Fluid buildup in the uterine cavity 12 days after delivery may be caused by poor uterine regeneration, uterine residue and infection in the uterine cavity. 1. Poor uterine regeneration: poor uterine regeneration after delivery can make the blood sinuses on the placental attachment surface reopen, causing uterine bleeding, ultrasound examination shows that there is blood in the uterine cavity, you need to follow the doctor’s advice to give medicines to promote uterine contraction treatment such as oxytocin. 2. Uterine residue: when there is placenta or fetal membrane tissue left in the uterine cavity after delivery, the blood vessels at the base are exposed due to the detachment of necrotic tissues, causing prolonged bloody discharge, ultrasonography can be a clear diagnosis, and medicines such as hysteronin can be given to promote uterine contraction and promote the discharge of residual tissues, and the uterus needs to be cleansed if necessary. 3. Uterine infection: due to the inflammatory stimulation can make the placental attachment surface of poor recovery and poor uterine contraction, there will also be a prolonged bloody discharge, accompanied by odor, the mother may also have fever, abdominal pain and other symptoms, combined with the history and ultrasonography, blood tests can be a clear diagnosis. When the ultrasound examination 12 days after delivery suggests fluid in the uterine cavity, it is necessary to find out the cause under the guidance of the doctor and actively deal with it.