On the 15th day after cesarean section, menstruation comes back suddenly after five days of cleansing, which may be caused by poor uterine regeneration, infection, retained placenta and fetal membranes and other reasons. 1. Poor uterine regeneration: menstruation usually does not occur 15 days after cesarean section, usually refers to the evil dew, when the uterus is not well regenerated after surgery, it will also be manifested as the performance of the evil dew suddenly after clean, through ultrasonography can assist in the diagnosis, and need to comply with the doctor’s instructions to give medicines to promote uterine contraction treatment such as hysteronin. 2. Infection: when there is a combination of postoperative infection, such as endometritis, it will also affect the contraction of the uterus and appear again after the discharge is clean, and may be accompanied by foul-smelling discharge, fever, etc. The combination of clinical manifestations and ultrasonography, blood tests can assist in the diagnosis. 3. Uterine cavity residue: when there is placenta and fetal membrane tissue left in the uterine cavity, it will also affect the uterine contraction, and there will be discharge after it is clean, which can be diagnosed by ultrasound examination. When the 15th day of cesarean section, the menstrual period is clean for five days and suddenly comes back, you need to go to the hospital in time to find out the cause and actively treat it.