Magnesium Sulfate and Premature Labor

Magnesium sulfate, or mag for short, is used in pregnancy to prevent seizures due to worsening preeclampsia, to slow or stop preterm labor, and to prevent injuries to a preterm baby's brain.

Magnesium sulfate is given as an intravenous infusion or intramuscular injection in the hospital over 12 to 48 hours. It relaxes smooth muscle tissues, which helps to prevent seizures and slow uterine contractions.

magnesium sulfate uses in labor
Verywell / Alexandra Gordon


Magnesium sulfate infusions have been a common practice on obstetrical floors for more than 60 years. It's a well-studied drug, so doctors know very well how it affects moms and babies. Here is how it is used:

In Preterm Labor: To Allow Time For Steroids

Magnesium sulfate is a tocolytic that has been used to slow or stop premature labor. Research shows that mag, like other tocolytics, doesn't work very well to actually prevent preterm birth, but it may help stall labor for a short time.

Doctors may treat preterm labor with 48 hours of magnesium sulfate, hoping to buy enough time to complete a course of steroids to help the baby's lungs develop.

In Preeclampsia: To Prevent Seizures

Preeclampsia is a common complication of pregnancy that causes high blood pressure and protein in the urine. If not treated, preeclampsia can develop into eclampsia, a seizure disorder.

The only cure for preeclampsia and eclampsia is delivery of the baby, but magnesium sulfate can help prevent seizures in women with severe preeclampsia.

In Preterm Babies: To Protect Newborn Brains

Premature babies, especially those who are born before about 32 weeks gestation, have immature brains at birth. As they grow, they are at risk for cerebral palsy, a disorder that affects movement and intelligence.

Short-term (24 hours or less) infusions of magnesium sulfate have been shown to help protect the baby's brain by reducing the incidence of cerebral palsy.

Possible Side Effects 

Magnesium sulfate infusion is safe and effective when administered for up to a week. However, its side effects can be very uncomfortable. In mothers, the side effects include:

  • Flushing or hot flashes
  • Feeling tired and lethargic
  • Nausea and vomiting
  • Dizziness
  • Blurred vision
  • Muscle weakness

These side effects during labor may make vaginal delivery difficult resulting in a C-section. However, magnesium sulfate administration is also shown to reduce post-operative pain. In addition, women who were given magnesium sulfate may experience a delay in milk production by up to 10 days.

In rare cases, respiratory depression can occur. This can be reversed with a calcium infusion and is more common in women with kidney problems.

Side Effects in Babies

Magnesium sulfate crosses the placenta to the baby, and babies may experience side effects that include poor muscle tone and low Apgar scores. These side effects are usually gone in a day or so and don't cause long-term problems.

Mag should not be given for longer than seven days, as long-term mag therapy can cause low calcium in the baby's bones.

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By Cheryl Bird, RN, BSN
Cheryl Bird, RN, BSN, is a registered nurse in a tertiary level neonatal intensive care unit at Mary Washington Hospital in Fredericksburg, Virginia.