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Magnesium sulfate versus nifedipine for acute tocolysis of preterm labor

      Objective

      To compare intravenous magnesium sulfate (MgSO4) to oral nifedipine for acute tocolysis of preterm labor (PTL).

      Study design

      A randomized clinical trial was performed. Patients with contractions, and cervical change or ruptured membranes, from 24 to 34 weeks were randomly assigned to MgSO4 or nifedipine. The primary endpoint was uterine quiescence, defined by 12 hours of 6 or fewer contractions/hour and no further cervical change within 48 hours of tocolytic initiation. Failure was defined by rupture of previously intact membranes, use of supplemental or alternate tocolytics, recurrent PTL or delivery within 48 hours. Intent to treat analysis was used with chi-square and t tests. We estimated that 192 patients would be required for 80% power to detect a 50% difference with alpha=.05 and beta=.2.

      Results

      192 patients were enrolled. More patients assigned to MgSO4 achieved uterine quiescence (table). Among patients who achieved uterine quiescence as defined above, time to quiescence was shorter with nifedipine. There were no differences in the proportion of patients delivering within the first 48 hours, episodes of recurrent PTL, gestational age (GA) at delivery, birth weight (BW), BW< 2500 grams or preterm birth prior to 37 or 32 weeks.

      Conclusion

      Uterine quiescence within 48 hours of initiation of tocolysis was more frequent with MgSO4 compared to nifedipine. Among patients who achieved uterine quiescence, time to quiescence was shorter with nifedipine. There were no differences in delivery within the first 48 hours, neonatal BW, GA at delivery, or rate of preterm birth between the groups.
      Tabled 1
      MgSO4Nifedipinep-value
      Uterine quiescence87%72%0.01
      Hours to quiescence8.46.10.02
      Delivery in 48 hours7.6%8.0%0.92
      Episodes recurrent PTL.44.400.32
      BW (grams)266027290.72
      BW <2500 grams44%37%0.35
      GA (weeks)35.836.00.61
      GA <37 weeks57%57%0.97
      GA <32 weeks11%8%0.39