Magnesium sulfate decreases the risk of eclampsia in patients with severe preeclampsia. There is a theoretical risk that magnesium sulfate increases the risk of intrapartum hemorrhage. We evaluated whether there was a quantitative difference in blood loss in patients receiving magnesium sulfate at the time of cesarean delivery (CD) compared to those who were not.A retrospective cohort study was performed using a database of patients with preeclampsia with severe features or eclampsia. The primary outcome was calculated estimated blood loss (cEBL) compared between patients who received magnesium sulfate during CD and those who did not. cEBL was derived through a validated equation by multiplying the patient's blood volume by percent of blood volume loss. Secondary outcomes were hematocrit change, visual EBL (vEBL), hemorrhage, cEBL >1500mL, Apgar <7 at 5 min, and NICU admission. Exclusion criteria were incomplete records or negative cEBL, as well as blood transfusion prior to collection of a postpartum hematocrit.We identified 124 patients with preeclampsia with severe features or eclampsia at time of CD. There were 57 (46%) that received magnesium sulfate during CD and 67 (54%) in which magnesium was stopped during the CD. The mean differences for hematocrit value (0.8, 95% confidence interval (CI) -0.3 to 1.8) and cEBL (108, 95% CI -102 to 318) were not significantly different after adjusting for obesity and history of CD.Magnesium sulfate administration for seizure prevention in patients with preeclampsia with severe features at the time of CD does not appear to be associated with an increase in the cEBL.