Pregnancy

Reasons Why a Doctor May Prescribe Magnesium Sulfate During Pregnancy

Learn why pregnant people who are diagnosed with preeclampsia or go into preterm labor may be treated with magnesium sulfate.

By Cheryl Bird, RN, BSN

Published on April 16, 2024

Medically reviewed by Alyssa Dweck, MS, MD, FACOG

What Is Magnesium Sulfate?

Magnesium sulfate is a drug that contains magnesium salt and sulfate. It can be used:

To treat seizures
To treat irregular heartbeats
To treat high blood pressure
As an anesthetic and analgesic
To prevent smooth muscle contractions

When used during pregnancy, 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.

Why Magnesium Sulfate Is Used in Pregnancy

Magnesium sulfate infusions were first used for treating preeclampsia in 1906 and popularized in the 1920s. Since then, it has become a mainstay on obstetrical floors. In fact, it is one of the most commonly used medications in obstetrics.

In addition to its use for preeclampsia, health care providers also use it to delay preterm birth and to to prevent fetal brain damage prior to preterm birth.

To prevent seizures in preeclampsia

Preeclampsia is a common complication of pregnancy. Symptoms include 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. However, magnesium sulfate can help prevent seizures in those with severe preeclampsia. In fact, research has found that it can reduce the risk of developing eclampsia and the risk of death by half or more.

To delay preterm labor

Magnesium sulfate is a tocolytic (an agent that inhibits uterine contractions) that has been used to delay 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.

Health care providers may treat preterm labor with up to 48 hours of magnesium sulfate. The goal is to buy enough time to complete a course of corticosteroids to help the fetal lungs develop prior to being born. These steroids are critical for preterm babies because breathing problems are the main cause of death and serious health problems in babies born very early.

To protect preterm fetal brains

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

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 of Magnesium Sulfate in Pregnancy

Magnesium sulfate infusion is safe and effective when administered for a short period. However, the Food and Drug Administration (FDA) has warned that using it for more than five to seven days could increase the risk of low calcium levels and bone problems in the fetus.

Using magnesium sulfate for delaying preterm labor is an "off-label" use, meaning it is not FDA-approved for that purpose. It is FDA-approved for treating preeclampsia.

Side effects in gestational parent

Magnesium sulfate side effects can be very uncomfortable for the pregnant person and can include:

Flushing or hot flashes
Feeling tired and lethargic
Nausea and vomiting
Headaches
Dizziness
Blurred vision
Muscle weakness
Injection site pain

These side effects during labor may make vaginal delivery difficult, resulting in a C-section. However, magnesium sulfate administration has also been shown to reduce post-operative pain. In addition, magnesium sulfate may delay milk production.

In rare cases, respiratory depression and cardiac arrest have occurred. This is usually associated with high magnesium serum levels.

Side effects in babies

Magnesium sulfate crosses the placenta to the fetus, and babies may experience side effects, including poor muscle tone and low Apgar scores. These side effects usually don't cause long-term problems.

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

How long do the side effects of magnesium sulfate last?

Most side effects associated with magnesium sulfate do not require treatment and will resolve on their own with time as your body adjusts to the medication. For example, many patients report facial flushing and warmth when treatment begins but those symptoms often resolve spontaneously. While most side effects don't require medical attention, be sure to keep your health care provider in the loop about any symptoms you're experiencing. They may have recommendations for getting relief.

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