The fourth day of menstruation is not allowed to cohabit, because at this time the uterine cavity of the menstrual blood has not been completely clean, the uterine cavity is still in the state of bleeding, small blood vessels are in the state of oozing blood. The endometrium is still in a state of recovery, the surface is not yet covered by epidermis, if strenuous sex will lead to endometrial abrasion, the menstrual period is not completely clean, the vagina’s resistance is weaker, but also cause unnecessary infection of the uterus, the endometrium is not yet recovered to cause damage. At this time if intercourse will stimulate uterine contraction, resulting in more bleeding, endometriosis and other conditions occur, intercourse during menstruation will also cause endometrial bleeding, aggravate pelvic stasis, resulting in increased menstrual flow, prolonged menstrual cycle, dysmenorrhea and other gynecological sequelae, in order to avoid gynecological disorders, you can have sex after three days after menstruation is clean.