If you have intercourse during menstruation, it will definitely have an effect, because it is easy for bacteria to infect the endometrium during menstruation; it may also cause uterine contraction and backflow of menstrual blood, which means that menstrual blood can enter into the abdominal cavity with the fallopian tubes. Menstrual blood contains a large amount of endometrial tissue, if this blood enters the abdominal cavity, it will cause endometriosis, causing dysmenorrhea and pelvic adhesions, which will have a serious impact on future pregnancy. Therefore, you should never have sex when you are menstruating. Once the disease is caused, it is sometimes difficult to cure, and it will affect your future menstruation and your future fertility.