Is magnesium sulphate (MgSO4) safe to use during pregnancy?

Magnesium Sulfate Safety in Pregnancy

Magnesium sulfate is safe and recommended for short-term use (typically <48 hours) in pregnancy for three specific indications: seizure prevention/treatment in preeclampsia/eclampsia, fetal neuroprotection before anticipated preterm delivery <32 weeks, and brief tocolysis to allow antenatal corticosteroid administration. 1, 2

Approved Safe Uses in Pregnancy

Seizure Prevention and Treatment (Primary Indication)

• Magnesium sulfate is the most effective agent for preventing and controlling eclamptic seizures, superior to phenytoin and diazepam, and is recommended as first-line therapy for women with severe preeclampsia. 3, 4
• The drug reduces cerebral palsy risk (relative risk 0.68,95% CI 0.54-0.87) without increasing mortality (relative risk 1.04,95% CI 0.92-1.17) when administered before 30 weeks gestation. 3
• Standard dosing: 4-6 g IV loading dose over 20-30 minutes, followed by 1-2 g/hour maintenance infusion. 5, 6

Fetal Neuroprotection

• Magnesium sulfate prophylaxis is recommended when delivery of a potentially viable infant is anticipated before 32 weeks gestation. 3, 5
• This indication is supported by 5 randomized controlled trials with enrollment starting as early as 24 weeks gestation. 3

Brief Tocolysis (Limited Role)

• Short-term use (up to 48 hours) to allow antenatal corticosteroid administration between 24-34 weeks is acceptable. 1, 2
• Prolonged use as a tocolytic agent is NOT recommended and is associated with increased infant mortality. 7

Critical Safety Warnings

Duration Limits - FDA Warning

• The FDA explicitly warns against continuous magnesium sulfate administration beyond 5-7 days in pregnancy due to risk of fetal hypocalcemia, skeletal demineralization, osteopenia, and neonatal fractures. 8
• The drug classification was changed from Category A to Category D specifically for prolonged use. 1, 2
• Short-term use (<48 hours) remains safe and appropriate for indicated conditions. 1, 2

Maternal Toxicity Monitoring

• Clinical monitoring (patellar reflexes, respiratory rate ≥16/min, urine output ≥100 mL/4 hours) is sufficient; routine serum magnesium levels are NOT necessary. 4, 6, 8
• Therapeutic serum levels: 3-6 mg/100 mL (2.5-5 mEq/L). 8
• Reflexes diminish at >4 mEq/L and may be absent at 10 mEq/L, where respiratory paralysis becomes a hazard. 8
• Maintain urine output ≥30 mL/hour, as magnesium is renally excreted and oliguria increases toxicity risk. 4, 8

Dangerous Drug Interactions

• NEVER combine magnesium sulfate with calcium channel blockers (especially IV or sublingual nifedipine) due to risk of severe myocardial depression and precipitous hypotension. 4, 6, 8
• Use caution with neuromuscular blocking agents due to excessive neuromuscular blockade. 8
• Reduce dosages of barbiturates, narcotics, and other CNS depressants due to additive effects. 8

Fluid Management

• Limit total fluid intake to 60-80 mL/hour to prevent pulmonary edema in preeclamptic patients who have capillary leak and reduced plasma volume. 6

Renal Impairment

• Use with extreme caution in renal impairment; dosage should not exceed 20 g in 48 hours in severe impairment. 8
• Serum magnesium monitoring is required in patients with renal dysfunction. 8

Neonatal Considerations

• Newborns exposed to maternal magnesium sulfate (especially >24 hours before delivery) may show neuromuscular or respiratory depression at birth. 8
• Premature neonates may have elevated magnesium levels in the first days of life due to limited renal excretion capacity. 5
• Have injectable calcium salt immediately available to counteract potential magnesium toxicity. 8

Practical Administration

• The 50% solution must be diluted to ≤20% concentration before IV infusion or IM injection. 8
• Administration can be performed by trained midwives or nursing staff, making it suitable for resource-limited settings. 4, 9
• Duration of treatment should not normally exceed 24 hours for seizure prophylaxis, and maintenance dose should not exceed 1 g/hour when using the IV route. 9

Common Pitfalls to Avoid

• Do not use magnesium sulfate for prolonged tocolysis (>5-7 days) - this is an unindicated use associated with fetal harm. 8, 1, 2
• Do not combine with calcium channel blockers without intensive monitoring. 4, 6
• Do not administer if patellar reflexes are absent or respiratory rate is <16/min. 8
• Do not use in digitalized patients without extreme caution due to risk of heart block if calcium administration becomes necessary. 8