Fetal pyelonephrosis refers to hydronephrosis in the fetus. There are many reasons for hydronephrosis in the fetus, and the more common reason is that the level of progesterone in the pregnant woman’s body is relatively high, which has a certain dilatation effect on the smooth muscle of the fetus’s ureter, which will lead to hydronephrosis in the fetus. Fetal hydronephrosis usually does not need treatment, wait until the newborn is born after urination, most fetal hydronephrosis can be recovered. However, if the fetus develops more severe hydronephrosis, this is the time to evaluate the hydronephrosis to see if it needs to be treated. In most cases, hydrocele does not need to be treated during the fetal period. Instead, we wait until the baby is born and passes urine, then observe the hydrocele and, if necessary, have it treated by a pediatric surgeon.