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Contemporary Usage of Obstetric Magnesium Sulfate: Indication, Contraindication, and Relevance of Dose

Pryde, Peter G. MD1; Mittendorf, Robert MD, DrPH2

doi: 10.1097/AOG.0b013e3181b43b0e
Current Commentary

Magnesium sulfate, a biologically potent compound, given sometimes in extraordinarily high doses, is among the most commonly used pharmaceuticals in American obstetric practice. Although most clinicians are in accord regarding its value for seizure prophylaxis in the setting of preeclampsia, such unanimity is not the case regarding its role in preterm labor. Credible scientific data indicate not only a lack of efficacy, but also toxicity to susceptible fetuses when magnesium sulfate is used in the high dosages found in tocolysis. In apparent contrast, three recent clinical trials, although individually inconclusive, provide data from which a very recent meta-analysis affirms a potential role for magnesium sulfate in prophylaxis against fetal neurologic injury. Comparing outcomes from these trials, with attention to dosage, relationships are revealed that unify observations previously regarded as conflicting: Magnesium sulfate indeed may have both neuroprotective and fetal toxic effects. The better, and safer, neuroprotection seems to occur at comparatively low antenatal doses (perhaps in a range between 4 g and 10.5 g), whereas increasing dosages exceed a “therapeutic window” whereby, as with most drugs, toxic sequelae begin to accrue.

Evidence supports using magnesium sulfate for preeclampsia, condemns its usage for tocolysis, and suggests a new role for low-dose antenatal magnesium sulfate in preventing cerebral palsy.

From 1The University of Wisconsin Medical School, Madison, Wisconsin; and the 2Departments of Obstetrics and Gynecology and of Pediatrics, Loyola University Medical Center, Maywood, Illinois.

See related editorial on page 500.

Corresponding author: Dr. Robert Mittendorf, Departments of Obstetrics and Gynecology and of Pediatrics, Loyola University Medical Center, 2160 South First Avenue, Maywood, IL, 60153; e-mail:

Financial Disclosure The authors did not report any potential conflicts of interest.

© 2009 The American College of Obstetricians and Gynecologists