It is best not to have intercourse during menstruation, because although most follicles do not develop during menstruation and there is no possibility of pregnancy, intercourse during menstruation may cause many complications. Because the uterine opening is open during menstruation and the endometrial blood sinuses are also open, sex during this time can easily lead to retrograde infection of pathogenic bacteria into the uterine cavity, causing endometritis, or even into the pelvis, causing pelvic inflammatory disease, or in serious cases, blocking the fallopian tubes, which may affect future pregnancies. It is possible to have intercourse during this period to make the active endometrium retrograde to the pelvis causing endometriosis and even chocolate cysts, which can also affect future pregnancies and even cause dysmenorrhea, so it is best to suggest having sex more than three days after menstruation.