Magnesium sulfate is most commonly used for the treatment
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
Magnesium sulfate is usually given through a vein (intravenously) until contractions have slowed and the
cervix has stopped thinning (effacing) or opening
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:
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
cervix and opened (dilated) it less than
4 cm, and the mother's
amniotic sac has not broken.
The mother is
The fetus is alive and not in
Another tocolytic medicine has not slowed uterine
Treatment with other tocolytic medicines has been stopped because
of side effects.
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
Common side effects of this medicine include:
Muscle weakness and lack of energy.
Nausea and vomiting.
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.
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.
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.
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):
Haas DM (2011). Preterm birth, search date June 2010.
BMJ Clinical Evidence. Available online:
American College of Obstetricians and Gynecologists
(2012). Management of preterm labor. ACOG Practice Bulletin
No. 127. Obstetrics and Gynecology, 119(6):
How this information was developed to help you make better health decisions.