Fetal renal severity index for predicting early surgical intervention in unilateral hydronephrosis: a retrospective case series with principal component analysis

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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. unilateral hydronephrosis fetal pyelocentesis fetal urine electrolytes fetal renal severity index principal component analysis anteroposterior diameter Figures Figure 1 Figure 2 Figure 3 Figure 4 1. Introduction Antenatal hydronephrosis is the most common fetal urinary tract abnormality, affecting approximately 1–2% of all pregnancies [ 1 ] . Mild and moderate cases often show spontaneous resolution during follow-up, but for severe ones, impaired urinary drainage may lead to elevated renal pelvic pressure, which progressively compresses the renal parenchyma, resulting in gradual kidney damage that may ultimately culminate in renal atrophy and loss of function [ 2 ] . Given the compensatory capacity of the contralateral healthy kidney, prenatal intervention is seldom pursued in unilateral cases [ 3 ] . However, even when obstruction is relieved postnatally in severe cases, evidence indicates that renal interstitial fibrosis may persist, with little improvement in renal morphology or function [ 4 ] . As fetal surgery advances rapidly, severe unilateral obstructive hydronephrosis may eventually become a candidate for in utero intervention. Critical challenges in this context include accurate assessment of fetal renal function, consideration of intrauterine treatment options, prediction of postnatal renal outcomes, and determination of optimal timing for postnatal surgery. Although fetal urine electrolytes represent one of the earliest biomarkers used to evaluate fetal renal status [ 5 ] , their utility in unilateral hydronephrosis remains underexplored [ 6 ] . This study retrospectively analyzes renal pelvic urine electrolyte profiles, prenatal ultrasound findings, and postnatal outcomes in fetuses with unilateral hydronephrosis, aiming to evaluate the predictive value of fetal electrolytes for surgical intervention and to summarize clinical management experience. 2. Materials and methods 2.1 Study design and participants This single-center study was conducted in the Fetal Medicine Department of Xiangya Hospital, Central South University. This retrospective study was in accordance with the ethical standards of the institutional and national research committee and Helsinki Declaration. Clinical data were collected from patients diagnosed with fetal unilateral hydronephrosis of the Society of Fetal Urology (SFU) grades 4 who underwent fetal pyelocentesis between September 2019 to July 2023. Postnatal outcomes were obtained via medical records and telephone follow-up. During the study period, pyelocentesis was performed on 37 fetuses. After applying the inclusion and exclusion criteria, 20 cases were ultimately included. Inclusion criteria were: ①Prenatal ultrasound indicating fetal unilateral hydronephrosis SFU grades 4; ②Performance of fetal pyelocentesis with complete electrolyte analysis of the urine sample; ③Availability of a detailed fetal urinary system ultrasound examination prior to the procedure; ④Blood tests for renal function and electrolytes within 48 hours after birth; ⑤Successful follow-up. Exclusion criteria included: ①Bilateral hydronephrosis; ②Concurrent other urinary system malformations; ③ Incomplete clinical data or loss to follow-up; ④abnormal karyotypes or CMA results. 2.2 Pre-procedure renal ultrasound examination Detailed fetal renal ultrasound was performed and included the following assessments: ①SFU grading; ②Anteroposterior diameter(APD) of the renal pelvis; ③Renal cortical thickness(RCT); ④ Status of the ureter and bladder; ⑤Status of the contralateral healthy kidney; ⑥The deepest vertical pocket (DVP) and amniotic fluid index(AFI). The APD was measured on a transverse image at the point of maximal pelviectasis, defined as the distance between the two parenchymal margins at the renal hilum [ 7 ] . RCT was measured as the distance from the corticomedullary junction to the renal capsule [ 8 ] . All fetuses were classified according to the SFU grading system [ 9 ] : Grade 0 - normal kidney with no hydronephrosis; Grade 1 - slightly dilated renal pelvis without caliectasis; Grade 2 - moderately dilated pelvis with mild caliectasis; Grade 3 - marked dilatation of the pelvis and calyces, with normal or slightly thinned renal parenchyma; Grade 4 - severe dilatation of the pelvis and calyces with thinning of the renal parenchyma. 2.3 Measurement of fetal urine electrolytes In this study, fetal pyelocentesis was reserved for SFU Grade 4 cases with progressive APD where parents strongly sought more definitive prognostic information to guide perinatal management. Written informed consent was obtained from the parents prior to puncture, which clearly stated that the intervention was intended for prognostic rather than therapeutic purposes, the associated risks and benefits had been thoroughly communicated. Under ultrasound guidance, fetal urine was aspirated from the affected renal pelvis using a biopsy needle (20G x 15cm, Batch No. DSN6847, Beijing Demaite Technology Development Co., Ltd.) via a transabdominal approach through the maternal abdominal wall and uterus into the fetal renal pelvis. The aspirated urine volume was recorded, and a 4ml sample was collected for measurement of sodium (Na + ), potassium (K + ), chloride (Cl − ), calcium (Ca 2+ ), urea, creatinine, and uric acid levels. 2.4 Neonatal outcomes and follow-up Data on gestational age (GA) at birth, sex, birth weight, and postnatal serum renal function and electrolytes were extracted from the medical records. Telephone follow-up was conducted in November 2025 to obtain the timing and type of any surgical intervention, as well as the children's current renal function status. The surgical indications were inquired in detail. 2.5 Statistical analysis Statistical analyses were conducted using R software (version 4.5.1). Continuous variables are presented as mean ± standard deviation (SD) or median [interquartile range, IQR] depending on data distribution. Categorical variables are expressed as frequencies and percentages (n, %). Group comparisons between the “early surgery” and “late” groups were performed using the Wilcoxon rank-sum test for continuous variables and Fisher’s exact test for categorical variables. To quantify the overall severity of renal impairment, a composite Fetal Renal Severity Index (FRSI) was constructed through principal component analysis (PCA). Five parameters were incorporated into the dimensionality reduction model: APD as an anatomical marker, along with fetal urinary Na⁺, Cl⁻, K⁺, and Ca²⁺ as functional markers. Prior to analysis, all variables were standardized by Z‑score normalization to eliminate scale differences. The first principal component (PC1) was extracted as the FRSI score and calibrated so that higher values reflect greater severity—integrating pronounced anatomical dilation with evidence of tubular salt-wasting. The predictive performance of the FRSI and individual markers was evaluated using receiver operating characteristic (ROC) curve analysis, from which the area under the curve (AUC), sensitivity, and specificity were derived. The optimal cutoff value was determined using the Youden index. Univariate logistic regression was applied to calculate the odds ratio (OR) for surgical urgency. A two‑sided p‑value < 0.05 was considered statistically significant. 3. Results 3.1 Study population Initial screening identified 31 cases with severe unilateral hydronephrosis that underwent fetal pyelocentesis. After excluding 4 cases with confounding anatomical anomalies and an additional 7 cases with incomplete clinical data or loss to follow-up, 20 fetuses were included in the final analysis. The study flowchart is shown in Fig. 1 . The maternal age was 30.60 ± 4.96 years old, gestational age at detection was 21.82 ± 5.74 weeks, the gravidity was 3 [IQR: 2, 3] and parity was 1 [IQR: 0.75, 2]. 3.2 Fetal renal ultrasound and urine electrolyte results Hydronephrosis was right-sided in 5 cases and left-sided in 15. The early surgery group and late surgery group were well-balanced regarding maternal age, gravidity, parity, DVP, AFI, SFU grade, APD, fetal urine Na+, K+, Cl-, Ca2+, urea, creatinine, and uric acid levels. A statistically significant difference was observed in the GA at first detection. The baseline characteristics of the study cohort, stratified by postnatal surgical urgency, are detailed in Table 1 . No procedure-related complications were recorded. Table 1 Baseline characteristics of the study population GA at detection (weeks), mean (SD) total cohort (n = 20) early ( 3m) (n = 8) p value 21.82 (5.74) 19.05 (4.73) 25.98 (4.63) 0.01 * affected side 1 left 15 (75.0%) 9 (75.0%) 6 (75.0%) right 5 (25.0%) 3 (25.0%) 2 (25.0%) DVP (mm), mean (SD) 472 (96.88) 485 (84.69) 452.5 (116.1) 0.22 AFI (mm), mean (SD) 155.05 (34.34) 156.33 (35.09) 153.13 (35.48) 0.85 APD (mm), median [Q1, Q3] 27 [19.3, 38.6] 29.5 [23, 46.5] 25 [19, 28] 0.16 RCT (mm), median [Q1, Q3] 2.55 [2.3, 2.92] 2.55 [2.42, 2.9] 2.6 [2.3, 2.92] 0.97 GA at puncture (weeks), median [Q1, Q3] 33.64 [29.32, 34.28] 33.07 [28.93, 34.17] 33.64 [32, 34.28] 0.70 fetal urine sample volume (ml), median [Q1, Q3] 26.5 [14.5, 56] 40 [20.75, 72.5] 15.5 [12.25, 30.5] 0.08 Na + (mmol/L),median [Q1, Q3] 42.4 [36.8, 44.75] 42.9 [41.6, 50.15] 39.35 [35.1, 42.7] 0.09 Cl − (mmol/L), median [Q1, Q3] 37.85 [32.25, 40.23] 40.2 [33.75, 41.62] 34.85 [31.22, 37.82] 0.06 K + (mmol/L) , median [Q1, Q3] 2.47 [1.97, 3.12] 2.55 [2.24, 3.12] 2.07 [1.8, 3.02] 0.37 Ca 2+ (mmol/L),median [Q1, Q3] 0.4 [0.28, 0.64] 0.42 [0.28, 0.77] 0.32 [0.26, 0.44] 0.33 urea (umol/L),median [Q1, Q3] 9.19 [8.11, 10.88] 9.25 [8.2, 11.16] 9.13 [8.11, 9.82] 0.64 creatinine (umol/L),median [Q1, Q3] 203.2 [160.4, 284.3] 219.85 [173.82, 349.6] 185.15 [158.5, 247.87] 0.33 uric acid (umol/L), median [Q1, Q3] 640.5 [597.68, 791.95] 698.9 [619.3, 864.75] 603.1 [597.68, 651.45] 0.23 FRSI, median [Q1, Q3] -0.17 [-0.69, 0.41] -0.61 [-1.13, -0.35] 0.23 [-0.11, 0.64] 0.01 * * p <0.05 3.3 Neonatal outcomes Postnatal characteristics, surgical interventions, and follow-up data are summarized in Table 2 . Neonates in the early surgery group were delivered at a significantly earlier gestational age compared to those in the late group. Indications for postnatal surgery were APD > 3 cm and/or progression of hydronephrosis during follow-up. No significant differences were observed between the two groups in terms of delivery mode, birth weight, sex distribution, or postnatal serum electrolyte and renal function profiles. All patients (12/12, 100%) in the early surgery group underwent pyeloplasty, whereas surgery was performed in 75% (6/8) of the late surgery group, with the remaining 25% (2/8) managed conservatively ( p = 0.15). Among those who underwent surgery, the timing of intervention differed significantly, the early group required decompression at a median age of 1.18 months (IQR: 1.11–2.04), while the late group at a median age of 12.27 months (IQR: 4.82–34.46) ( p < 0.01). The median follow-up duration was comparable between the two groups (28.5 months vs. 38.5 months, p = 0.37), indicating a sufficient observation period for both cohorts. At the last follow-up in December 2025, renal function was reported to be within normal limits for all children. However, due to inconsistent methodologies (e.g., ultrasound, diuretic renography, MR urography) used across different follow-up hospitals, standardized indicators for evaluating surgical efficacy were not available. Table 2 Neonatal outcomes and biochemical characteristics stratified by surgical urgency GA at delivery (weeks),median [Q1, Q3] total cohort (n = 20) early ( 3m) (n = 8) p value 37.22 [35.86, 38.32] 36.07 [35.43, 36.89] 38.64 [38.21, 39.68] 0.01 * delivery mode 1 vaginal delivery 4 (20.0%) 2 (16.7%) 2 (25.0%) cesarean section 16 (80.0%) 10 (83.3%) 6 (75.0%) birth weight (g),median [Q1, Q3] 3180 [3062.5, 3513.75] 3110 [3062.5, 3247.5] 3417.5 [3071.25, 3750] 0.33 neonate sex 1 female 2 (10.0%) 1 (8.3%) 1 (12.5%) male 18 (90.0%) 11 (91.7%) 7 (87.5%) neonatal blood sample Na + (mmol/L),median [Q1, Q3] 142.85 [141.38, 145.08] 143.75 [141.68, 144.65] 141.95 [140.7, 147.12] 0.82 Cl − (mmol/L),median [Q1, Q3] 108.4 [106.48, 110.85] 108.8 [108.25, 110.85] 106.95 [104.7, 110.35] 0.32 K + (mmol/L),median [Q1, Q3] 4.52 [4.4, 4.76] 4.48 [4.36, 4.65] 4.8 [4.41, 5.3] 0.18 Ca 2+ (mmol/L), median [Q1, Q3] 2.15 [2.12, 2.28] 2.13 [2.12, 2.27] 2.24 [2.04, 2.29] 0.85 urea (umol/L),median [Q1, Q3] 3.41 [2.84, 4.14] 3.41 [2.84, 3.8] 3.54 [2.84, 4.63] 0.56 creatinine (umol/L),median [Q1, Q3] 83.75 [73.72, 95.8] 74.8 [70.9, 88.42] 92.6 [84.23, 98] 0.07 uric acid (umol/L),median [Q1, Q3] 343.15 [304.7, 423.45] 321.10 [290.2, 385.42] 379.80 [342.7, 430.45] 0.11 surgery performed 0.15 yes 18 (90.0%) 12 (100%) 6 (75.0%) no 2 (10.0%) 0 (0%) 2 (25.0%) age at surgery (months) 0.01 * median [Q1, Q3] 2.40[1.13, 5.72] 1.18 [1.11, 2.04] 12.27 [4.82, 34.46] missing 2 (10.0%) 0 (0%) 2 (25.0%) follow-up duration (months) median [Q1, Q3] 35 [16, 56.75] 28.5 [14.25, 57] 38.5 [31, 49.25] 0.37 * p <0.05 3.4 Correlation analysis of clinical and biochemical parameters To explore the relationships between anatomical markers, biochemical indices, and clinical outcomes, Pearson correlation analysis was conducted (Fig. 2 ). Fetal urinary metabolites (creatinine and uric acid) demonstrated moderate to strong positive correlations with urinary electrolyte levels, indicating potential shared renal transport or handling pathways. Renal cortical thickness was inversely correlated with both serum creatinine (r = − 0.53) and urinary creatinine (r = − 0.46). Notably, the gestational age at initial diagnosis correlated positively with the postnatal timing of surgery (r = 0.61), suggesting that earlier prenatal detection may predict a need for earlier surgical intervention. In contrast, none of the individual urinary electrolytes (Na⁺, Cl⁻, K⁺, or Ca²⁺) showed a strong linear correlation (|r| > 0.4) with the timing of surgery. 3.5 Construction and validation of FRSI Principal component analysis was performed on five standardized parameters: APD and fetal urinary levels of Na⁺, Cl⁻, K⁺, and Ca²⁺. As illustrated in the loading plot (Fig. 3 a), the first principal component (PC1) explained 55.9% of the total variance in the dataset, effectively representing the dominant pathological pattern. The loading vectors indicate the relative contribution of each variable to PC1. Fetal urinary chloride and sodium exhibited the strongest loadings (highlighted by red/orange vectors), supporting the role of tubular salt-wasting as a hallmark of severe obstruction. Prenatal APD and urinary calcium also loaded positively on PC1, demonstrating that the resulting FRSI integrates both anatomical dilation and functional tubular loss into a single continuous score. The clinical validity of the FRSI was further assessed by comparing scores between outcome groups (Fig. 3 b). The FRSI was significantly higher in the early-surgery group (median > 0) than in the late-surgery group (median < 0) (p< 0.05). Most fetuses in the late-surgery group had negative scores, reflecting preserved tubular function and less pronounced anatomical dilation relative to the cohort average. In contrast, the early-surgery group clustered in the positive range, confirming that elevated FRSI values are strongly linked to the need for prompt postnatal intervention. 3.6 Analysis of the predictive value of FRSI for early surgery The predictive performance of FRSI was compared against fetal urinary sodium to validate the clinical utility. As demonstrated in the ROC analysis (Fig. 4 a), an AUC of 0.86 was achieved by the FRSI (red line), significantly outperforming urinary sodium, which yielded an AUC of 0.73 (grey line). GA at initial detection of hydronephrosis was also examined as a prognostic factor. As shown in Fig. 4 b, fetuses in the early surgery group were identified significantly earlier compared to the late group (19.0 ± 4.73 vs 26.0 ± 4.63 weeks, p = 0.01). 4. Discussion In the management of fetal urological anomalies, distinguishing unilateral hydronephrosis cases that are approaching a critical threshold remains a formidable challenge for perinatal medicine. Although current guidelines favor conservative management for asymptomatic unilateral hydronephrosis [ 10 ] , concerns persist among clinicians regarding a “watchful waiting” approach in high‑risk cases [ 11 ] . The number of nephrons in humans peaks at approximately 36 weeks of gestation and does not increase after birth. Persistent severe obstruction in utero, such as in ureteropelvic junction obstruction (UPJO), may cause dysplastic changes or cystic degeneration of the metanephric tissue, leading to permanent and irreversible renal impairment [ 12 ] . Conservative observation, in effect, merely shifts the window of renal injury from prenatal to postnatal period. Postnatal surgical intervention, such as pyeloplasty, can only salvage existing function, not reverse the loss of nephrons sustained during gestation [ 13 ] . Fetal intervention has traditionally been limited to a few life-threatening anomalies, such as posterior urethral valves, owing to high procedural risks and complications. Yet, ongoing advances in fetoscopy, ultrasound-guided techniques, and perinatal anesthesia are reshaping its risk–benefit balance. With expanding indications and reduced complications in the era of fetal therapy [ 14 ] , unilateral hydronephrosis merits reevaluation to improve risk stratification and intervention timing. No procedure-related maternal or fetal complications were observed in this study. This favorable safety profile is likely attributable to rigorous patient selection, standardized procedural protocols, and the expertise of the multidisciplinary fetal intervention team. These findings suggest that, in appropriately selected cases, fetal pyelocentesis offers an acceptable safety margin, supporting its potential role in managing carefully stratified severe unilateral hydronephrosis. Our analysis reaffirms established demographic patterns, with a predominance of male and left-sided cases, likely attributable to spatiotemporal differences in ureteric bud development [ 15 ] . Enrolled fetuses were screened by prenatal amniocentesis to exclude known genetic abnormalities. By excluding cases of genetic renal dysplasia, we minimized confounding from primary genetic defects, supporting the interpretation that the observed biochemical changes and elevated index values resulted from secondary pathophysiological effects of mechanical urinary obstruction. Nevertheless, the primary concern for clinicians remains the prenatal prediction of renal prognosis [ 16 ] . Our data reveal a harsh reality: postnatal serum markers are subject to a significant "compensatory masking effect." Owing to the robust filtration capacity of the contralateral healthy kidney [ 17 ] , neonatal serum creatinine and electrolyte levels may remain within normal ranges even when the affected kidney has sustained significant damage [ 18 ] . This systemic biochemical "illusion" renders serum markers of limited utility for early warning in unilateral disease [ 19 ] , compelling us to shift our focus to a more specific medium reflecting the local pathophysiological state of the obstructed kidney—fetal urine. Urine collected directly from the affected renal pelvis is unaffected by the contralateral healthy kidney, thereby providing an earlier and more accurate reflection of local pathophysiological changes than systemic serum biomarkers [ 20 ] . Under normal physiological conditions, plasma electrolytes are filtered through the glomerulus to form the initial filtrate [ 21 ] . The proximal tubule reabsorbs Na⁺, K⁺, Cl⁻, HCO 3 − and other solutes isosmotically. Subsequently, the loop of Henle actively reabsorbs Na⁺ and Cl⁻. The distal tubule further reabsorbs Na⁺, Cl⁻, and Ca²⁺, while the collecting duct, under the regulation of hormones like aldosterone and antidiuretic hormone, performs the final fine-tuning of Na⁺, K⁺, H⁺, and water balance, ultimately forming the final urine [ 22 , 23 ] . In the setting of mechanical obstruction, the elevated intrapelvic pressure leads to dilation and deformation of the renal tubules, causing epithelial cell damage and a reduced responsiveness to antidiuretic hormone [ 24 ] . Impairment in water reabsorption occurs, and the concentrating function is the earliest to be compromised [ 25 ] . If obstruction persists, sustained high pressure compromises glomerular capillary integrity, disrupts the filtration barrier, and promotes interstitial inflammation and fibrosis [ 26 ] . Ultimately, glomeruli undergo sclerosis, effective filtration declines, the kidney loses its functions of reabsorption, secretion, concentration, dilution, and acidification entirely. Consistent with this pathophysiology, the concentrating ability of renal tubules is impaired early in obstructive disease [ 27 ] . Consequently, urinary electrolyte levels may paradoxically decrease rather than increase, reflecting tubular dysfunction that precedes overt glomerular failure. While fetal urinalysis has long been viewed as a promising diagnostic window [ 28 ] , the limitations of traditional assessment criteria were clearly demonstrated in this study. Conventional thresholds, such as fetal urinary Na + < 100 mmol/L, Cl − < 90 mmol/L, are largely derived from end-stage renal disease and proved insensitive in our cohort. Even in cases where the APD reached 40.4 mm and the cortical thickness was measured 2.1 mm, urinary electrolyte levels remained within traditionally defined "normal ranges", despite postnatal confirmation of renal impairment. This observation suggests that the aforementioned reference criteria may have limited utility in predicting future kidney function [ 29 ] . Single-parameter assessments, whether biochemical or sonographic, fail to capture signals of renal impairment during the "subclinical window," leading to potential underestimation of disease severity [ 30 ] . To address these limitations, this study innovatively employed principal component analysis to construct the FRSI. The core value of FRSI lies in its integration of a “structural–functional coupling” framework: APD quantifies the physical intensity of obstruction, while urinary electrolytes capture the metabolic response of renal tubules under sustained pressure. Sustained high pressure induces ischemia‑reperfusion injury and downregulates key transporters in tubular epithelial cells [ 31 ] , resulting in subtle electrolyte wasting before overt renal failure occurs [ 32 ] . The FRSI consolidates these early pathological signals through a weighted multivariate algorithm. Our findings demonstrate that the FRSI was significantly higher in the early‑surgery group (< 3 months) than in the late‑surgery group, indicating that an elevated FRSI score not only reflects the current degree of renal compromise but also predicts a progressive pathophysiological trajectory—a feature that no single anatomical or biochemical marker can independently delineate. The translational potential of the FRSI is particularly promising, especially its high specificity, which offers strong support for clinical decision‑making. An optimal cutoff of − 0.12 was identified; an FRSI above this threshold achieved 100% specificity for predicting the need for surgery within the first three months of life. As a quantitative tool, the FRSI aligns with the pathophysiological understanding of early tubular injury. For fetuses with high FRSI scores, prenatal counseling should be more proactive, including shortened ultrasound surveillance intervals and early postnatal surgical evaluation to mitigate ongoing renal deterioration. The introduction of this index not only empowers clinicians to more accurately identify high-risk fetuses requiring early surgical intervention, thus optimizing treatment decision-making, but also opens new avenues for research in fetal hydronephrosis. It emphasizes the importance of a comprehensive, multi-parameter assessment strategy. We acknowledge several limitations in this study. First, fetal pyelocentesis is invasive and introduces selection bias, as only severe cases requiring this procedure were included; thus, findings may not extend to mild or moderate hydronephrosis. Second, ethical restrictions precluded obtaining urine from healthy fetuses, so no control group was available to define normal fetal urinary electrolyte ranges. Third, the FRSI reflects only a single time point and cannot track dynamic renal changes. Sequential sampling, while theoretically informative, is clinically unsafe due to risks such as membrane rupture or infection. Finally, Surgical decisions, although protocol‑based, could also be influenced by clinician judgment or parental concerns in borderline cases. Therefore, Research in this area remains challenging. Future work should prioritize prospective cohorts, standardized renal assessment, validation of the FRSI in larger populations, and exploration of its utility for guiding fetal intervention. Conclusion FRSI provides better risk assessment for surgical urgency in prenatal unilateral hydronephrosis than single indicator, and holds potential as a new tool to guide prenatal counseling and postnatal surgical timing. Abbreviations PCA principal component analysis FRSI fetal renal severity index SFU society of Fetal Urology APD anteroposterior diameter RCT renal cortical thickness DVP the deepest vertical pocket GA gestational age SD standard deviation OQR interquartile range ROC receiver operating characteristic AUC the area under the curve OR the odds ratio Declarations Ethics approval and consent to participate This retrospective study was in accordance with the ethical standards of the institutional and national research committee and Helsinki Declaration. This study protocol was reviewed and approved by the Ethics Committee of Xiangya Hospital, protocol code 2025091320, date of approval was Sep. 1st, 2025. Informed consent was obtained from all subjects involved in the study. Consent for publication Not applicable. Data Availability All data generated or analyzed during this study are available from the corresponding author on reasonable request. Competing interests The authors declare no conflict of interests. Funding sources This research was funded by HSTD | Natural Science Foundation of Hunan Province (Grant number 2023JJ40951); HSTD | Natural Science Foundation of Hunan Province (Grant number 2026JJ50626); Science and Technology Project of Changsha Science and Technology Bureau (Grant No. kq2502061). Authors' contributions Conceptualization, Wenyan Jian, Ruojin Yao, Yanhua Zhao, and Dewei Guo; methodology,Wenyan Jian and Dewei Guo; software, Mi Pei, Wenjing Yong, Gang Chen; validation, Yimei Fu, Ronghui Tang and Fuyan Tan; formal analysis, Mi Pei, Wenjing Yong, Gang Chen and Wenyan Jian; investigation, Ruojin Yao, Yanhua Zhao and Yimei Fu; resources, Wenyan Jian, Ruojin Yao, Yanhua Zhao and Dewei Guo; data curation, Mi Pei, Wenjing Yong, Gang Chen, Ronghui Tang and Fuyan Tan; writing—original draft preparation,Wenyan Jian and Dewei Guo ; writing—review and editing, Ruojin Yao, Yanhua Zhao; visualization,Mi Pei, Wenjing Yong, Gang Chen; supervision, Ruojin Yao and Yanhua Zhao; project administration, Ruojin Yao, Yanhua Zhao, Ronghui Tang and Fuyan Tan; funding acquisition, Wenyan Jian and Dewei Guo. All authors have read and agreed to the published version of the manuscript. References Gu SL, Yang XQ, Zhai YH, Xu WL, Guo WX, Shen T. 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J Pediatr Urol. 2016;12(5):305. .e1-305.e5.DOI:10.1016/j.jpurol.2016.06.012 . Cheng,Hong-Lin. Surgical indications for unilateral neonatal hydronephrosis in considering ureteropelvic junction obstruction[J]. Urol Sci. 2014;25(3):73–6. 10.1016/j.urols.2012.05.002 . Riccabona M. Assessment and management of newborn hydronephrosis. World J Urol. 2004;22(2):73–8. https://doi.org/10.1007/s00345-004-0405-0 . Elmaci AM, Dönmez Mİ. Time to resolution of isolated antenatal hydronephrosis with anteroposterior diameter ≤ 20 mm. Eur J Pediatrics. 2019;178(6):823–8. https://doi.org/10.1007/s00431-019-03359-y . Gordon I. Diuretic renography in infants with prenatal unilateral hydronephrosis: an explanation for the controversy about poor drainage. BJU Int. 2001;87(6):551–5. https://doi.org/10.1046/j.1464-410x.2001.00081.x . Al-Refai A, Ryan G, Van Mieghem T. Maternal risks of fetal therapy. Curr Opin Obst Gynecol. 2017;29(2):80–4. https://doi.org/10.1097/GCO.0000000000000346 . Chevalier RL. (2015). Congenital urinary tract obstruction: the long view. Advances in chronic kidney disease, 22(4), 312–9. https://doi.org/10.1053/j.ackd.2015.01.012 Nulens K, Lorenzo AJ, Dos Santos J et al. Fetal urinary tract dilation: What to tell the parents[J].Prenatal Diagnosis, 2024, 44(2). 10.1002/pd.6497 Huseynov M, Özcan R, Emre Ş, Canpolat N, Kuruğoğlu S, Sayman HB, Eliçevik M, Söylet Y, Büyükünal C, Emir H. Transient minimal hydronephrosis on contralateral kidney in infants with unilateral hydronephrosis: Is it an early sign of worsening of the affected kidney? Turk J Med Sci. 2021;51(4):2029–35. https://doi.org/10.3906/sag-2012-99 . Eskild-Jensen A, Gordon I, Piepsz A, Frøkiaer J. Congenital unilateral hydronephrosis: a review of the impact of diuretic renography on clinical treatment. J Urol. 2005;173(5):1471–6. https://doi.org/10.1097/01.ju.0000157384.32215.fe . Xu ZM, Li MJ, Tao C. (2017). Serum and urinary thioredoxin concentrations are associated with severity of children hydronephrosis. Clinica chimica acta; international journal of clinical chemistry, 466, 127–32. https://doi.org/10.1016/j.cca.2017.01.020 Shokeir AA, Nijman RJ. Antenatal hydronephrosis: changing concepts in diagnosis and subsequent management. BJU Int. 2000;85(8):987–94. https://doi.org/10.1046/j.1464-410x.2000.00645.x . Chávez-Iñiguez JS, Navarro-Gallardo GJ, Medina-González R, Alcantar-Vallin L, García-García G. (2020). Acute Kidney Injury Caused by Obstructive Nephropathy. International journal of nephrology, 2020, 8846622. https://doi.org/10.1155/2020/8846622 Blaine J, Chonchol M, Levi M. Renal control of calcium, phosphate, and magnesium homeostasis. Clin J Am Soc Nephrology: CJASN. 2015;10(7):1257–72. https://doi.org/10.2215/CJN.09750913 . Zhang SY, Mahler GJ. Modelling Renal Filtration and Reabsorption Processes in a Human Glomerulus and Proximal Tubule Microphysiological System. Micromachines. 2021;12(8):983. https://doi.org/10.3390/mi12080983 . Zhang SY, Mahler GJ. (2023). A glomerulus and proximal tubule microphysiological system simulating renal filtration, reabsorption, secretion, and toxicity.Lab on a chip, 23(2), 272–84. https://doi.org/10.1039/d2lc00887d Bastos FM, Leal CRV, Vaz de Castro PAS, Vaz KKB, Oliveira KKR, Silva Filha RD, Pereira AK, Simões E, Silva AC. Tubular and glomerular biomarkers of renal tissue function in the urine of fetuses with posterior urethral valves. J Pediatr Urol. 2022;18(3):e3681–9. https://doi.org/10.1016/j.jpurol.2022.03.019 . Klahr S, Tokyo. Japan), 39(5), 355–61. https://doi.org/10.2169/internalmedicine.39.355 . Chevalier RL, Forbes MS, Thornhill BA. J] Kidney Int. 2009;75(11):1145–52. 10.1038/ki.2009.86 . .Ureteral obstruction as a model of renal interstitial fibrosis and obstructive nephropathy.[. Madsen MG. Urinary biomarkers in hydronephrosis. Dan Med J. 2013;60(2):B4582. Ruano R, Safdar A, Au J, Koh CJ, Gargollo P, Shamshirsaz AA, Espinoza J, Cass DL, Olutoye OO, Olutoye OA, Welty S, Roth DR, Belfort MA, Braun MC. Defining and predicting 'intrauterine fetal renal failure' in congenital lower urinary tract obstruction. Pediatr Nephrol. 2016;31(4):605–12. https://doi.org/10.1007/s00467-015-3246-8 . Lee RS, Cendron M, Kinnamon DD, Nguyen HT. Antenatal hydronephrosis as a predictor of postnatal outcome: a meta-analysis. Pediatrics. 2006;118(2):586–93. https://doi.org/10.1542/peds.2006-0120 . Cheung KW, Morris RK, Kilby MD. Congenital urinary tract obstruction. Best practice & research. Clin Obstet Gynecol. 2019;58:78–92. https://doi.org/10.1016/j.bpobgyn.2019.01.003 . Hester AG, Krill A, Shalaby-Rana E, Rushton HG. Initial observational management of hydronephrosis in infants with reduced differential renal function and non-obstructive drainage parameters. J Pediatr Urol. 2022;18(5):e6611–6. https://doi.org/10.1016/j.jpurol.2022.07.023 . Additional Declarations No competing interests reported. Cite Share Download PDF Status: Under Revision Version 1 posted Editorial decision: Revision requested 29 Apr, 2026 Reviews received at journal 24 Apr, 2026 Reviews received at journal 22 Apr, 2026 Reviewers agreed at journal 20 Apr, 2026 Reviewers agreed at journal 19 Apr, 2026 Reviewers agreed at journal 14 Apr, 2026 Reviewers invited by journal 14 Apr, 2026 Editor invited by journal 13 Apr, 2026 Editor assigned by journal 10 Apr, 2026 Submission checks completed at journal 10 Apr, 2026 First submitted to journal 07 Apr, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9343823","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":626420947,"identity":"e11776cf-2049-44bf-8d8c-2ea482834f0b","order_by":0,"name":"Wenyan Jian","email":"","orcid":"","institution":"Xiangya Hospital Central South University","correspondingAuthor":false,"prefix":"","firstName":"Wenyan","middleName":"","lastName":"Jian","suffix":""},{"id":626420948,"identity":"7cfe9f9c-3a29-4b41-b2f9-cb1fa292bcce","order_by":1,"name":"Ruojin Yao","email":"","orcid":"","institution":"Xiangya Hospital Central South University","correspondingAuthor":false,"prefix":"","firstName":"Ruojin","middleName":"","lastName":"Yao","suffix":""},{"id":626420949,"identity":"8b7836fc-f368-421d-a765-7a6277179b97","order_by":2,"name":"Yanhua Zhao","email":"","orcid":"","institution":"Xiangya Hospital Central South University","correspondingAuthor":false,"prefix":"","firstName":"Yanhua","middleName":"","lastName":"Zhao","suffix":""},{"id":626420950,"identity":"a956f07b-af8d-4a35-a504-47d57db1fcc2","order_by":3,"name":"Mi Pei","email":"","orcid":"","institution":"Changsha Huayi Middle School","correspondingAuthor":false,"prefix":"","firstName":"Mi","middleName":"","lastName":"Pei","suffix":""},{"id":626420951,"identity":"5dee52b3-e272-407b-b654-19bbe0cd4a6f","order_by":4,"name":"Wenjing Yong","email":"","orcid":"","institution":"Xiangya Hospital Central South University","correspondingAuthor":false,"prefix":"","firstName":"Wenjing","middleName":"","lastName":"Yong","suffix":""},{"id":626420952,"identity":"a205e45b-0741-4c08-93a1-499eb42127d3","order_by":5,"name":"Gang Chen","email":"","orcid":"","institution":"Xiangya Hospital Central South University","correspondingAuthor":false,"prefix":"","firstName":"Gang","middleName":"","lastName":"Chen","suffix":""},{"id":626420953,"identity":"aa7e6a2a-9ee0-4ea3-b57f-591f800f4e83","order_by":6,"name":"Yimei Fu","email":"","orcid":"","institution":"Xiangya Hospital Central South University","correspondingAuthor":false,"prefix":"","firstName":"Yimei","middleName":"","lastName":"Fu","suffix":""},{"id":626420958,"identity":"dfb63862-a8d9-4be0-96fb-e79698081916","order_by":7,"name":"Ronghui Tang","email":"","orcid":"","institution":"General Hospital, Hunan University of Medicine","correspondingAuthor":false,"prefix":"","firstName":"Ronghui","middleName":"","lastName":"Tang","suffix":""},{"id":626420960,"identity":"dd112e4f-c2ae-4b8a-a561-9920945dda3a","order_by":8,"name":"Fuyan Tan","email":"","orcid":"","institution":"General Hospital, Hunan University of Medicine","correspondingAuthor":false,"prefix":"","firstName":"Fuyan","middleName":"","lastName":"Tan","suffix":""},{"id":626420962,"identity":"e5adac0a-895b-4fe4-9f30-1e8944e13b54","order_by":9,"name":"Dewei Guo","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA0UlEQVRIiWNgGAWjYJACCSDm4YewmUnQItlAqhYGgwPEapGPSD5442ObjYzx8ePPJBgqrBMb2M8ewKvF8EZasuXMtjQeszM5ZhIMZ9ITG3jyEvBrmZFjJs277TCP2Q0eNgnGtsOJDRI8BoS1/N32n8d4BvszCcZ/RGiRlwBqYdx2gMdAgsFMgrGBCC0GPM+SLXv/JfNInMkxtkg4lm7cxpNDwJZ2YIj9OGNnz99+/OGNDzXWsv3sZwjYcgCZlwDEbHjVg2xpIKRiFIyCUTAKRgEAwww/dl+WVj4AAAAASUVORK5CYII=","orcid":"","institution":"Xiangya Hospital Central South University","correspondingAuthor":true,"prefix":"","firstName":"Dewei","middleName":"","lastName":"Guo","suffix":""}],"badges":[],"createdAt":"2026-04-07 10:55:14","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9343823/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9343823/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":107616359,"identity":"5f1540cf-6c96-4261-92ff-f478d73771b5","added_by":"auto","created_at":"2026-04-23 09:13:08","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":168275,"visible":true,"origin":"","legend":"\u003cp\u003eFlowchart of study population selection.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-9343823/v1/07a890ee945fefc5f4760e10.png"},{"id":107616395,"identity":"e24652f5-493e-4407-9f41-47683583dcb3","added_by":"auto","created_at":"2026-04-23 09:13:11","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":343933,"visible":true,"origin":"","legend":"\u003cp\u003eCorrelation matrix of clinical, anatomical, and biochemical parameters. The heatmap shows Pearson correlation coefficients (r) for key variables. Red and blue colors denote positive and negative correlations, respectively, with intensity reflecting the strength of the association. Only statistically significant correlations (P \u0026lt; 0.05) are displayed.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-9343823/v1/b127c73bc9bec57df6ea6723.png"},{"id":107616353,"identity":"1b53e180-0f99-4abc-ac4a-ebbd6533edec","added_by":"auto","created_at":"2026-04-23 09:13:08","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":243648,"visible":true,"origin":"","legend":"\u003cp\u003eConstruction and validation of the FRSI. \u003cstrong\u003ea:\u003c/strong\u003e PCA loading plot. Visual representation of the contribution of five standardized parameters to PC1, which constitutes the FRSI. The length and color of the arrows represent the contribution of each variable to the index. \u003cstrong\u003eb:\u003c/strong\u003e Comparison of FRSI stratified by surgical urgency.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-9343823/v1/961c5694ecae3de5460996a5.png"},{"id":107616410,"identity":"794e33cf-e35d-464b-8628-f0d521beae90","added_by":"auto","created_at":"2026-04-23 09:13:14","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":181252,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ea: \u003c/strong\u003eDiagnostic performance of FRSI versus urinary sodium alone. \u003cstrong\u003eb: \u003c/strong\u003eGestational age at first detection stratified by surgical outcome.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-9343823/v1/df0b016577a732209159b741.png"},{"id":107616438,"identity":"7d97d119-1505-427e-a040-0b17312da245","added_by":"auto","created_at":"2026-04-23 09:13:21","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1513273,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9343823/v1/aecab0ba-f329-4583-8b19-678cb87de8ee.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Fetal renal severity index for predicting early surgical intervention in unilateral hydronephrosis: a retrospective case series with principal component analysis","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eAntenatal hydronephrosis is the most common fetal urinary tract abnormality, affecting approximately 1\u0026ndash;2% of all pregnancies\u003csup\u003e[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]\u003c/sup\u003e. Mild and moderate cases often show spontaneous resolution during follow-up, but for severe ones, impaired urinary drainage may lead to elevated renal pelvic pressure, which progressively compresses the renal parenchyma, resulting in gradual kidney damage that may ultimately culminate in renal atrophy and loss of function\u003csup\u003e[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]\u003c/sup\u003e. Given the compensatory capacity of the contralateral healthy kidney, prenatal intervention is seldom pursued in unilateral cases\u003csup\u003e[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]\u003c/sup\u003e. However, even when obstruction is relieved postnatally in severe cases, evidence indicates that renal interstitial fibrosis may persist, with little improvement in renal morphology or function\u003csup\u003e[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]\u003c/sup\u003e. As fetal surgery advances rapidly, severe unilateral obstructive hydronephrosis may eventually become a candidate for in utero intervention. Critical challenges in this context include accurate assessment of fetal renal function, consideration of intrauterine treatment options, prediction of postnatal renal outcomes, and determination of optimal timing for postnatal surgery. Although fetal urine electrolytes represent one of the earliest biomarkers used to evaluate fetal renal status\u003csup\u003e[\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]\u003c/sup\u003e, their utility in unilateral hydronephrosis remains underexplored\u003csup\u003e[\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]\u003c/sup\u003e. This study retrospectively analyzes renal pelvic urine electrolyte profiles, prenatal ultrasound findings, and postnatal outcomes in fetuses with unilateral hydronephrosis, aiming to evaluate the predictive value of fetal electrolytes for surgical intervention and to summarize clinical management experience.\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Study design and participants\u003c/h2\u003e \u003cp\u003eThis single-center study was conducted in the Fetal Medicine Department of Xiangya Hospital, Central South University. This retrospective study was in accordance with the ethical standards of the institutional and national research committee and Helsinki Declaration. Clinical data were collected from patients diagnosed with fetal unilateral hydronephrosis of the Society of Fetal Urology (SFU) grades 4 who underwent fetal pyelocentesis between September 2019 to July 2023. Postnatal outcomes were obtained via medical records and telephone follow-up. During the study period, pyelocentesis was performed on 37 fetuses. After applying the inclusion and exclusion criteria, 20 cases were ultimately included. Inclusion criteria were: ①Prenatal ultrasound indicating fetal unilateral hydronephrosis SFU grades 4; ②Performance of fetal pyelocentesis with complete electrolyte analysis of the urine sample; ③Availability of a detailed fetal urinary system ultrasound examination prior to the procedure; ④Blood tests for renal function and electrolytes within 48 hours after birth; ⑤Successful follow-up. Exclusion criteria included: ①Bilateral hydronephrosis; ②Concurrent other urinary system malformations; ③ Incomplete clinical data or loss to follow-up; ④abnormal karyotypes or CMA results.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Pre-procedure renal ultrasound examination\u003c/h2\u003e \u003cp\u003eDetailed fetal renal ultrasound was performed and included the following assessments: ①SFU grading; ②Anteroposterior diameter(APD) of the renal pelvis; ③Renal cortical thickness(RCT); ④ Status of the ureter and bladder; ⑤Status of the contralateral healthy kidney; ⑥The deepest vertical pocket (DVP) and amniotic fluid index(AFI). The APD was measured on a transverse image at the point of maximal pelviectasis, defined as the distance between the two parenchymal margins at the renal hilum\u003csup\u003e[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/sup\u003e. RCT was measured as the distance from the corticomedullary junction to the renal capsule\u003csup\u003e[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/sup\u003e. All fetuses were classified according to the SFU grading system\u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/sup\u003e: Grade 0 - normal kidney with no hydronephrosis; Grade 1 - slightly dilated renal pelvis without caliectasis; Grade 2 - moderately dilated pelvis with mild caliectasis; Grade 3 - marked dilatation of the pelvis and calyces, with normal or slightly thinned renal parenchyma; Grade 4 - severe dilatation of the pelvis and calyces with thinning of the renal parenchyma.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Measurement of fetal urine electrolytes\u003c/h2\u003e \u003cp\u003eIn this study, fetal pyelocentesis was reserved for SFU Grade 4 cases with progressive APD where parents strongly sought more definitive prognostic information to guide perinatal management. Written informed consent was obtained from the parents prior to puncture, which clearly stated that the intervention was intended for prognostic rather than therapeutic purposes, the associated risks and benefits had been thoroughly communicated. Under ultrasound guidance, fetal urine was aspirated from the affected renal pelvis using a biopsy needle (20G x 15cm, Batch No. DSN6847, Beijing Demaite Technology Development Co., Ltd.) via a transabdominal approach through the maternal abdominal wall and uterus into the fetal renal pelvis. The aspirated urine volume was recorded, and a 4ml sample was collected for measurement of sodium (Na\u003csup\u003e+\u003c/sup\u003e), potassium (K\u003csup\u003e+\u003c/sup\u003e), chloride (Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e), calcium (Ca\u003csup\u003e2+\u003c/sup\u003e), urea, creatinine, and uric acid levels.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Neonatal outcomes and follow-up\u003c/h2\u003e \u003cp\u003eData on gestational age (GA) at birth, sex, birth weight, and postnatal serum renal function and electrolytes were extracted from the medical records. Telephone follow-up was conducted in November 2025 to obtain the timing and type of any surgical intervention, as well as the children's current renal function status. The surgical indications were inquired in detail.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.5 Statistical analysis\u003c/h2\u003e \u003cp\u003eStatistical analyses were conducted using R software (version 4.5.1). Continuous variables are presented as mean\u0026thinsp;\u0026plusmn;\u0026thinsp;standard deviation (SD) or median [interquartile range, IQR] depending on data distribution. Categorical variables are expressed as frequencies and percentages (n, %). Group comparisons between the \u0026ldquo;early surgery\u0026rdquo; and \u0026ldquo;late\u0026rdquo; groups were performed using the Wilcoxon rank-sum test for continuous variables and Fisher\u0026rsquo;s exact test for categorical variables.\u003c/p\u003e \u003cp\u003eTo quantify the overall severity of renal impairment, a composite Fetal Renal Severity Index (FRSI) was constructed through principal component analysis (PCA). Five parameters were incorporated into the dimensionality reduction model: APD as an anatomical marker, along with fetal urinary Na⁺, Cl⁻, K⁺, and Ca\u0026sup2;⁺ as functional markers. Prior to analysis, all variables were standardized by Z‑score normalization to eliminate scale differences. The first principal component (PC1) was extracted as the FRSI score and calibrated so that higher values reflect greater severity\u0026mdash;integrating pronounced anatomical dilation with evidence of tubular salt-wasting. The predictive performance of the FRSI and individual markers was evaluated using receiver operating characteristic (ROC) curve analysis, from which the area under the curve (AUC), sensitivity, and specificity were derived. The optimal cutoff value was determined using the Youden index. Univariate logistic regression was applied to calculate the odds ratio (OR) for surgical urgency. A two‑sided p‑value\u0026thinsp;\u0026lt;\u0026thinsp;0.05 was considered statistically significant.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Study population\u003c/h2\u003e \u003cp\u003eInitial screening identified 31 cases with severe unilateral hydronephrosis that underwent fetal pyelocentesis. After excluding 4 cases with confounding anatomical anomalies and an additional 7 cases with incomplete clinical data or loss to follow-up, 20 fetuses were included in the final analysis. The study flowchart is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The maternal age was 30.60\u0026thinsp;\u0026plusmn;\u0026thinsp;4.96 years old, gestational age at detection was 21.82\u0026thinsp;\u0026plusmn;\u0026thinsp;5.74 weeks, the gravidity was 3 [IQR: 2, 3] and parity was 1 [IQR: 0.75, 2].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Fetal renal ultrasound and urine electrolyte results\u003c/h2\u003e \u003cp\u003eHydronephrosis was right-sided in 5 cases and left-sided in 15. The early surgery group and late surgery group were well-balanced regarding maternal age, gravidity, parity, DVP, AFI, SFU grade, APD, fetal urine Na+, K+, Cl-, Ca2+, urea, creatinine, and uric acid levels. A statistically significant difference was observed in the GA at first detection. The baseline characteristics of the study cohort, stratified by postnatal surgical urgency, are detailed in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. No procedure-related complications were recorded.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eBaseline characteristics of the study population\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eGA at detection (weeks), mean (SD)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003etotal cohort\u003c/p\u003e \u003cp\u003e(n\u0026thinsp;=\u0026thinsp;20)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eearly (\u0026lt;\u0026thinsp;3m)\u003c/p\u003e \u003cp\u003e(n\u0026thinsp;=\u0026thinsp;12)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003elate (\u0026gt;\u0026thinsp;3m)\u003c/p\u003e \u003cp\u003e(n\u0026thinsp;=\u0026thinsp;8)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cem\u003ep\u003c/em\u003e value\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e21.82 (5.74)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e19.05 (4.73)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e25.98 (4.63)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.01\u003csup\u003e*\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eaffected side\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eleft\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e15 (75.0%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e9 (75.0%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6 (75.0%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eright\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e5 (25.0%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3 (25.0%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2 (25.0%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDVP (mm), mean (SD)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e472 (96.88)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e485 (84.69)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e452.5 (116.1)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.22\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAFI (mm), mean (SD)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e155.05 (34.34)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e156.33 (35.09)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e153.13 (35.48)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.85\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAPD (mm),\u0026nbsp;median [Q1, Q3]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e27 [19.3, 38.6]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e29.5 [23, 46.5]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e25 [19, 28]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.16\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRCT (mm),\u0026nbsp;median [Q1, Q3]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2.55 [2.3, 2.92]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.55 [2.42, 2.9]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2.6 [2.3, 2.92]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.97\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGA at puncture (weeks),\u003c/p\u003e \u003cp\u003emedian [Q1, Q3]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e33.64 [29.32, 34.28]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e33.07 [28.93, 34.17]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e33.64 [32, 34.28]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.70\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003efetal urine sample\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003evolume (ml),\u0026nbsp;median [Q1, Q3]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e26.5 [14.5, 56]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e40 [20.75, 72.5]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e15.5 [12.25, 30.5]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.08\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNa\u003csup\u003e+\u003c/sup\u003e (mmol/L),median [Q1, Q3]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e42.4 [36.8, 44.75]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e42.9 [41.6, 50.15]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e39.35 [35.1, 42.7]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.09\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCl\u003csup\u003e\u0026minus;\u003c/sup\u003e (mmol/L),\u0026nbsp;\u0026nbsp;median [Q1, Q3]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e37.85 [32.25, 40.23]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e40.2 [33.75, 41.62]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e34.85 [31.22, 37.82]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.06\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eK\u003csup\u003e+\u003c/sup\u003e (mmol/L)\u0026nbsp;,\u0026nbsp;median [Q1, Q3]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2.47 [1.97, 3.12]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.55 [2.24, 3.12]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2.07 [1.8, 3.02]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.37\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCa \u003csup\u003e2+\u003c/sup\u003e (mmol/L),median [Q1, Q3]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.4 [0.28, 0.64]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.42 [0.28, 0.77]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.32 [0.26, 0.44]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.33\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eurea (umol/L),median [Q1, Q3]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e9.19 [8.11, 10.88]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e9.25 [8.2, 11.16]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e9.13 [8.11, 9.82]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.64\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ecreatinine (umol/L),median [Q1, Q3]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e203.2 [160.4, 284.3]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e219.85 [173.82, 349.6]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e185.15 [158.5, 247.87]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.33\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003euric acid (umol/L),\u0026nbsp;\u0026nbsp;median [Q1, Q3]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e640.5 [597.68, 791.95]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e698.9 [619.3, 864.75]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e603.1 [597.68, 651.45]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.23\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFRSI, median [Q1, Q3]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e-0.17 [-0.69, 0.41]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e-0.61 [-1.13, -0.35]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.23 [-0.11, 0.64]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.01\u003csup\u003e*\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"5\"\u003e* \u003cem\u003ep\u003c/em\u003e \u0026lt;0.05\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Neonatal outcomes\u003c/h2\u003e \u003cp\u003ePostnatal characteristics, surgical interventions, and follow-up data are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Neonates in the early surgery group were delivered at a significantly earlier gestational age compared to those in the late group. Indications for postnatal surgery were APD\u0026thinsp;\u0026gt;\u0026thinsp;3 cm and/or progression of hydronephrosis during follow-up. No significant differences were observed between the two groups in terms of delivery mode, birth weight, sex distribution, or postnatal serum electrolyte and renal function profiles. All patients (12/12, 100%) in the early surgery group underwent pyeloplasty, whereas surgery was performed in 75% (6/8) of the late surgery group, with the remaining 25% (2/8) managed conservatively (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.15). Among those who underwent surgery, the timing of intervention differed significantly, the early group required decompression at a median age of 1.18 months (IQR: 1.11\u0026ndash;2.04), while the late group at a median age of 12.27 months (IQR: 4.82\u0026ndash;34.46) (\u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.01). The median follow-up duration was comparable between the two groups (28.5 months vs. 38.5 months, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.37), indicating a sufficient observation period for both cohorts. At the last follow-up in December 2025, renal function was reported to be within normal limits for all children. However, due to inconsistent methodologies (e.g., ultrasound, diuretic renography, MR urography) used across different follow-up hospitals, standardized indicators for evaluating surgical efficacy were not available.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eNeonatal outcomes and biochemical characteristics stratified by surgical urgency\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eGA at delivery (weeks),median [Q1, Q3]\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003etotal cohort\u003c/p\u003e \u003cp\u003e(n\u0026thinsp;=\u0026thinsp;20)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eearly (\u0026lt;\u0026thinsp;3m)\u003c/p\u003e \u003cp\u003e(n\u0026thinsp;=\u0026thinsp;12)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003elate (\u0026gt;\u0026thinsp;3m)\u003c/p\u003e \u003cp\u003e(n\u0026thinsp;=\u0026thinsp;8)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cem\u003ep\u003c/em\u003e value\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e37.22 [35.86, 38.32]\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e36.07 [35.43, 36.89]\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e38.64 [38.21, 39.68]\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.01\u003csup\u003e*\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003edelivery mode\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003evaginal delivery\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e4 (20.0%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2 (16.7%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2 (25.0%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ecesarean section\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e16 (80.0%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e10 (83.3%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e6 (75.0%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ebirth weight (g),median [Q1, Q3]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e3180 [3062.5, 3513.75]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3110 [3062.5, 3247.5]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e3417.5 [3071.25, 3750]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.33\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eneonate sex\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003efemale\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2 (10.0%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1 (8.3%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1 (12.5%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003emale\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e18 (90.0%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e11 (91.7%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e7 (87.5%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eneonatal blood sample\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNa\u003csup\u003e+\u003c/sup\u003e (mmol/L),median [Q1, Q3]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e142.85 [141.38, 145.08]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e143.75 [141.68, 144.65]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e141.95 [140.7, 147.12]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.82\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCl\u003csup\u003e\u0026minus;\u003c/sup\u003e (mmol/L),median [Q1, Q3]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e108.4 [106.48, 110.85]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e108.8 [108.25, 110.85]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e106.95 [104.7, 110.35]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.32\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eK\u003csup\u003e+\u003c/sup\u003e (mmol/L),median [Q1, Q3]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e4.52 [4.4, 4.76]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e4.48 [4.36, 4.65]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e4.8 [4.41, 5.3]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.18\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCa\u003csup\u003e2+\u003c/sup\u003e (mmol/L),\u0026nbsp;median [Q1, Q3]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2.15 [2.12, 2.28]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2.13 [2.12, 2.27]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2.24 [2.04, 2.29]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.85\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eurea (umol/L),median [Q1, Q3]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e3.41 [2.84, 4.14]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3.41 [2.84, 3.8]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e3.54 [2.84, 4.63]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.56\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ecreatinine (umol/L),median [Q1, Q3]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e83.75 [73.72, 95.8]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e74.8 [70.9, 88.42]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e92.6 [84.23, 98]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.07\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003euric acid (umol/L),median [Q1, Q3]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e343.15 [304.7, 423.45]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e321.10 [290.2, 385.42]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e379.80 [342.7, 430.45]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.11\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003esurgery performed\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.15\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eyes\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e18 (90.0%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e12 (100%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e6 (75.0%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eno\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2 (10.0%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0 (0%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2 (25.0%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eage at surgery (months)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.01\u003csup\u003e*\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003emedian [Q1, Q3]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2.40[1.13, 5.72]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.18 [1.11, 2.04]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e12.27 [4.82, 34.46]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003emissing\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2 (10.0%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0 (0%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2 (25.0%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003efollow-up duration (months)\u003c/p\u003e \u003cp\u003emedian [Q1, Q3]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e35 [16, 56.75]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e28.5 [14.25, 57]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e38.5 [31, 49.25]\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.37\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"5\"\u003e* \u003cem\u003ep\u003c/em\u003e\u0026lt;0.05\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Correlation analysis of clinical and biochemical parameters\u003c/h2\u003e \u003cp\u003eTo explore the relationships between anatomical markers, biochemical indices, and clinical outcomes, Pearson correlation analysis was conducted (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Fetal urinary metabolites (creatinine and uric acid) demonstrated moderate to strong positive correlations with urinary electrolyte levels, indicating potential shared renal transport or handling pathways. Renal cortical thickness was inversely correlated with both serum creatinine (r = \u0026minus;\u0026thinsp;0.53) and urinary creatinine (r = \u0026minus;\u0026thinsp;0.46). Notably, the gestational age at initial diagnosis correlated positively with the postnatal timing of surgery (r\u0026thinsp;=\u0026thinsp;0.61), suggesting that earlier prenatal detection may predict a need for earlier surgical intervention. In contrast, none of the individual urinary electrolytes (Na⁺, Cl⁻, K⁺, or Ca\u0026sup2;⁺) showed a strong linear correlation (|r| \u0026gt; 0.4) with the timing of surgery.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003e3.5 Construction and validation of FRSI\u003c/h2\u003e \u003cp\u003ePrincipal component analysis was performed on five standardized parameters: APD and fetal urinary levels of Na⁺, Cl⁻, K⁺, and Ca\u0026sup2;⁺. As illustrated in the loading plot (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea), the first principal component (PC1) explained 55.9% of the total variance in the dataset, effectively representing the dominant pathological pattern. The loading vectors indicate the relative contribution of each variable to PC1. Fetal urinary chloride and sodium exhibited the strongest loadings (highlighted by red/orange vectors), supporting the role of tubular salt-wasting as a hallmark of severe obstruction. Prenatal APD and urinary calcium also loaded positively on PC1, demonstrating that the resulting FRSI integrates both anatomical dilation and functional tubular loss into a single continuous score. The clinical validity of the FRSI was further assessed by comparing scores between outcome groups (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). The FRSI was significantly higher in the early-surgery group (median\u0026thinsp;\u0026gt;\u0026thinsp;0) than in the late-surgery group (median\u0026thinsp;\u0026lt;\u0026thinsp;0) (p\u0026lt; 0.05). Most fetuses in the late-surgery group had negative scores, reflecting preserved tubular function and less pronounced anatomical dilation relative to the cohort average. In contrast, the early-surgery group clustered in the positive range, confirming that elevated FRSI values are strongly linked to the need for prompt postnatal intervention.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e3.6 Analysis of the predictive value of FRSI for early surgery\u003c/h2\u003e \u003cp\u003eThe predictive performance of FRSI was compared against fetal urinary sodium to validate the clinical utility. As demonstrated in the ROC analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea), an AUC of 0.86 was achieved by the FRSI (red line), significantly outperforming urinary sodium, which yielded an AUC of 0.73 (grey line). GA at initial detection of hydronephrosis was also examined as a prognostic factor. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb, fetuses in the early surgery group were identified significantly earlier compared to the late group (19.0\u0026thinsp;\u0026plusmn;\u0026thinsp;4.73 vs 26.0\u0026thinsp;\u0026plusmn;\u0026thinsp;4.63 weeks, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.01).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4. Discussion","content":"\u003cp\u003eIn the management of fetal urological anomalies, distinguishing unilateral hydronephrosis cases that are approaching a critical threshold remains a formidable challenge for perinatal medicine. Although current guidelines favor conservative management for asymptomatic unilateral hydronephrosis\u003csup\u003e[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/sup\u003e, concerns persist among clinicians regarding a \u0026ldquo;watchful waiting\u0026rdquo; approach in high‑risk cases\u003csup\u003e[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/sup\u003e. The number of nephrons in humans peaks at approximately 36 weeks of gestation and does not increase after birth. Persistent severe obstruction in utero, such as in ureteropelvic junction obstruction (UPJO), may cause dysplastic changes or cystic degeneration of the metanephric tissue, leading to permanent and irreversible renal impairment\u003csup\u003e[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]\u003c/sup\u003e. Conservative observation, in effect, merely shifts the window of renal injury from prenatal to postnatal period. Postnatal surgical intervention, such as pyeloplasty, can only salvage existing function, not reverse the loss of nephrons sustained during gestation\u003csup\u003e[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]\u003c/sup\u003e. Fetal intervention has traditionally been limited to a few life-threatening anomalies, such as posterior urethral valves, owing to high procedural risks and complications. Yet, ongoing advances in fetoscopy, ultrasound-guided techniques, and perinatal anesthesia are reshaping its risk\u0026ndash;benefit balance. With expanding indications and reduced complications in the era of fetal therapy\u003csup\u003e[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/sup\u003e, unilateral hydronephrosis merits reevaluation to improve risk stratification and intervention timing. No procedure-related maternal or fetal complications were observed in this study. This favorable safety profile is likely attributable to rigorous patient selection, standardized procedural protocols, and the expertise of the multidisciplinary fetal intervention team. These findings suggest that, in appropriately selected cases, fetal pyelocentesis offers an acceptable safety margin, supporting its potential role in managing carefully stratified severe unilateral hydronephrosis.\u003c/p\u003e \u003cp\u003eOur analysis reaffirms established demographic patterns, with a predominance of male and left-sided cases, likely attributable to spatiotemporal differences in ureteric bud development\u003csup\u003e[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/sup\u003e. Enrolled fetuses were screened by prenatal amniocentesis to exclude known genetic abnormalities. By excluding cases of genetic renal dysplasia, we minimized confounding from primary genetic defects, supporting the interpretation that the observed biochemical changes and elevated index values resulted from secondary pathophysiological effects of mechanical urinary obstruction. Nevertheless, the primary concern for clinicians remains the prenatal prediction of renal prognosis\u003csup\u003e[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]\u003c/sup\u003e. Our data reveal a harsh reality: postnatal serum markers are subject to a significant \"compensatory masking effect.\" Owing to the robust filtration capacity of the contralateral healthy kidney\u003csup\u003e[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]\u003c/sup\u003e, neonatal serum creatinine and electrolyte levels may remain within normal ranges even when the affected kidney has sustained significant damage\u003csup\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e. This systemic biochemical \"illusion\" renders serum markers of limited utility for early warning in unilateral disease\u003csup\u003e[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]\u003c/sup\u003e, compelling us to shift our focus to a more specific medium reflecting the local pathophysiological state of the obstructed kidney\u0026mdash;fetal urine.\u003c/p\u003e \u003cp\u003eUrine collected directly from the affected renal pelvis is unaffected by the contralateral healthy kidney, thereby providing an earlier and more accurate reflection of local pathophysiological changes than systemic serum biomarkers\u003csup\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e. Under normal physiological conditions, plasma electrolytes are filtered through the glomerulus to form the initial filtrate\u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]\u003c/sup\u003e. The proximal tubule reabsorbs Na⁺, K⁺, Cl⁻, HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e and other solutes isosmotically. Subsequently, the loop of Henle actively reabsorbs Na⁺ and Cl⁻. The distal tubule further reabsorbs Na⁺, Cl⁻, and Ca\u0026sup2;⁺, while the collecting duct, under the regulation of hormones like aldosterone and antidiuretic hormone, performs the final fine-tuning of Na⁺, K⁺, H⁺, and water balance, ultimately forming the final urine\u003csup\u003e[\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e. In the setting of mechanical obstruction, the elevated intrapelvic pressure leads to dilation and deformation of the renal tubules, causing epithelial cell damage and a reduced responsiveness to antidiuretic hormone\u003csup\u003e[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]\u003c/sup\u003e. Impairment in water reabsorption occurs, and the concentrating function is the earliest to be compromised\u003csup\u003e[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]\u003c/sup\u003e. If obstruction persists, sustained high pressure compromises glomerular capillary integrity, disrupts the filtration barrier, and promotes interstitial inflammation and fibrosis\u003csup\u003e[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/sup\u003e. Ultimately, glomeruli undergo sclerosis, effective filtration declines, the kidney loses its functions of reabsorption, secretion, concentration, dilution, and acidification entirely. Consistent with this pathophysiology, the concentrating ability of renal tubules is impaired early in obstructive disease\u003csup\u003e[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]\u003c/sup\u003e. Consequently, urinary electrolyte levels may paradoxically decrease rather than increase, reflecting tubular dysfunction that precedes overt glomerular failure.\u003c/p\u003e \u003cp\u003eWhile fetal urinalysis has long been viewed as a promising diagnostic window\u003csup\u003e[\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]\u003c/sup\u003e, the limitations of traditional assessment criteria were clearly demonstrated in this study. Conventional thresholds, such as fetal urinary Na\u003csup\u003e+\u003c/sup\u003e \u0026lt; 100 mmol/L, Cl \u003csup\u003e\u0026minus;\u003c/sup\u003e \u0026lt; 90 mmol/L, are largely derived from end-stage renal disease and proved insensitive in our cohort. Even in cases where the APD reached 40.4 mm and the cortical thickness was measured 2.1 mm, urinary electrolyte levels remained within traditionally defined \"normal ranges\", despite postnatal confirmation of renal impairment. This observation suggests that the aforementioned reference criteria may have limited utility in predicting future kidney function\u003csup\u003e[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]\u003c/sup\u003e. Single-parameter assessments, whether biochemical or sonographic, fail to capture signals of renal impairment during the \"subclinical window,\" leading to potential underestimation of disease severity\u003csup\u003e[\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eTo address these limitations, this study innovatively employed principal component analysis to construct the FRSI. The core value of FRSI lies in its integration of a \u0026ldquo;structural\u0026ndash;functional coupling\u0026rdquo; framework: APD quantifies the physical intensity of obstruction, while urinary electrolytes capture the metabolic response of renal tubules under sustained pressure. Sustained high pressure induces ischemia‑reperfusion injury and downregulates key transporters in tubular epithelial cells\u003csup\u003e[\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]\u003c/sup\u003e, resulting in subtle electrolyte wasting before overt renal failure occurs\u003csup\u003e[\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]\u003c/sup\u003e. The FRSI consolidates these early pathological signals through a weighted multivariate algorithm. Our findings demonstrate that the FRSI was significantly higher in the early‑surgery group (\u0026lt;\u0026thinsp;3 months) than in the late‑surgery group, indicating that an elevated FRSI score not only reflects the current degree of renal compromise but also predicts a progressive pathophysiological trajectory\u0026mdash;a feature that no single anatomical or biochemical marker can independently delineate. The translational potential of the FRSI is particularly promising, especially its high specificity, which offers strong support for clinical decision‑making. An optimal cutoff of \u0026minus;\u0026thinsp;0.12 was identified; an FRSI above this threshold achieved 100% specificity for predicting the need for surgery within the first three months of life. As a quantitative tool, the FRSI aligns with the pathophysiological understanding of early tubular injury. For fetuses with high FRSI scores, prenatal counseling should be more proactive, including shortened ultrasound surveillance intervals and early postnatal surgical evaluation to mitigate ongoing renal deterioration. The introduction of this index not only empowers clinicians to more accurately identify high-risk fetuses requiring early surgical intervention, thus optimizing treatment decision-making, but also opens new avenues for research in fetal hydronephrosis. It emphasizes the importance of a comprehensive, multi-parameter assessment strategy.\u003c/p\u003e \u003cp\u003eWe acknowledge several limitations in this study. First, fetal pyelocentesis is invasive and introduces selection bias, as only severe cases requiring this procedure were included; thus, findings may not extend to mild or moderate hydronephrosis. Second, ethical restrictions precluded obtaining urine from healthy fetuses, so no control group was available to define normal fetal urinary electrolyte ranges. Third, the FRSI reflects only a single time point and cannot track dynamic renal changes. Sequential sampling, while theoretically informative, is clinically unsafe due to risks such as membrane rupture or infection. Finally, Surgical decisions, although protocol‑based, could also be influenced by clinician judgment or parental concerns in borderline cases. Therefore, Research in this area remains challenging. Future work should prioritize prospective cohorts, standardized renal assessment, validation of the FRSI in larger populations, and exploration of its utility for guiding fetal intervention.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003e FRSI provides better risk assessment for surgical urgency in prenatal unilateral hydronephrosis than single indicator, and holds potential as a new tool to guide prenatal counseling and postnatal surgical timing.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cdiv\u003e\n \u003ctable border=\"0\" cellspacing=\"0\" cellpadding=\"0\" width=\"524\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 26.3862%;\"\u003e\n \u003cp\u003ePCA\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 73.6138%;\"\u003e\n \u003cp\u003eprincipal component analysis\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 26.3862%;\"\u003e\n \u003cp\u003eFRSI\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 73.6138%;\"\u003e\n \u003cp\u003efetal renal severity index\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 26.3862%;\"\u003e\n \u003cp\u003eSFU\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 73.6138%;\"\u003e\n \u003cp\u003esociety of Fetal Urology\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 26.3862%;\"\u003e\n \u003cp\u003eAPD\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 73.6138%;\"\u003e\n \u003cp\u003eanteroposterior diameter\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 26.3862%;\"\u003e\n \u003cp\u003eRCT\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 73.6138%;\"\u003e\n \u003cp\u003erenal cortical thickness\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 26.3862%;\"\u003e\n \u003cp\u003eDVP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 73.6138%;\"\u003e\n \u003cp\u003ethe deepest vertical pocket\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 26.3862%;\"\u003e\n \u003cp\u003eGA\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 73.6138%;\"\u003e\n \u003cp\u003egestational age\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 26.3862%;\"\u003e\n \u003cp\u003eSD\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 73.6138%;\"\u003e\n \u003cp\u003estandard deviation\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 26.3862%;\"\u003e\n \u003cp\u003eOQR\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 73.6138%;\"\u003e\n \u003cp\u003einterquartile range\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 26.3862%;\"\u003e\n \u003cp\u003eROC\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 73.6138%;\"\u003e\n \u003cp\u003ereceiver operating characteristic\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 26.3862%;\"\u003e\n \u003cp\u003eAUC\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 73.6138%;\"\u003e\n \u003cp\u003ethe area under the curve\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 26.3862%;\"\u003e\n \u003cp\u003eOR\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 73.6138%;\"\u003e\n \u003cp\u003ethe odds ratio\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e"},{"header":"Declarations","content":"\u003ch3\u003e\u0026nbsp;Ethics approval and consent to participate\u003c/h3\u003e\n\u003cp\u003eThis retrospective study was in accordance with the ethical standards of the institutional and national research committee and Helsinki Declaration. This study protocol was reviewed and approved by the Ethics Committee of Xiangya Hospital, protocol code 2025091320, date of approval was Sep. 1st, 2025. Informed consent was obtained from all subjects involved in the study.\u0026nbsp;\u003c/p\u003e\n\u003ch4\u003eConsent for publication\u003c/h4\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll data generated or analyzed during this study are available from the corresponding author on reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no conflict of interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding sources\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research was funded by HSTD | Natural Science Foundation of Hunan Province (Grant number 2023JJ40951); HSTD | Natural Science Foundation of Hunan Province (Grant number 2026JJ50626); Science and Technology Project of Changsha Science and Technology Bureau (Grant No. kq2502061).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors\u0026apos; contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eConceptualization, Wenyan Jian, Ruojin Yao, Yanhua Zhao, and Dewei Guo; methodology,Wenyan Jian and Dewei Guo; software, Mi Pei, Wenjing Yong, Gang Chen; validation, Yimei Fu, Ronghui Tang and Fuyan Tan; formal analysis, Mi Pei, Wenjing Yong, Gang Chen and Wenyan Jian; investigation, Ruojin Yao, Yanhua Zhao and Yimei Fu; resources, Wenyan Jian, Ruojin Yao, Yanhua Zhao and Dewei Guo; data curation, Mi Pei, Wenjing Yong, Gang Chen, Ronghui Tang and Fuyan Tan; writing\u0026mdash;original draft preparation,Wenyan Jian and Dewei Guo ; writing\u0026mdash;review and editing, Ruojin Yao, Yanhua Zhao; visualization,Mi Pei, Wenjing Yong, Gang Chen; supervision, Ruojin Yao and Yanhua Zhao; project administration, Ruojin Yao, Yanhua Zhao, Ronghui Tang and Fuyan Tan; funding acquisition, Wenyan Jian and Dewei Guo. All authors have read and agreed to the published version of the manuscript.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eGu SL, Yang XQ, Zhai YH, Xu WL, Guo WX, Shen T. Clinical characteristics of patients with prenatal hydronephrosis in early postnatal period: a single center retrospective study. 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J Pediatr Urol. 2022;18(5):e6611\u0026ndash;6. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jpurol.2022.07.023\u003c/span\u003e\u003cspan address=\"10.1016/j.jpurol.2022.07.023\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"bmc-pregnancy-and-childbirth","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"prch","sideBox":"Learn more about [BMC Pregnancy and Childbirth](http://bmcpregnancychildbirth.biomedcentral.com/)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/prch/default.aspx","title":"BMC Pregnancy and Childbirth","twitterHandle":"@BMC_series","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"em","reportingPortfolio":"BMC Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"unilateral hydronephrosis, fetal pyelocentesis, fetal urine electrolytes, fetal renal severity index, principal component analysis, anteroposterior diameter","lastPublishedDoi":"10.21203/rs.3.rs-9343823/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9343823/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003ch2\u003eBackground\u003c/h2\u003e \u003cp\u003ePredicting 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.\u003c/p\u003e\u003ch2\u003eMethods\u003c/h2\u003e \u003cp\u003eA 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⁺, Ca\u0026sup2;⁺) 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 (\u0026lt;\u0026thinsp;3 months) and late group. Diagnostic performance was evaluated using ROC curve analysis.\u003c/p\u003e\u003ch2\u003eResults\u003c/h2\u003e \u003cp\u003eThe 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\u0026thinsp;\u0026gt;\u0026thinsp;0) versus the late group (median\u0026thinsp;\u0026lt;\u0026thinsp;0, p\u0026thinsp;=\u0026thinsp;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\u0026thinsp;=\u0026thinsp;0.73). Fetuses requiring early intervention were diagnosed significantly earlier in gestation (19.0\u0026thinsp;\u0026plusmn;\u0026thinsp;4.73 vs. 26.0\u0026thinsp;\u0026plusmn;\u0026thinsp;4.63 weeks, p\u0026thinsp;=\u0026thinsp;0.01). An FRSI cutoff\u0026thinsp;\u0026minus;\u0026thinsp;0.12 yielded 100% specificity for identifying cases necessitating urgent postnatal surgery.\u003c/p\u003e\u003ch2\u003eConclusion\u003c/h2\u003e \u003cp\u003eFRSI 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.\u003c/p\u003e","manuscriptTitle":"Fetal renal severity index for predicting early surgical intervention in unilateral hydronephrosis: a retrospective case series with principal component analysis","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-04-23 09:12:35","doi":"10.21203/rs.3.rs-9343823/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-04-29T20:29:42+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-04-24T22:23:59+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-04-22T20:18:00+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"289603956002203935283700486455033574544","date":"2026-04-20T14:45:28+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"171336793920593798132882048302582397927","date":"2026-04-19T23:39:57+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"26253570581180293784908142079008966783","date":"2026-04-14T17:05:51+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-04-14T16:59:18+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2026-04-13T13:42:41+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-04-10T10:53:37+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-04-10T10:53:30+00:00","index":"","fulltext":""},{"type":"submitted","content":"BMC Pregnancy and Childbirth","date":"2026-04-07T10:48:06+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"bmc-pregnancy-and-childbirth","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"prch","sideBox":"Learn more about [BMC Pregnancy and Childbirth](http://bmcpregnancychildbirth.biomedcentral.com/)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/prch/default.aspx","title":"BMC Pregnancy and Childbirth","twitterHandle":"@BMC_series","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"em","reportingPortfolio":"BMC Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"ebbe0489-d583-4437-a3f0-b12b94764747","owner":[],"postedDate":"April 23rd, 2026","published":true,"recentEditorialEvents":[{"type":"decision","content":"Revision requested","date":"2026-04-29T20:29:42+00:00","index":"","fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"in-revision","subjectAreas":[],"tags":[],"updatedAt":"2026-04-29T20:38:58+00:00","versionOfRecord":[],"versionCreatedAt":"2026-04-23 09:12:35","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9343823","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9343823","identity":"rs-9343823","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}