Guidelines for Magnesium Sulfate Administration in Preterm Pregnancy for Neuroprotection
Magnesium sulfate should be administered for fetal neuroprotection when preterm delivery is anticipated before 32 weeks' gestation, as it significantly reduces the risk of cerebral palsy without increasing mortality. 1
Indications and Timing
- Magnesium sulfate is recommended by the Society for Maternal-Fetal Medicine as an intrapartum neuroprotective agent for pregnancies that are <32 weeks of gestation 2
- Although data specific to the periviable period (22-25 weeks) are limited, magnesium sulfate prophylaxis is still recommended if delivery of a potentially viable infant is anticipated 1
- For women with preeclampsia, magnesium sulfate serves dual purposes: prevention of eclamptic seizures and fetal neuroprotection 1
Dosing Regimens
ACOG Recommendation
- Loading dose of 4-6g intravenously over 20-30 minutes followed by a maintenance infusion of 1-2g/hour until delivery 1
- Continue for a minimum of 4 hours and maximum of 24 hours 3
WHO Recommendation
- Standard regimen of 4g loading dose given intravenously over 15-20 minutes, followed by 1g/hour maintenance dose 4
- Maximum duration of 24 hours and minimum of 4 hours 4
RCOG Recommendation
- 4g loading dose intravenously over 20 minutes followed by a maintenance dose of 1g/hour for up to 12 hours 5
- Maximum cumulative dose should not exceed 50g 5
Optimal Therapeutic Targets
- The maternal serum magnesium concentration associated with the lowest probability of delivering an infant with cerebral palsy is 4.1 mg/dL (95% CI 3.7 to 4.4) 6
- Pharmacokinetic studies suggest that magnesium sulfate administration for duration longer than 18 hours, given within 12 hours of delivery, may maximize neuroprotective benefits 7
Special Considerations
- In women with preterm preeclampsia, magnesium sulfate dosing needs to be adapted to postnatal blood concentrations if the mother received magnesium therapy before delivery 2
- For women with fetal growth restriction and absent end-diastolic flow, delivery should be considered by 34 weeks gestation, with magnesium sulfate administration for neuroprotection 1
- For fetal growth restriction with reversed end-diastolic flow, delivery should be considered by 30-32 weeks gestation, with magnesium sulfate administration for neuroprotection 2, 1
Clinical Pitfalls to Avoid
- Avoid using calcium channel blockers (especially nifedipine) with magnesium sulfate without careful monitoring, as this combination may cause precipitous blood pressure drop 3
- In preterm infants with intrauterine growth restriction on parenteral nutrition, careful monitoring of plasma phosphate concentration within the first days of life is required to prevent severe hypophosphatemia when the mother has received magnesium sulfate 2
- Avoid "running dry" a preeclamptic woman receiving magnesium sulfate, as she is already at risk of acute kidney injury; maintain adequate hydration while avoiding fluid overload 3
- Limit total fluid intake to 60-80 mL/hour to avoid risks of pulmonary edema in women with preeclampsia receiving magnesium sulfate 3