Limited data suggest magnesium sulfate infusion may be associated with deficits in information processing and memory, potentially due to side effects or alterations in cerebral vasodilation. As such changes may have implications in medical decision making, our objective was to determine if postpartum cognitive function (information processing speed and working memory capacity) differed based on receipt of magnesium sulfate in women with hypertensive disorders of pregnancy (HDP). In this prospective observational study, women with singleton gestations who were diagnosed with HDP at ≥34 weeks of gestation were enrolled on admission for delivery. Magnesium prophylaxis was administered at obstetrician discretion. During the postpartum hospitalization, women were administered two validated assessments of cognitive function: the Salthouse Digit Comparison (SDC), which measures speed of information processing, and the Size Judgment Span (SJS), which measures working memory capacity. Scores were analyzed as continuous measures and by quartile (of increasing cognitive function). Bivariable and multivariable analyses compared cognitive function scores based on active receipt of magnesium sulfate at the time of the interview. Of 336 enrolled women, 30.1% (N=101) were receiving magnesium sulfate at the time of the cognitive function tests. Women on magnesium sulfate were more likely to be publicly insured (21.8% vs 10.7%, p=0.007) and to have delivered at an earlier gestational age (37.8 vs 38.5 weeks, p<0.001). Compared to women not on magnesium sulfate, women receiving magnesium sulfate had significantly lower median scores for the SDC and the SJS (Table). After adjusting for potential confounders, women on magnesium sulfate had significantly lower SDC scores (β -5.46, 95% CI -8.44 to -2.48) and lower odds of scoring in the highest SDC quartile (aOR 0.28, 95% CI 0.13-0.57). The SJS did not differ by whether women were receiving magnesium in multivariable analyses (Table). Among women with HDP, active receipt of magnesium sulfate is associated with slower maternal processing speed but no significant differences in memory. These findings suggest women receiving magnesium sulfate may benefit from tailored counseling strategies or increased time to make decisions in order to optimize shared decision making and quality of care.