The main reason is that the day before menstruation, the ovulation period is over and there are no viable eggs in the woman’s body, so when the male sperm enters the woman’s uterus, there is no way to form a fertilized egg, so the chances of getting pregnant at this time are relatively small. The most noteworthy thing is that women should be firmly prohibited from having sex during menstruation, because the cervix is open during menstruation, and if you have intercourse during menstruation, infection may occur, and pathogenic bacteria may lead to endometritis and pelvic inflammatory disease if the infection travels upstream. In more serious cases, endometriosis may occur, which may lead to dysmenorrhea, secondary infertility, and have a great impact on women’s health.