Background:Maternal mortality from severe eclampsia and preeclampsia has reduced significantly in developed nations from revolutionary management by magnesium sulfate, however, it is high in developing nations such as India. Various factors including BMI can significantly affect serum magnesium levels in subjects getting therapeutic and prophylactic management using the Pritchard regimen. Aim: The present study aimed to assess the effect of BMI (Body mass index) on serum magnesium levels in Pritchard regimen subjects and magnesium sulfate therapy subjects for prophylaxis of seizures. Methods: The present study assessed 320 subjects with severe preeclampsia and eclampsia who were under the Pritchard regimen within the defined study period. In all the included subjects, serum magnesium levels were assessed after the loading dose, the third dose, and 24 hours after the last dose. Subjects were divided into various groups based on the WHO BMI category. The data gathered were statistically analyzed. Results: The study results showed that following loading dose, mean serum magnesium levels in subjects for BMI <18.5, 18.5-24.99, 25-29.99, and ≥30 were 3.89±1.07, 3.40±0.95, 3.18±0.79, and 2.75±0.65 mg/dl respectively. However, the difference was statistically non-significant with p=0.0614 and serum magnesium level decrease was seen with increased BMI. Following 3rd dose, the mean serum magnesium level was 6.74±0.93, 6.32±1.13, 5.811.00, and 5.56±0.98mg/dl for BMI <18.5, 18.5-24.99, 25-29.99, and ≥30 respectively with statistically significant difference and p=0.0001. Higher convulsion rates were seen in obese subjects compared to non-obese subjects with 13.04% and 2.91% subjects respectively. Conclusion: The present study concludes that BMI plays a significant role in serum magnesium levels in pregnant females under magnesium sulfate therapy for seizure prophylaxis.