The main reasons why it is not recommended for women to have sex on the fourth day of their menstrual period include the risk of infection and the risk of endometriosis. 1. cause infection: because the blood discharged during a woman’s menstrual period is a good environment for bacterial reproduction, especially in the case of sexual intercourse without cleanliness and hygiene, it is easy to lead to bacterial infections, and at this time the body of the woman’s resistance is weaker, which may lead to infections resulting in inflammation of the consequences of the more serious. 2. lead to endometriosis: women may have contractions during sex, in which case it is easy to retrograde the blood discharged during menstruation back to the pelvic cavity, and if planted on the surface of other organs in the pelvis such as the ovaries, it is easy to develop the symptoms of endometriosis. Therefore, it is not recommended to have sex on the fourth day of menstruation. If the patient experiences any discomfort, it is recommended to go to the hospital for early and regular treatment.