With the widespread use of prenatal ultrasound, hydronephrosis is the most common abnormality on prenatal ultrasound, accounting for about 50% of all prenatal defects. The problem of hydronephrosis detected during fetal life is a hot issue for many families, so what exactly should be done about hydronephrosis detected during fetal life? Definition of hydronephrosis A dilatation of the renal collecting system due to inadequate drainage or urinary reflux is often thought to be due to mechanical factors alone, but this is not quite accurate. More precisely, hydronephrosis is a comprehensive condition caused by the interaction of glomerular hemodynamics and tubular function. Diagnostic ultrasound criteria for hydronephrosis 1. Fetal stage: American Society of Fetal Urology (SFU) grading: Grade 0 – no renal pelvis dilatation; Grade I – only mild renal pelvis dilatation; Grade II – Grade II – dilated renal pelvis with visible calyces; Grade III – dilated renal pelvis and calyces; Grade IV – more severe dilatation of renal pelvis and calyces and thinning of renal parenchyma; 2. Neonatal period: Grignon et al. graded according to the degree of dilatation of renal pelvis and calyces: Grade 1: dilated renal pelvis 1.5 cm Grade 4: marked dilatation of the renal pelvis with moderate dilatation of the renal calyces; Grade 5: severe dilatation of the renal calyces with parenchymal thinning. Hydronephrosis detected in the fetus does have a tendency to remit spontaneously. A review of the domestic and international literature reveals that reports of remission rates vary and differ widely: 30%-85% are available, which may be explained by the different selection of study subjects and the determination of surgical indications. And the trend of remission of hydronephrosis seems to be more pronounced than that of hydronephrosis alone? Follow-up means are ultrasonography, diuretic nephrography, or VCUG, and IUV is not recommended as a routine method of examination and follow-up because of the greater radiation. The first examination of postnatal ultrasonography is 3-7 days after birth, and postnatal ultrasonography should be performed at least 2 days after birth, because the relative dehydrated state and physiological oliguria in the postnatal period may lead to false-negative results. The exception to this rule is in neonates with bilateral hydronephrosis with bladder wall thickening, a condition that often suggests bladder outlet obstruction due to a posterior urethral valve. Surgical intervention is required if there are signs, symptoms or impaired renal function or progressive decline in renal function or bilateral hydronephrosis. Definition of impaired renal function or progressive decline in renal function: impaired renal function (40%?) is defined as a decrease in renal function of less than 35%-30%. A decrease in renal function of 10%-5% is defined as a progressive decrease in renal function. Other factors suggesting the need for surgery include: intermediate to high SFU classification, hydronephrosis detected at mid-pregnancy, progressive enlargement of hydronephrosis, relatively large hydronephrosis, and also according to the type of renal chart excretion curve. The frequency of follow-up must be close within 2 years of age, with ultrasound every 3 months and diuretic nephrogram every 6 months. Long-term stable hydronephrosis also requires follow-up with staged renal nuclear scan in addition to ultrasound follow-up. In conclusion, according to the SFU classification, there is a clear tendency for spontaneous remission of hydronephrosis below grade III that appears in the fetus; ultrasound and diuretic nephrogram follow-up after birth; surgical intervention at the onset of associated symptoms or decline in renal function; and early surgical intervention for hydronephrosis above grade IV.