Magnesium Sulfate Compared with Nifedipine for Acute Tocolysis of Preterm Labor: A Randomized Controlled Trial

Abstract

Although not proved to be effective, intravenous magnesium sulfate is the most frequently used first-line tocolytic agent in North America. Its overuse may have adverse effects on both the mother and newborn infant. Nifedipine, which is given orally, has comparatively few adverse effects, although severe dyspnea, myocardial infarction, and fetal death have been reported. Nifedipine causes fewer side effects than betamimetic agents and yields better neonatal outcomes. This multicenter randomized trial enrolled 192 women who presented in active preterm labor at 24 weeks’ to 33 weeks + 6 days’ gestation. They were randomly assigned to receive either 4 g of magnesium sulfate, followed by an infusion of 2 g/h and added boluses as needed at an infusion rate up to 4 g/h; or 10 mg of nifedipine sublingually at 20-minute intervals totaling 3 doses, followed by the same dose every 4 or 6 hours. The primary outcome was no delivery for 48 hours and uterine quiescence—defined as 6 or fewer contractions per hour for 12 hours and no further cervical change within 48 hours of the start of treatment. Demographics and obstetrical characteristics were similar for the 92 patients given magnesium and the 100 treated with nifedipine. Significantly more magnesium-treated patients than those given nifedipine achieved the primary outcome (87% vs. 72%). Approximately 8% of women in each group delivered within 48 hours. Patients in the 2 treatment groups did not differ significantly with regard to gestational age at the time of delivery, birth before 37 or before 32 weeks’ gestation, or recurrent preterm labor. Maternal side effects were significantly more frequent in magnesium-treated women, 3 of whom had pulmonary edema. Neonatal outcomes were similar, but multivariable analysis, controlling for numerous possible confounding factors, showed that infants exposed to magnesium sulfate were 3 times likelier to be admitted to neonatal intensive care.