Magnesium sulfate for seizure prevention during pregnancy


Generic Name
magnesium sulfate

Magnesium sulfate is used to prevent or stop seizures (eclampsia) during pregnancy. It is usually given through a vein (intravenously, or IV) or injected into a muscle (intramuscularly, or IM). Treatment to prevent seizures is usually continued for 24 hours after delivery.

Getting magnesium sulfate through a vein (IV) is less painful than IM injection because the IM injections must be given frequently. The IM delivery route may be used when:

  • A pump for adjustable and reliable IV delivery is not available.
  • Continuous monitoring of the health of the mother and fetus is not possible.
  • The mother must be transferred to another location for treatment.

Magnesium sulfate is also used during pregnancy to stop uterine contractions during preterm labor. But the dosage used for preventing eclampsia does not seem to have this effect during labor.1 For more information, see the topic Preterm Labor.

How It Works

The action of magnesium in preventing or stopping seizures during pregnancy is not known. This is primarily because the causes of preeclampsia and eclampsia are poorly understood.1

Why It Is Used

Magnesium sulfate is used to:

  • Prevent seizures in a woman with moderate to severe preeclampsia. When magnesium sulfate is used during labor and delivery, it is usually continued for at least 24 hours after delivery.
  • Stop seizures when they are occurring.

How Well It Works

A large, worldwide study has also shown that, magnesium sulfate treatment cuts the risk of eclampsia (seizures) in half for women with preeclampsia. In the study, the chance of eclampsia decreased regardless of how severe the woman's preeclampsia was.1

Side Effects

Side effects of magnesium sulfate that may affect the mother include:

  • Muscle weakness.
  • Lack of energy.
  • Low blood pressure (rare).
  • Headache.
  • Nausea and vomiting.
  • Stuffy nose.
  • Chest pain.
  • Buildup of fluid in the lungs (pulmonary edema) and slowed or difficult breathing.
  • Blurry vision.
  • Slurred speech.
  • Flushing.

Rare side effects of magnesium sulfate that may affect the fetus include:

  • Low Apgar scores at birth.
  • Low blood pressure.
  • Buildup of fluid in the lungs (pulmonary edema).

See Drug Reference for a full list of side effects. (Drug Reference is not available in all systems.)

What To Think About

In rare cases, symptoms of magnesium toxicity (nausea, muscle weakness, loss of reflexes) occur during magnesium sulfate treatment. The medicine calcium gluconate is given to treat the problem.

Magnesium sulfate and nifedipine are not used together because this combination can cause dangerously low blood pressure. (Nifedipine is a blood pressure medicine that is also used to control preterm labor.)

Magnesium sulfate:

  • Affects the central nervous system (brain and spinal cord) of the mother. Part of normal care when intravenous magnesium sulfate is given includes checking the mother's reflexes. If too much magnesium sulfate is given, the mother's reflexes will be slowed. Reflexes are usually checked about every 2 to 4 hours while the mother is on this medicine.
  • Affects the fetus's central nervous system. If this medicine has been given to the mother in large doses and the baby is born before the drug has had time to clear the mother's body, the baby may have temporary problems with breathing right after birth. These problems are quickly reversed with medicine.
  • Leaves the mother's body in her urine. The amount of urine she produces is closely monitored to ensure that this medicine does not build up in her bloodstream.

Mothers on magnesium sulfate are closely monitored. Blood pressure and pulse are checked about every 30 minutes for at least the first few hours of treatment.

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  1. Magpie Trial Collaborative Group (2002). Do women with pre-eclampsia, and their babies, benefit from magnesium sulphate? The Magpie Trial: A randomised placebo-controlled trial. Lancet, 359(9321): 1877–1890.

Last Updated: November 14, 2008

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