There are many reasons why there is blood in intercourse on the first day after menstruation, considering that it is caused by residual blood or gynecological diseases. 1. Residual blood: when a woman’s period is just over, there may be a small amount of residual blood in the uterine cavity or vagina, and coitus may stimulate the discharge of the residual blood, thus forming the phenomenon of bleeding. 2. Gynecological diseases: If you suffer from cervicitis, cervical lesions and other gynecological diseases, coitus may also lead to vaginal bleeding. The body is still relatively weak at the end of menstruation, it is not recommended to have intercourse at this time, so as not to have a negative impact on women’s reproductive health. If you have gynecological inflammation, it may aggravate the infection and may cause irregular menstruation. In addition, the first day after the first day of menstruation, there may be other reasons, it is recommended that timely medical treatment, improve the relevant examination, under the guidance of a professional doctor to clarify the cause of the disease, targeted treatment.