A Thesis Submitted to the School of Graduate Studies in Partial Fulfilment of the Requirements for the Degree Master of Science in Clinical Epidemiology (Health Research Methodology) / Introduction: There is no single agreed-upon tool to forecast hydronephrosis obstruction and surgical risk in the literature. Previous scoring systems have relied on renal scan parameters for this task, which are not performed or read consistently across hospitals. We aim to determine the utility of a hydronephrosis obstruction early scoring system in identifying severe UPJO-like hydronephrosis suggestive of surgery. Methods: We reviewed a prenatal hydronephrosis database (2008-23) to identify patients with unilateral UPJO-like hydronephrosis. Patients with vesicoureteral reflux (VUR), megaureter, or other anomalies were excluded. We collected data on the first ultrasound and surgical outcome for the Society for Fetal Urology (SFU) grade, anteroposterior pelvic diameter (APD), and percent difference in renal length – the renal length index (RLI). I undertook a multivariable logistic regression to determine the predicted value of these sonographic measurements. Model development and internal validation were performed, and model performance (calibration and discrimination) was reported. Results: Of the 465 UPJO-like hydronephrosis patients, 76.1% (354) were male and had a mean age at baseline of 3.8 ± 3.8 months. 34.8% (162) had an obstructive renal scan and 27.7% (129) underwent pyeloplasty. The developed model to predict surgery using the aforementioned variables with excellent model performance (AUC 0.92). A simplified model using SFU 3-4, APD > 13mm and RLI >12% also predicted surgery with great model performance (AUC 0.89) and improved clinical user-friendliness. Conclusion: Our clinical predictive model can accurately predict surgery in both derivation and validation cohorts of patients with antenatal UPJO-like hydronephrosis. This non-invasive tool can greatly aid in patient counselling. / Thesis / Master of Health Sciences (MSc) / Hydronephrosis (HN) is a kidney condition prevalent in 1-5% of pregnancies, typically detected through third-trimester ultrasounds. Although 70% of cases resolve spontaneously, the remaining 30% are at higher risk of renal failure if unaddressed. Traditional scoring systems have depended on renal scans, which expose patients to radiation and suffer from inconsistency across healthcare facilities. Using data from pediatric patients with HN, this study identified key predictive factors for surgical management such as severe dilation of kidneys associated with thinning, affected kidney length more than 12% longer than healthy kidney and anteroposterior diameter of the renal pelvis > 13mm. A new scoring system based solely on renal ultrasound parameters was formulated. This system establishes different risk categories, which have been validated for use in daily clinical practice. This offers urologists a reliable, radiation-free tool to gauge individual surgical risks and provides a foundation for tailored patient counselling.