Duration of Magnesium Sulfate Intramuscular Administration for Preeclampsia
For pregnant women with preeclampsia receiving intramuscular magnesium sulfate, the standard duration is 24 hours postpartum using the Pritchard regimen: 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 for 24 hours. 1, 2
Standard IM Protocol (Pritchard Regimen)
The FDA-approved intramuscular regimen for severe preeclampsia/eclampsia consists of: 2
Loading dose: 10-14 grams total
- 4-5 grams IV in 250 mL fluid OR
- 4 grams IV plus 10 grams IM (5 grams in each buttock simultaneously) 2
Maintenance dose: 4-5 grams (8-10 mL of 50% solution) IM into alternate buttocks every 4 hours 2
Duration: Continue until paroxysms cease, typically 24 hours postpartum 1, 2
Maximum daily dose: Do not exceed 30-40 grams per 24 hours 2
Clinical Context and Indications
Magnesium sulfate should be administered to women with preeclampsia who have proteinuria with either severe hypertension (≥160/110 mmHg) OR hypertension with neurological signs/symptoms for seizure prophylaxis. 3, 4
The IM route is particularly valuable in resource-limited settings where continuous IV infusion may not be feasible, as it can be administered by trained midwives or nursing staff. 4 IM administration of undiluted 50% solution achieves therapeutic plasma levels within 60 minutes. 2
Duration Considerations: The 24-Hour Standard
The 24-hour postpartum protocol remains the safer standard and is recommended by international guidelines. 1 This recommendation is based on: 3, 1
- ISSHP (International Society for the Study of Hypertension in Pregnancy) 2018 guidelines endorse 24-hour postpartum continuation 1
- The therapeutic magnesium level target is 6 mg/100 mL for seizure control 2
Evidence for Shortened Duration
Recent research has explored shorter durations, but with important caveats:
One RCT (n=1176) comparing 12-hour vs 24-hour IM regimens found no difference in seizure rates (0.3% vs 0.9%, p=0.29), with the 12-hour group experiencing fewer injection site complications and shorter hospital stays 5
Another RCT (n=1113) found women who received ≥8 grams (minimum 8 hours) of magnesium sulfate before delivery may not benefit from continuing 24 hours postpartum, with no eclampsia difference between groups 6
However, a systematic review and meta-analysis (n=1369) reported that both cases of postpartum eclampsia occurred in the <24-hour group, supporting continued use of the 24-hour protocol 7
Despite some evidence suggesting shorter courses may be adequate in select patients, the 24-hour protocol remains the recommended standard because eclampsia is rare and studies are underpowered to definitively prove non-inferiority of shortened regimens. 1, 7
Critical Safety Monitoring During IM Administration
Before each IM maintenance dose, verify: 2
- Patellar reflex present (loss occurs at 4-5 mmol/L magnesium) 8
- Respiratory rate ≥12 breaths/minute 4
- Urine output ≥30 mL/hour (oliguria dramatically increases toxicity risk) 4, 8
Do not routinely check serum magnesium levels; clinical monitoring is sufficient unless renal impairment is present. 1 In severe renal insufficiency, maximum dosage is 20 grams/48 hours with frequent serum level monitoring. 2
Important Safety Warnings
Never combine magnesium sulfate with calcium channel blockers (especially nifedipine) due to risk of severe hypotension and myocardial depression 1, 4
Do not continue magnesium sulfate beyond 5-7 days in pregnancy as it can cause fetal abnormalities 2
Have calcium gluconate 10% (15-30 mL IV) or calcium chloride 10% (5-10 mL IV) immediately available as the antidote for magnesium toxicity 8
Toxicity signs progress predictably: loss of reflexes (4-5 mmol/L) → respiratory depression (4-5 mmol/L) → cardiac conduction abnormalities (2.5-5 mmol/L) → severe bradycardia and arrest (6-10 mmol/L) 8
Practical Administration Tips
- Deep IM injection of undiluted 50% solution is appropriate for adults 2
- Injection site pain, inflammation, and bruising are common with IM administration (occurring in >90% of patients), but are significantly reduced with shorter duration protocols 5
- Limit total fluid intake to 60-80 mL/hour to prevent pulmonary edema in preeclamptic patients 1
- Avoid NSAIDs for postpartum pain when possible, as they worsen hypertension and increase acute kidney injury risk 1