Fetal hydronephrosis is an increase in the amount of urine at the fetal renal pelvis, most of which is not serious, while it is more serious if the amount of fluid is larger. The urinary system is not well developed in the fetal period, and conditions such as ureteral stenosis may exist. Ureteral stenosis can prevent urine produced by the kidneys from being eliminated from the body through the ureter and bladder in a timely manner, and it can be retained in the renal pelvis and calyces and other locations, which manifests itself as hydronephrosis when ultrasound is performed. Mild hydronephrosis suggests that the ureteral stenosis is not serious, and the hydronephrosis can be recovered on its own with the development of the urinary system. If the hydronephrosis is more serious, it suggests that the ureteral stenosis is more serious, and there is a possibility that the hydronephrosis compresses the renal tissues, leading to renal function damage. If the fetus is found to have hydronephrosis, it is necessary to pay attention to the regular review of gynecological ultrasound, and if necessary, surgical treatment can be considered.