Dark brown blood 20 days after cesarean section may be related to poor healing of uterine incision, incomplete uterine regeneration, infection, etc. At this time, it is recommended to consult a doctor in time. 1. Poor healing of the uterine incision: If the location of the uterine incision is not properly selected during the cesarean section, the uterine incision is not properly stitched, or the uterine incision is infected after delivery, it may lead to black-brown blood 20 days after the cesarean section. 2. Incomplete uterine regeneration: if the pregnant woman has placental adhesions, placental implantation, retained meconium and endometritis, it may also lead to black-brown blood again after 20 days of caesarean section. 3. Infection: Pregnant women with weakened immunity after cesarean section are prone to infections, such as endometritis and cervicitis, which may also occur. Therefore, it is recommended to consult a doctor in time to find out the cause of the dark brown blood 20 days after the cesarean section, and carry out appropriate treatment under the guidance of the doctor.