Background Predicting the need for early postnatal surgery in severe fetal unilateral hydronephrosis remains challenging. Traditional fetal urinary electrolyte thresholds, derived from end-stage renal disease, lack sensitivity for early impairment. This study aimed to develop and validate a novel multivariate index integrating anatomical and functional parameters to stratify surgical risk.Methods A single-center retrospective study was conducted on 20 fetuses with isolated unilateral ureteropelvic junction obstruction who underwent ultrasound-guided renal pyelocentesis. Prenatal sonographic data and fetal urinary electrolytes (Na⁺, Cl⁻, K⁺, Ca²⁺) were analyzed. A fetal renal severity index (FRSI) was constructed using principal component analysis (PCA) on five standardized variables. Postnatal outcomes were categorized into early surgery (< 3 months) and late group. Diagnostic performance was evaluated using ROC curve analysis.Results The PCA-derived FRSI, capturing 55.9% of total variance, integrated anteroposterior diameter with tubular electrolyte concentration. FRSI scores were significantly higher in the early surgery group (median > 0) versus the late group (median < 0, p = 0.01). For predicting the need for surgery within 3 months, the FRSI achieved an AUC of 0.86, significantly outperforming urinary sodium alone (AUC = 0.73). Fetuses requiring early intervention were diagnosed significantly earlier in gestation (19.0 ± 4.73 vs. 26.0 ± 4.63 weeks, p = 0.01). An FRSI cutoff − 0.12 yielded 100% specificity for identifying cases necessitating urgent postnatal surgery.Conclusion FRSI provides superior risk stratification for surgical urgency in prenatal unilateral hydronephrosis compared to single indicators. It offers a potential tool to optimize prenatal counseling and postnatal management timing.