With the increase of health care and screening during pregnancy, more and more fetal hydronephrosis is being detected, which causes anxiety to some expectant fathers and mothers because most people do not know enough about it. In fact, most of the hydrocele appearing in the fetus has the tendency to resolve or disappear on its own. Fetal hydronephrosis is defined as a significant separation of the collecting system of the renal pelvis during fetal life. Most fetal hydronephrosis is physiological, and we call it physiological hydronephrosis. This hydronephrosis can often change with fetal development, renal physiology and ureteral tissue anatomy, and spontaneously resolve before the end of pregnancy or in the first year after birth without further intervention. According to the latest statistics, more than 50% of hydronephrosis occurring in fetal life can resolve or disappear on their own. In contrast, fetal hydronephrosis due to truly obstructive factors is pathological and can result in serious consequences such as decreased renal function and urinary tract infection. Incomplete obstruction of the upper urinary tract during embryonic development will grow significantly as the fetus grows up, leading to decreased renal function. If complete obstruction of the upper urinary tract will lead to a non-functional kidney with severe cystic dysplasia, the result of this damage is similar to congenital renal dysplasia. 1. Diagnostic methods of fetal hydronephrosis Prenatal examination: Ultrasound is preferred. The degree of renal pelvis dilatation is mainly expressed by the anterior-posterior diameter (APD) of the renal pelvis. It is usually considered that an APD >6 mm before 20 weeks of gestation, an APD >8 mm at 20-30 weeks of gestation, and an APD >10 mm after 30 weeks of gestation can be diagnosed as hydronephrosis. Grading method of hydronephrosis: mild hydronephrosis, APD<15mm, normal renal calyces; moderate hydronephrosis, APD>15mm, dilated renal calyces; severe hydronephrosis, APD>15mm, dilated renal calyces and thinning renal parenchyma. Further postnatal examination methods: ultrasound, such as intravenous pyelogram (IVP), diuretic nephrography (ECT) and other laboratory tests. The diagnosis of most hydronephrosis depends on repeated ultrasound and other tests and changes in indicators during long-term follow-up to decide and whether surgical intervention is needed. 2.Fetal intervention In most cases, the diagnosis of fetal hydronephrosis is made by ultrasound examination in the fourth to sixth month of pregnancy, after which regular (semi- to monthly) follow-up examinations should be performed, and ultrasound examination should be performed again before full term. For severe malformations such as bilateral polycystic kidneys and renal dysplasia with reduced amniotic fluid found during regular checkups, the pregnancy is terminated after prenatal specialist consultation. Generally, the effect of surgical treatment after the birth of hydronephrosis is more certain and does not affect the growth and development of the child in the future, so termination of pregnancy is not required under close follow-up by pediatric urologists. Only a small percentage of hydrocele requires prenatal therapeutic intervention and should be performed by an experienced provider. In the case of long-term fetal bladder drainage, the indications for the procedure are: a male fetus between 4 and 6 months of gestation, the presence of severe bilateral ureterorenal hydrops, urinary indicators that meet the criteria, and progressive amniotic fluid reduction. 3.Post-birth treatment After birth, for a few symptomatic children such as abdominal masses, urinary tract infections, or young children complaining of recurrent abdominal pain, surgery is required as early as possible. The management of asymptomatic children after birth has become a new issue for pediatric urologists: which children can resolve on their own? Which ones need surgery? Generally speaking, it is very unlikely that a kidney with a pelvic separation of less than 10 mm in the second trimester will require surgery, but it is important to be followed closely by a pediatric urologist, and if the pelvic separation increases progressively and renal function decreases during the follow-up, surgical intervention will be required. If the renal pelvis separation is greater than 20 mm on postnatal ultrasound, it should be considered a high risk factor for surgery and IVP and ECT should be performed as soon as possible to decide whether to take surgical treatment as soon as possible. Close postnatal follow-up of hydronephrosis present in the fetus has two purposes: to preserve renal function and to avoid unnecessary surgical interventions. In asymptomatic children, the most definitive method to determine the need for surgery is the diuretic nephrogram (ECT). In a diuretic nephrogram, surgery is indicated if there is a significant decrease in renal function, which is the accepted standard if the fractional renal function is below 35%. If a decrease in renal function of 5% or more is found in the dynamic diuretic nephrogram follow-up examination, it is considered that a decrease in renal function also requires surgical intervention. 4.Frequency of follow-up and follow-up of stable hydronephrosis Too high a frequency of follow-up may lead to a decrease in parental compliance and discontinuation of follow-up, while too low a frequency may lead to a significant decrease in renal function. Therefore, we recommend close follow-up up to two years of age, with an interval of three months, or less in special cases. Our clinical statistics show that about 70% of simple hydronephrosis procedures occur within 2 years of age, so we believe that 2 years of age is a critical period for the progression of hydronephrosis and close follow-up is necessary. after 2 years of age, the frequency of follow-up can be reduced to once every 6 months for ultrasound and once a year for diuretic nephrogram. In the actual follow-up work, we encountered many long-term stable hydronephrosis with no progression and no significant decrease in hydronephrosis. For these children, ultrasonography-based follow-up can be used. Statistics show that if two consecutive ultrasonographic examinations reveal an increase in the degree of hydronephrosis in the kidney, the diuretic nephrogram must be accompanied by a decrease in renal function of the kidney by more than 5%.