This project aims to systematically delineate the natural progression of congenital hydronephrosis diagnosed within the critical window of 0-6 months through a prospective, multicenter, observational cohort study. The focus will be on analyzing the resolution rates, progression rates, and influencing factors of hydronephrosis of varying severities based on the UTD grading system.

Congenital hydronephrosis is one of the most common congenital urinary system abnormalities in children, with a high prenatal detection rate. However, its postnatal natural course is highly heterogeneous, leading to significant controversy in clinical management regarding follow-up intensity and intervention timing. Currently, there is a lack of prospective, large-sample, multicenter natural history data in China. By establishing a standardized follow-up system and collecting high-quality clinical and imaging data, this study aims to provide high-level evidence-based medical support for developing individualized and precise clinical management strategies, thereby reducing unnecessary interventions and delayed treatment. Consequently, conducting this multicenter study holds significant clinical and scientific value.

Inclusion Criteria:

• Diagnostic Criteria: Congenital hydronephrosis is diagnosed by abdominal ultrasound examination and meets the UTD grading system criteria (Grades I-III). This is defined as an anterior-posterior renal pelvis diameter (APD) ≥4 mm during the fetal period or ≥7 mm after birth, or accompanied by calyceal dilation, renal parenchymal changes, and other manifestations.
• Informed Consent: The legal guardian voluntarily agrees to participate in the study and provides written informed consent.
• Follow-up Feasibility: The guardian commits to cooperating with the complete 3-year follow-up period, including attending regular examinations at the research center, and maintains stable contact information.

Exclusion Criteria:

• Presence of other severe congenital malformations that may affect follow-up or prognosis assessment, such as congenital heart disease, biliary atresia, spina bifida, etc.
• Secondary hydronephrosis caused by acquired factors (e.g., urinary system tumors, stones, trauma) or well-defined genetic metabolic diseases.
• Having received interventional treatments prior to enrollment, such as surgical procedures related to hydronephrosis (e.g., pyeloplasty) or pharmacological interventions (e.g., long-term use of diuretics).
• Severe underlying diseases that preclude tolerance for long-term follow-up, such as severe infections, respiratory failure, or renal failure (e.g., glomerular filtration rate < 30 ml/min/1.73m²).
• Inability of the legal guardian to cooperate due to mental illness, cognitive impairment, or refusal to comply with follow-up schedules and data collection requirements.

Infants diagnosed with congenital hydronephrosis by abdominal ultrasound within 0-6 months after birth, meeting the UTD classification criteria (Grades I-III).