Posterior uterine effusion after pregnancy usually refers to pelvic fluid, which may be caused by pelvic inflammatory disease or ectopic pregnancy. 1. Pelvic inflammatory disease: If a pregnant woman has a combination of pelvic inflammatory disease, a small amount of fluid at the back of the uterus will be found when she undergoes ultrasound examination at the beginning of pregnancy. If the pregnant woman usually does not have abdominal pain, lumbago, abnormal vaginal discharge and other uncomfortable symptoms, it can be observed for the time being. 2. Ectopic pregnancy: In the early stage of pregnancy, the fertilized ovum that has been deposited in a place other than the uterus is called ectopic pregnancy. If a woman has an ectopic pregnancy, the local organs and tissues will rupture and bleed as the embryo grows. The blood collects at the back of the uterus, and the presence of fluid at the back of the uterus is detected during an ultrasound. This condition should be treated immediately by a doctor and given appropriate treatment. Pregnant women should go to regular hospitals for regular maternity checkups during pregnancy. If abdominal pain or abnormal vaginal bleeding occurs, they should go to the hospital in time to clarify the cause and deal with it actively.