Most of the mild hydronephrosis in children will heal spontaneously, and requires regular observation and close follow-up. Some children with hydronephrosis found during fetal life may have functional, transient hydronephrosis without mechanical obstruction, and regular renal ultrasound examinations are performed 1 week, 1 month, and 3 months after birth to closely track the anterior and posterior pelvic diameters. If the renal pelvis width keeps decreasing during the postnatal review, it means that the hydronephrosis is functional and most of them are able to heal on their own. If the ultrasound indicates that the anteroposterior diameter of the renal pelvis is getting heavier during the follow-up or follow-up, most patients will not heal on their own and should undergo surgical intervention in a timely manner.