You can’t get pregnant if you have intercourse on the fourth day of your period because it’s not yet time to ovulate. Even though sperm can only survive in the body for 2-3 days after entering the vagina, ovulation occurs around the 16th day of menstruation. So by the time ovulation occurs, the sperm is dead and the sperm and egg cannot combine to get pregnant. However, it is recommended that you should not have sex if your period has not yet cleared because sex during menstruation may cause inflammation of the genitals, chronic cervicitis or endometritis, or even inflammation of the pelvic cavity. In addition, if you have intercourse before the menstrual blood is clean, it may also cause the menstrual blood to reflux into the pelvis and cause endometriosis, which will clinically appear as lower abdominal pain during menstruation.