Magnesium Sulfate for Preterm Labor

Browse By All Topics


Magnesium Sulfate for Preterm Labor


Generic Name
magnesium sulfate

Magnesium sulfate is most commonly used for the treatment of preeclampsia during pregnancy. But magnesium sulfate is sometimes used as a tocolytic medicine to slow uterine contractions or to help protect the baby's brain during preterm labor.

Magnesium sulfate is usually given through a vein (intravenously) until contractions have slowed and the mother's cervix has stopped thinning (effacing) or opening (dilating).

How It Works

This medicine is thought to affect the action of calcium in the body, and calcium must be present for the muscles of the uterus to contract.

Why It Is Used

Magnesium sulfate may be used to stop preterm labor when:

  • Labor needs to be delayed for 24 to 48 hours to:
    • Let corticosteroids given to the mother help fetal lungs mature.
    • Provide time to move a mother to a hospital that offers neonatal intensive care, if her local hospital does not.
  • Regular contractions of the uterus have thinned (effaced) the cervix and opened (dilated) it less than 4 cm, and the mother's amniotic sac has not broken.
  • The mother is healthy.
  • The fetus is alive and not in distress.
  • Another tocolytic medicine has not slowed uterine contractions.
  • Treatment with other tocolytic medicines has been stopped because of side effects.

If preterm labor is likely to lead to preterm delivery and the mother is less than 32 weeks pregnant, magnesium sulfate may be used to reduce the risk of certain problems with the baby's brain, such as cerebral palsy.1, 2

How Well It Works

Studies have shown that magnesium sulfate is unlikely to stop preterm labor.3 But if it's given to women in preterm labor who are less than 32 weeks pregnant, it may help reduce the risk of cerebral palsy in babies who are born preterm.4

Side Effects

Common side effects of this medicine include:

  • Muscle weakness and lack of energy.
  • Blurry vision.
  • Slurred speech.
  • Headache.
  • Nausea and vomiting.
  • Flushing.
  • Stuffy nose.

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:

  • 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 through 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.

Complete the new medication information form (PDF)(What is a PDF document?) to help you understand this medication.



  1. American College of Obstetricians and Gynecologists (2012). Prediction and prevention of preterm birth. ACOG Practice Bulletin No. 130. Obstetrics and Gynecology, 120(4): 964–973.
  2. Rouse DJ, et al. (2008). A randomized, controlled trial of magnesium sulfate for the prevention of cerebral palsy. New England Journal of Medicine, 359(9): 895–905.
  3. Haas DM (2011). Preterm birth, search date June 2010. BMJ Clinical Evidence. Available online:
  4. American College of Obstetricians and Gynecologists (2012). Management of preterm labor. ACOG Practice Bulletin No. 127. Obstetrics and Gynecology, 119(6): 1308–1317.


ByHealthwise Staff
Primary Medical ReviewerSarah Marshall, MD - Family Medicine
Specialist Medical ReviewerWilliam Gilbert, MD - Maternal and Fetal Medicine
Last RevisedJanuary 8, 2013

This information does not replace the advice of a doctor. Healthwise, Incorporated disclaims any warranty or liability for your use of this information. Your use of this information means that you agree to the Terms of Use.

How this information was developed to help you make better health decisions.

© 1995-2012 Healthwise, Incorporated. Healthwise, Healthwise for every health decision, and the Healthwise logo are trademarks of Healthwise, Incorporated.