Magnesium sulfate for preterm labor


Generic Name
magnesium sulfate

Magnesium sulfate is most commonly used for the treatment of preeclampsia during pregnancy. Use of magnesium for the treatment of preterm labor or preeclampsia is an unlabeled use of the medicine.

Magnesium sulfate is sometimes used as a tocolytic medicine to slow uterine contractions during preterm labor. But studies show it does not stop preterm labor and it may cause complications for both mother and baby.1

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

Some hospitals use magnesium sulfate for preterm labor.2 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.

How Well It Works

Studies have shown that magnesium sulfate is unlikely to stop preterm labor.3 It may also cause complications for mother and baby.1

Side Effects

Side effects are common with magnesium sulfate and can affect both the mother and fetus. 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.

These side effects can decrease over the duration of treatment and go away when the medicine is stopped.

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

Magnesium sulfate affects both a mother and her fetus. This medicine:

  • May reduce risks of cerebral palsy in the premature newborn if preterm labor progresses on to preterm delivery during treatment.4
  • Affects a mother's central nervous system. Part of normal care when intravenous magnesium sulfate is given includes checking the mother's reflexes, usually every 2 to 4 hours, while she is on this medicine.
  • Affects the fetus's central nervous system. If a mother is given large doses of magnesium sulfate, the newborn may have trouble breathing immediately after birth. But this problem is easily treated with medicine. Some studies show that magnesium sulfate may increase the risk of death for the baby.1
  • Can lower a mother's blood pressure and increase her heart rate. Her blood pressure and pulse are checked frequently for the first few hours of treatment.
  • Leaves the mother's body through her urine. The amount of urine she produces is closely watched to ensure that the medicine does not build up in her blood.

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  1. Grimes DA, Nanda K (2006). Magnesium sulfate tocolysis: Time to quit. Obstetrics and Gynecology, 108(4): 986–989.
  2. Cunningham FG, et al. (2005). Preterm birth. In Williams Obstetrics, 22nd ed., pp. 855–880. New York: McGraw-Hill.
  3. Haas DM (2008). Preterm birth, search date June 2007. Online version of BMJ Clinical Evidence:
  4. 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.

Last Updated: January 14, 2009

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