Blood in the underpants after intercourse may be caused by seminal vesiculitis and tuberculosis of the reproductive system in men, cervical inflammation, cervical cancer, submucosal uterine fibroids and other causes in women. 1. Male causes (1) Seminal vesiculitis: due to inflammatory stimulation can make the seminal vesicles and other red, swollen, painful, may appear blood sperm, can be assisted in the diagnosis by clinical manifestations, physical examination and laboratory tests. (2) Tuberculosis of the reproductive system: it can be manifested as frequent urination, urgency, pain in urination, etc. In severe cases, hematospermia and reduced semen volume can occur, which can lead to blood in the underwear after intercourse, and can be diagnosed with the assistance of rectal palpation, hard nodules in the prostate and seminal vesicles, and culture of urinary tuberculosis bacilli. 2. Female causes (1) Inflammation of the cervix: such as acute cervicitis, cervical polyps, etc., contact bleeding can occur, which can be diagnosed with the help of history and gynecological examination, vaginal secretion examination, ultrasonography and so on. (2) Cervical cancer: contact bleeding is most common, and it can also be accompanied by vaginal flow of rice slop-like fluid with fishy smell, which can be diagnosed with the assistance of cervical tissue biopsy and cervical cytology examination. (3) Submucosal fibroids: for the uterine submucosal fibroids that come out of the cervix and enter into the vagina, they can also cause contact bleeding after coitus, which can be diagnosed by ultrasonography and so on. (4) Trauma: If it is the first time to have intercourse or rough action during intercourse, it will bleed due to hymen rupture or perineal soft tissue injury, which can be clearly diagnosed by body examination. When there is blood in the underwear after intercourse and accompanied by other discomforts, you can go to the hospital for consultation and active treatment.