Does magnesium sulfate given for prevention of eclampsia affect the outcome of labor?

OBJECTIVE: The purpose of this study was to determine whether magnesium sulfate given for prevention of eclampsia affected labor outcomes compared with phenytoin, which is not known to impede uterine activity when given in anticonvulsant doses. STUDY DESIGN: Secondary analysis was performed of a study of women with pregnancy-induced hypertension who were admitted for delivery and randomly assigned to receive either magnesium sulfate or phenytoin for eclampsia prophylaxis. Nulliparous women with a singleton pregnancy in cephalic presentation at term were selected for analysis in an effort to limit the influence of confounding variables such as preterm birth and malpresentations on labor management and outcomes. Similarly, women who had severe preeclampsia and who received labor epidural analgesia were excluded. RESULTS: A total of 2138 women were randomized to receive magnesium sulfate or phenytoin in the primary study. A total of 905 nulliparous women met the inclusion criteria for this secondary analysis; 480 had been randomized to phenytoin and 425 were given magnesium sulfate. The two groups were similar demographically. Labor outcomes such as (1) oxytocin stimulation, (2) admission-to-delivery intervals, (3) prolonged second-stage labor, (4) forceps delivery, and (5) cesarean delivery were not affected by maternal treatment with magnesium sulfate. CONCLUSION: Compared with phenytoin, magnesium sulfate given for intrapartum treatment of pregnancy induced hypertension does not significantly affect labor outcomes.