Uterine fluid in pregnancy may be caused by preeclampsia, subchorionic hematoma, rejection reaction, pelvic inflammatory exudation and so on. 1. Pre-eclampsia: temporary deficiency of progesterone in the body after pregnancy, which does not support the pregnancy will cause vaginal bleeding, and ultrasound will find uterine fluid when bleeding. 2. Subchorionic hematoma: Subchorionic hematoma occurs after pregnancy, the bleeding after placenta has no channel to flow out of the cervical canal, and accumulates inside the uterine cavity, which will also show fluid in the uterine cavity. 3. Rejection reaction: after pregnancy, the embryo produces rejection reaction in the uterus, which will trigger aseptic inflammation and lead to the situation of uterine cavity fluid. 4. Pelvic inflammatory exudation: during pregnancy there are pelvic inflammatory diseases, such as endometrial inflammatory, tubal inflammatory diseases, inflammation leads to localized exudation of the endometrium, which can also lead to the formation of uterine cavity fluid. There are more reasons for uterine fluid after pregnancy, and the pathological ones may affect the growth and development of the fetus, so it is necessary to seek medical treatment in time.