Sudden arrival of blood 12 days after cesarean section is late postpartum hemorrhage, which may be caused by retained placenta and membranes, retained meconium, incomplete restoration of the placental attachment surface, infection, poor healing of uterine incision after cesarean section, and so on. 1. Retained placenta and membranes: the most common cause of postpartum hemorrhage in the late stage of vaginal delivery, but cannot be completely excluded after cesarean section. 2. Retained meconium: the meconium is mostly detached about one week after delivery. If the meconium is incompletely detached, part of the meconium remains, causing poor uterine contraction and sudden bleeding 12 days after cesarean section. 3. Incomplete restoration of placental attachment: thrombosis of placental attachment after cesarean section. Exhaustion, anemia and other reasons cause poor uterine contraction, placental attachment surface thrombus dislodgement, causing the opening of blood sinuses, resulting in sudden bleeding 12 days after cesarean section. 4. Infection: mainly endometritis. Bacterial infection of the uterine cavity causes endometritis, which leads to poor uterine contraction and poor recovery of the placental attachment surface, causing the opening of the blood sinuses, resulting in the sudden onset of blood 12 days after cesarean section. 5. Poor healing of uterine incision after cesarean section: the uterine artery is cut off from both ends of the transverse incision in the lower uterine segment, resulting in insufficient local blood supply, the choice of transverse incision is too low or too high, the suture is not proper, and the incision is infected, which results in poor healing of uterine incision and leads to late postpartum hemorrhage. When blood suddenly comes in 12 days after cesarean section, the mother should consult the doctor in time to find out the cause under the doctor’s guidance and give appropriate treatment.