Magnesium Sulphate Dosing for Fetal Neuroprotection
Administer a 4-6 gram IV loading dose over 20-30 minutes, followed by a 1-2 gram per hour maintenance infusion when delivery is anticipated before 32 weeks' gestation. 1, 2
Gestational Age Criteria
MgSO4 for fetal neuroprotection should be administered if delivery is planned before 32 weeks gestation. 1
- The strongest evidence supports use from viability through 32 weeks' gestation, with the greatest benefit seen in the most premature infants 1
- Administration is recommended when preterm birth is planned or expected within 24 hours 3
- The intervention reduces cerebral palsy risk by approximately 32% without increasing neonatal mortality 4, 5
Standard Dosing Protocol
Loading Dose
- Administer 4-6 grams IV over 20-30 minutes to achieve immediate therapeutic levels 2, 6, 3
- Begin administration as close as possible to 4 hours before anticipated delivery 3
- If delivery is expected sooner than 4 hours, still administer—there remains benefit even with shorter exposure 3
Maintenance Infusion
- Continue with 1-2 grams per hour IV infusion 2, 7, 3
- For patients with BMI ≥25 kg/m², use 2 grams per hour rather than 1 gram per hour to achieve therapeutic levels 2, 6
- Continue until delivery but stop after 24 hours if undelivered 3
Minimum Duration
- The infusion should run for a minimum of 4 hours before delivery for optimal neuroprotection 3
Critical Safety Monitoring
Monitor for magnesium toxicity at least every 4 hours by recording pulse, blood pressure, respiratory rate, and deep tendon reflexes. 3
- Maintain respiratory rate ≥12 breaths per minute 7
- Ensure patellar reflexes remain present 6
- Monitor urine output (minimum 30 mL/hour) 7
- Limit total fluid intake to 60-80 mL/hour to prevent pulmonary edema, particularly important in preeclamptic patients 2, 6
Absolute Contraindications
Never combine magnesium sulfate with calcium channel blockers (especially nifedipine) due to severe risk of hypotension and myocardial depression. 1, 2, 7, 6
Dual Indication Scenarios
When preeclampsia coexists with preterm labor before 32 weeks, magnesium sulfate serves both eclampsia prophylaxis and fetal neuroprotection—use the same dosing regimen for both indications. 2, 6
Alternative Regimen (Resource-Limited Settings)
The Pritchard protocol may be used when continuous IV access is limited: 4 grams IV plus 10 grams IM (5 grams in each buttock) as loading dose, followed by 5 grams IM every 4 hours in alternate buttocks. 7
Common Pitfalls to Avoid
- Do not use prolonged high-dose regimens (>48 hours or >2 g/hour for extended periods)—these increase risk of neonatal complications without additional benefit 4, 8
- The optimal "therapeutic window" appears to be between 4-10.5 grams total dose; higher cumulative doses may cause fetal toxicity 8
- Do not withhold treatment in periviable gestations (22-25 weeks) if resuscitation is planned—neuroprotection remains beneficial 1, 2
- Premature newborns may have elevated magnesium levels in first days of life due to limited renal excretion capacity 2
Concurrent Therapies
Administer antenatal corticosteroids between 24+0 and 34+0 weeks when preterm delivery is anticipated—these therapies are complementary. 1