BACKGROUND: Many studies have demonstrated the association between low birth weight (LBW) and chronic kidney disease, estimated glomerular filtration rate (eGFR) and kidney volume (KV). However, studies on twins and those investigating numerous perinatal factors beyond LBW, and their associations with various kidney parameters are scarce. METHODS: A two-center cross-sectional study on five-year-old LBW children was conducted between 2021 and 2023. 110 children were enrolled (8 LBW, 58 very LBW (VLBW), 44 extremely LBW (ELBW)); 56 were twins. We examined associations between birth weight (BW), various prenatal, perinatal and postnatal factors, and eGFR, KV, tubular abnormalities and kidney ultrasound abnormalities, both in singletons and twins. RESULTS: In children with ELBW, eGFR correlated with BW (r = 0.55, P = 0.0018), while in those with BW ≥ 1000 g, eGFR remained constant. Other factors associated with decreased eGFR were hypertensive disorder of pregnancy (93.86 vs. 87.26 ml/min/1.73m2, P = 0.0285) in singletons, decreased growth velocity (β = 0.83, P = 0.0277) in twins, and lower total KV (tKV) and relative KV (rKV) in both singletons (r = 0.60, P < 0.0001 for tKV and r = 0.45, P = 0.0010 for rKV) and twins (β = 0.34, P < 0.0001 for tKV and β = 0.23, P = 0.0002 for rKV). Based on the multivariable models excluding KV, BW and gestational age were associated with eGFR in singletons, while male gender, BW, growth velocity, and coffee drinking during pregnancy were associated with eGFR in twins. However, in models that included KV, BW, gestational age and growth velocity were no longer significant. Total KV was associated with BW (r = 0.39, P = 0.0050 for singletons; β = 2.85, P < 0.0001 for twins), body mass index (r = 0.34, P = 0.0145 for singletons; β = 8.44, P < 0.0001 for twins), and growth velocity (β = 1.43, P = 0.0078). Twins born small for gestational age had lower tKV (70.88 vs 89.20 ml, P < 0.0001). Relative KV showed similar associations. Relative kidney volumes were significantly lower for both kidneys compared to the reference population (55.02 vs 65.42 ml/m2, P < 0.0001 for right kidney and 61.12 vs 66.25 ml/m2, P = 0.0015 for left kidney); however, only 8.6% of children had rKV below 10th percentile. CONCLUSION: Many factors affect eGFR and KV, some of them differ between twins and singletons. Based on multivariable models, eGFR seems to be better predicted by KV than by BW and gestational age in LBW children. Relative kidney volumes were significantly lower in our cohort compared to the reference population, but only 8.6% of rKV were below 10th percentile.

3rd Faculty of Medicine Charles University Prague Czech Republic

Department of Medical Chemistry and Clinical Biochemistry 2nd Faculty of Medicine Charles University and Motol University Hospital Prague Czech Republic

Department of Obstetrics and Gynecology 2nd Faculty of Medicine Neonatal Unit Charles University and Motol University Hospital Prague Czech Republic

Department of Pediatrics 2nd Faculty of Medicine Charles University and Motol University Hospital 5 Úvalu 84 Praha 5 Prague 15006 Czech Republic

Department of Radiology 2nd Faculty of Medicine Charles University and Motol University Hospital Prague Czech Republic

Institute for the Care of Mother and Child Neonatal Intensive Care Unit Prague Czech Republic

Institute of Applied Mathematics and Information Technologies Faculty of Science Charles University Prague Czech Republic

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