About 90% of pediatric hydronephrosis is due to congenital developmental malformations that result in excessive urine in the kidney. Most of them are caused by congenital obstruction at the junction of the renal pelvis and ureter and obstruction at the end of the ureter, while hydronephrosis caused by obstruction at the entrance of the ureter to the bladder is also seen, but is relatively rare. Congenital hydronephrosis can be detected during pregnancy if regular obstetric examinations are performed. Therefore, regular maternity checkups during pregnancy can monitor the changes of the condition for timely treatment. Of course, there is a special case where a small percentage of children have hydronephrosis due to polyps in the connection between the renal pelvis and ureter. Such children may not have hydronephrosis at the time of delivery, but as the polyp grows, the edema around the polyp causes obstruction in the area where the renal pelvis and ureter join, and only after birth does the dilated renal pelvis become more dilated, and even symptoms such as vomiting, back pain, and hematuria appear. The symptoms usually do not appear until the child is a little older, three or four years old or even six or seven years old. When children have symptoms, they may already have kidney damage, and the damaged kidney function will not be restored, so this situation requires timely surgery. What is the decision of when to operate when my child has hydronephrosis? The first thing to consider when to operate for hydrocele in children is the severity of the condition. As mentioned above, if the hydronephrosis has caused kidney function damage, surgery should be done immediately and not delayed. Generally speaking, whether to operate is mainly considered in these aspects: first, the kidney function is less than 40% to do surgery, if the kidney function is impaired has not been operated, may be after a few months to check the kidney function is only 30%, or even 10%; second, there is obvious anatomical evidence, that is, through ultrasound, nuclear magnetic examination, etc., clearly found that the renal pelvis and ureteral junction obstruction, you can operate; third, the child with vomiting, abdominal pain, hematuria and other symptoms, surgery can also be considered. Can I delay the surgery for my child with hydronephrosis? Can the surgery be done later? This is a question that many parents are concerned about. Indeed, if it is said that the kidney function is not below 40%, the child is still very young less than two months, or even only one month, the renal pelvis is not heavily dilated, and the kidney cortex is not thin, then surgery can be temporarily not done. The nephrogram is not 100% accurate because the kidneys of newborns are not yet well developed and the nephrogram does not necessarily reflect the true kidney function. There may be a small amount of error in assessing the kidney function of newborns according to the international kidney chart test results. At present, the initial domestic standard is to operate if the kidney function is below 40%, but if parents want to wait a little, they can only monitor the kidney function regularly by nuclear examination method. Now the kidney function is below 40% parents do not want to do surgery, two or three months later to do another check, if the kidney function is even lower, the rate of progress is more than 10%, it is necessary to do surgery; if this check kidney function is 40%, the next time is also 40%, this situation is temporary not to do surgery is also possible.