Magnesium Sulfate More Effective Than Nifedipine in Preventing Preterm Delivery

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June 29, 2007 — Magnesium sulfate is more effective than nifedipine in preventing preterm delivery for 48 hours, but nifedipine is linked with fewer maternal adverse events, according to the results of a randomized controlled trial published in the July issue of Obstetrics & Gynecology.

"Magnesium sulfate is the most commonly used first-line tocolytic in North America although it has not been demonstrated to be superior to saline infusion, and its use has been a source of controversy," write Deirdre J. Lyell, MD, from Stanford University Medical Center and Lucile S. Packard Children's Hospital in California, and colleagues. "Magnesium sulfate requires intravenous administration, has potential for overmedication with serious maternal adverse effects and may be associated with adverse neonatal effects.... Nifedipine may be more easily tolerated, is administered orally, and appears to have few adverse effects."

The objective of this multicenter randomized study was to compare the efficacy and adverse effects of intravenous magnesium with those of oral nifedipine for acute tocolysis of preterm labor in 192 patients in active preterm labor who were at 24 to 33 weeks and 6 days of gestation. The main endpoint was arrest of preterm labor, defined as prevention of delivery for 48 hours with uterine quiescence.

Compared with women treated with nifedipine, more women treated with magnesium sulfate achieved the main endpoint (87% vs 72%; P = .01). Both groups were similar in delivery within 48 hours (7.6% magnesium sulfate vs 8.0% nifedipine; P = .92), gestational age at delivery (35.8 vs 36.0 weeks; P = .61), birth before 37 and 32 weeks (57% vs 57%; P = .97 and 11% vs 8%; P = .39, respectively), and episodes of recurrent preterm labor.

In the magnesium sulfate group, mild and severe maternal adverse effects were significantly more frequent than in the nifedipine group. Although average birth weight, birth weight less than 2500 g, and neonatal morbidities were similar in both groups, newborns of mothers in the magnesium sulfate group spent longer in the neonatal intensive care unit (8.8 ± 17.7 vs 4.2 ± 8.2 days; P = .007).

Study limitations include lack of double-blinding or placebo control and failure to establish the optimal doses of magnesium sulfate and nifedipine.

"Patients who received magnesium sulfate achieved the primary outcome more frequently," the authors write. "However, delay of delivery, gestational age at delivery, and neonatal outcomes were similar between groups. Nifedipine was associated with fewer maternal adverse effects."

The Division of Maternal–Fetal Medicine at Stanford University and the Department of Obstetrics and Gynecology at Santa Clara Valley Medical Center supported this study. The authors have disclosed no relevant financial relationships.

Obstet Gynecol. 2007;110:61-67.