In Western countries, magnesium sulfate (MgSO₄) for threatened preterm labor is recommended as short-term tocolysis (₄ tocolysis on maternal renal function. This retrospective observational study included pregnant women hospitalized for threatened preterm labor who received MgSO₄ therapy at Fukuoka University Hospital between January and December 2021. Eligible patients were ≥18 years old, ≥22 weeks of gestation, and treated with MgSO₄ for ≥24 h. A total of 66 patients were classified into short-term (n = 26) and long-term (≥48 h) group (n = 40). The primary outcome was the incidence of acute kidney injury (AKI). Secondary outcomes included changes in serum creatinine level (ΔCr; peak – baseline) and correlations between ΔCr and Mg exposure (duration, daily dose, serum Mg concentration). Neonatal laboratory values at birth were also assessed. AKI occurred in 0% and 2.5% (1/40) of the short- and long-term groups, respectively; the affected case met creatinine-based AKI criteria but impairment was not clinically significant. ΔCr showed a weak positive correlation with treatment duration (ρ = 0.321, P = 0.009), while daily Mg dose and serum Mg levels were not correlated. Long-term group neonates showed higher serum Mg (P = 0.038) and lower Ca (P = 0.045). Long-term MgSO₄ tocolysis may cause mild renal burden but appears unlikely to induce clinically significant maternal renal dysfunction under careful monitoring. Maternal and neonatal electrolyte monitoring is advisable during prolonged MgSO₄ therapy.