You should not have intercourse during menstruation because the opening of the uterus is slightly open during menstruation and there is also menstrual blood flowing from the uterus. Blood is the best medium for bacteria to multiply, so if you have intercourse during menstruation, it can lead to cervicitis, endometritis, pelvic inflammatory disease and tubal inflammation. Inflammation in severe cases can lead to lower abdominal pain, increased discharge, fever and even blockage in the fallopian tubes. Another point is that having intercourse during menstruation can cause menstrual blood to flow backwards, which can lead to endometriosis. Normally, the endometrium is supposed to cover the uterine cavity. If you have intercourse during menstruation, it can cause the endometrium to implant outside the uterine cavity, which can lead to endometriosis. The main clinical manifestations of endometriosis are abdominal pain, increased menstrual flow, and in some cases, long-term chronic abdominal pain.