Objective <p>To compare the median effective dose (ED₅₀) of prophylactic continuous infusion of metaraminol in parturients with different body mass index (BMI) undergoing cesarean delivery under spinal anesthesia, and to investigate the influence of maternal BMI on the required dose of metaraminol for preventing post-spinal hypotension, thereby providing a basis for clinical individualized medication.</p> Methods <p>Eighty full-term singleton parturients scheduled for elective cesarean delivery at Suzhou Municipal Hospital Affiliated with Nanjing Medical University from July to September 2024 were selected and divided into a standard group (Group S, BMI &lt; 30&#xa0;kg/m²) and an obese group (Group O, BMI ≥ 30&#xa0;kg/m²) based on BMI. All parturients underwent spinal anesthesia. Immediately after successful dural puncture, a continuous infusion of metaraminol was initiated. The initial dose for both groups was set at 2&#xa0;mg, with a concentration of 0.02&#xa0;mg/mL and a fixed infusion rate of 100 mL/h. The sequential method was used to adjust the dose for subsequent parturients, with a dose gradient of 0.05&#xa0;mg: if the previous parturient developed hypotension, the dose for the next parturient was increased by 0.05&#xa0;mg; if hypotension did not occur, the dose for the next parturient was decreased by 0.05&#xa0;mg. Hypotension was defined as a systolic blood pressure (SBP) decrease of ≥ 20% from baseline or SBP &lt; 90 mmHg. Probit regression analysis was used to calculate the ED₅₀ and 95% confidence interval (95% CI) of metaraminol for both groups.</p> Results <p>The ED₅₀ and 95% CI of metaraminol for preventing post-spinal hypotension in Group S was 1.84&#xa0;mg/h (95% CI: 1.76–1.89&#xa0;mg/h); the corresponding value for Group O was 2.28&#xa0;mg/h (95% CI: 2.19–2.43&#xa0;mg/h). The relative potency ratio between Group O and Group S was 1.24. There were no statistically significant differences in the incidence of maternal adverse reactions or fetal umbilical cord blood gas analysis parameters between the two groups .</p> Conclusion <p>The dose of metaraminol required to prevent hypotension after spinal anesthesia for cesarean delivery is influenced by maternal BMI. Obese parturients (BMI ≥ 30&#xa0;kg/m²) require a higher dose of metaraminol. Therefore, it is reasonable to individualize the prophylactic dose of metaraminol based on maternal weight (BMI) in clinical practice.</p> Trial registration <p>The trial protocol was registered in the Chinese Clinical Trial Registry on June 29, 2024, with the registration number ChiCTR2400086366.</p>