It is absolutely forbidden to have intercourse during menstruation because of the following hazards: first, during menstruation, intercourse can cause pathogenic bacteria to enter the uterine cavity along the cervix, then into the fallopian tubes and finally into the pelvis, causing inflammation of the pelvis and leading to adhesion of the fallopian tubes, resulting in infertility. The second danger is that intercourse during menstruation can cause menstrual blood to flow backwards, which results in active endometrial cells entering the pelvic cavity, causing endometriosis and leading to pelvic chocolate cysts or infertility. This is because endometriosis can lead to infertility in 40% to 50% of patients.