Objective: To estimate the effect of maternal exposure to magnesium sulfate on fetal heart rate characteristics during active labor. Methods: Within a 4-year retrospective cohort study of consecutive term deliveries reaching the second stage of labor, we compared women exposed to magnesium for severe preeclampsia to all women not exposed. Primary outcome was the electronic fetal monitoring tracing in the 30 minutes preceding delivery. Secondary outcomes were fetal acidemia and nursery disposition. Attributable risk estimates and multivariable logistic regression were used to estimate the association between magnesium exposure and fetal heart rate characteristics. Unadjusted risk estimates for the association between fetal heart rate characteristics and neonatal outcomes were generated stratified by group. Results: Of 5,387 women, 248 (4.6%) were exposed to magnesium. Magnesium exposure was associated with lower fetal heart rate baseline (136.9±12.3 beats per minute compared with 139.0±13.5 beats per minute; P=.02), increased risk of baseline less than 120 beats per minute (adjusted odds ratio [OR] 1.76, 95% confidence interval [CI] 1.21-2.56), and increased risk of absent or minimal variability (adjusted OR 2.41, 95% CI 1.78-3.27). More than 20% increased frequency of ever absent or minimal variability was attributable to magnesium (attributable risk 0.21, 95% CI 0.15-0.27). There were no significant differences in presence or number of accelerations or decelerations; however, magnesium was associated with fewer prolonged decelerations (adjusted OR 0.64, 95% CI 0.49-0.84). After excluding women with adverse neonatal outcomes, these associations remained. Conclusion: Maternal exposure to magnesium is associated with lower fetal heart rate baseline within the accepted normal range, decreased variability, and fewer prolonged decelerations without evidence of adverse effect on neonatal outcome.