological period for women, when the body’s resistance is poor, the pelvic cavity is congested, and the female hormone level is at its lowest value, the defense capacity of female reproductive organs is at its lowest, and the vagina’s own self-cleaning capacity is also reduced to its lowest. Sexual intercourse at this time may lead to the invasion of exogenous pathogens, which may result in vaginal inflammation. Vaginal inflammation may then reach the endometrium through the menstrual blood, through the open cervical opening, and reach the pelvis, where endometritis, adnexitis, and even inflammation of the pelvis may occur. Furthermore, intercourse at this time may lead to the retrograde flow of menstrual blood into the abdominal cavity and thus the possibility of pelvic endometriosis.