Fetal congenital hydronephrosis (Hydronephrosis) refers to a series of clinical manifestations such as separation of the renal pelvic collecting system or the presence of dilated renal calyces or ureters. Fetal hydronephrosis is the most common urological abnormality detected during prenatal ultrasonography. It is defined as a clinical manifestation of fetal hydronephrosis with separation of the renal pelvic collecting system and hydronephrosis detected during intrauterine ultrasonography, and its incidence is 1%. In the past, most children with hydronephrosis presented with clinical symptoms during natural growth and then were seen or operated on. In recent years, due to the widespread use of fetal ultrasonography, hydronephrosis can be detected at 17-18 weeks of gestation, thus increasing the detection rate of asymptomatic hydronephrosis and thus improving the evaluation and management of fetal hydronephrosis and neonatal hydronephrosis. It is of great practical importance for the improvement of renal function in normally surviving newborns. Under normal conditions, the fetal renal pelvis is not dilated and the ureter cannot be seen. In hydronephrosis, dilated renal pelvis and calyces can be seen on ultrasound, as well as dilated ureters. It is generally accepted that a dilated renal pelvis greater than 1 cm in diameter can be diagnosed as hydronephrosis, but the current classification is rather vague, and the clinical significance and management opinions differ depending on the degree of pelvic dilatation. The degree of pelvis and calyces dilatation is classified into five levels: level 1: pelvis dilatation (1. 0 cm); level 2: pelvis dilatation (1. 0-1. 5 cm) and no calyces dilatation; level 3: marked pelvis dilatation (1. 5 cm) and mild calyces dilatation; level 4: marked pelvis dilatation ( > 1. 5 cm) and moderate calyces dilatation; and level 5: severe calyces dilatation and thinning of the renal cortex. The most common cause of hydronephrosis is pelvic ureteral junction obstruction (UPJO), with posterior urethral l membrane and obstructive ectopic ureteral cysts being the second and third causes, respectively. In addition, non-obstructive fetal hydronephrosis is a common problem. This condition is mostly physiological or secondary to vesicoureteral reflux; therefore, fetal hydronephrosis diagnosed prenatally, either bilaterally or unilaterally, partially resolves spontaneously after delivery, and hydronephrosis does not imply obstruction and does not imply impaired renal function or a postnatal lesion. However, the exacerbation of hydronephrosis necessitates further clinical intervention. Therefore, it is necessary to make a comprehensive evaluation of the condition to decide whether it is obstructive or non-obstructive and to achieve the correct clinical management for fetuses with hydrocele found prenatally and for newborns after birth. Deterioration of renal function is the main indication for surgery. Benign mild hydronephrosis does not require intervention, but moderate or severe hydronephrosis often has a poor outcome and requires early surgical treatment. Prenatal diagnosis of hydronephrosis and early confirmation of the diagnosis in the newborn determine the direction of further treatment. The earlier the treatment, the better the recovery of renal function.