In the past, pediatric hydronephrosis was usually detected incidentally during abdominal ultrasound due to abdominal masses, abdominal pain, or for other reasons. However, with improved imaging techniques, advances in perinatal medicine, and the strengthening of the concept of eugenics, more and more pelvic hydrocele is detected early in the fetal period and is closely monitored and followed up until after birth. At present, precisely, pediatric pyelonephrosis is under our control for follow-up observation and diagnostic treatment, so the understanding of this disease should be changed in contrast to the previous concept, especially the choice of follow-up and timing of surgery. Fetal hydronephrosis can be classified as physiologic or pathologic. Physiological hydronephrosis can subside spontaneously, probably due to low fetal renal vascular resistance, high glomerular filtration rate and low concentration capacity, high ureteral compliance and easy expansion, immature fetal and neonatal kidney development, and transparent renal cones and medulla on ultrasonography, which can be mistaken for hydronephrosis images. In contrast, pathologic hydronephrosis is due to the presence of true obstructive factors that cause separation of the collecting system, and these obstructions do not disappear on their own; and because fetal and neonatal renal tubules are short and straight, the same obstructive factors can cause more serious consequences than in adults. Therefore, a distinction needs to be made between physiologic and pathologic hydronephrosis, with close follow-up observation and appropriate treatment. According to SFU principles, all children with fetal hydronephrosis should have an ultrasound examination about 1 week after birth, along with comparison with the last prenatal ultrasound findings to further clarify the grading of the hydronephrosis in the child. However, Wiener et al. suggest that the grading of pelvic hydrocele may differ between 48 h and 7-10 d postnatal ultrasound, and that the grading of pelvic hydrocele may be underestimated by the first ultrasound examination too early; whereas urinary tract abnormalities detected at 6 weeks postnatally are more specific. Therefore, we recommend that in fetuses with prenatal pyelocele, the initial postnatal ultrasound screening should be performed 7 to 10 d after birth and the second examination should be performed 42 d after birth, although it may be earlier in more severe cases. The fetuses are then grouped accordingly and proceed to the next step of follow-up observation. Ultrasound is the primary and preferred adjunct both in prenatal screening and in the follow-up observation of the child after birth. Although ultrasonography is limited in predicting the extent and etiology of impaired renal function, it can play a role in diagnosis and evaluation. Studies have shown that the glomeruli, tubules and cellular structures are basically normal in renal parenchyma with a thickness of 4 mm or more; in those with a thickness of 3 mm, some glomeruli and tubules are still present, but the cells are deformed and the nuclei are irregular. Ultrasound can not only clearly show the kidney structure of the fetus after 24 weeks of gestation, but also directly measure the degree of expansion of the pelvic collecting system and the thickness of the renal cortex, and it is non-invasive, simple and easy to accept and operate. Of course, there are also various examination methods such as ECT, MRU, IVP, etc. However, it should be noted that the results are for comparison purposes only, as they are affected by the young age and lack of cooperation of the child, as well as the small veins and difficulties in injection. Treatment of hydronephrosis in young infants remains controversial. It is believed that most young infants with mild to moderate hydronephrosis do not require surgery and may improve on their own during follow-up observations. However, the SFU reported that 25% of children <6 months of age with grade 3-4 hydronephrosis who had relative renal function greater than 40% still required surgery at follow-up, while those who were operated upon detection had significantly better reduction in hydronephrosis and improved pelvic emptying than those who were conservatively observed. In practical application, we mastered the selection of the timing of surgery mainly considering: (i) SFU grade 3 to 4; (ii) 3-6 months of close observation and continuous expansion of the renal pelvic collecting system; (iii) renal cortical thickness less than 5 mm; and (iv) the clear presence of obstructive factors with reference to comparative ECT and MRU and a significant decrease in GFR in a single kidney (due to the difficulty of ECT and MRU examination in practical application, it can be used as a reference). In conclusion, with the changes in diagnosis, follow-up and treatment of fetal hydronephrosis after birth, many concepts should be distinguished from the past and need to be more accurately conceptualized and standardized.