In the U.S., magnesium sulfate has become the most commonly used drug for treating preterm labor. Magnesium sulfate is given only intravenously. A woman is given an initial infusion of 4 to 6 grams over 15 to 30 minutes, and then a maintenance dose of 2 to 3 grams per hour.

How Does Magnesium Sulfate Work?

Doctors do not know exactly how magnesium sulfate inhibits contractions. The most common explanation is that magnesium lowers calcium levels in uterine muscle cells. Since calcium is necessary for muscle cells to contract, this is thought to relax the uterine muscle.

How Effective Is Magnesium Sulfate?

Magnesium sulfate is often quite effective in slowing contractions, although this effect and how long it lasts varies from woman to woman. Like all tocolytic medications, however, magnesium sulfate does not consistently prevent or delay preterm delivery for a significant period of time.

Even so, studies have shown that magnesium sulfate can delay delivery for at least several days (depending on how far dilated a woman's cervix is when the medication is started). This isn't a lot of time, but it can make a big difference for the fetus if the mother is given steroids along with magnesium sulfate. After 48 hours, steroids improve a baby's lung function and reduce his or her risk of dying by 40%.

What Are the Potential Side Effects of Magnesium Sulfate?

For the Mother

About half of the women who receive magnesium sulfate have some side effects. Potential side effects include flushing, feeling uncomfortably warm, headache, dry mouth, nausea, and blurred vision. Women often say they feel wiped out, as though they have the flu. These side effects can be uncomfortable, but they are not dangerous.

When given in high doses, magnesium sulfate can cause cardiac arrest and respiratory failure. Fortunately, women can be monitored for increases in the magnesium blood levels. If the levels become too high, the dose can be lowered. One of the most common signs that doctors watch for is the loss of the knee-jerk reflex (a jerk that usually occurs when your leg is tapped just below the knee). If for some reason the levels get too high, another medication, called calcium gluconate, can help reverse the effects of magnesium sulfate.

For the Baby

Since magnesium sulfate relaxes most muscles, babies who have been exposed to magnesium for an extended period of time may be listless or floppy at birth. This effect typically goes away as the drug clears from the baby's system.

Are There Women Who Should Not Take Magnesium Sulfate?

Women with medical conditions that could be made worse by the side effects described above should not be given magnesium sulfate or similar drugs. This includes women with myasthenia gravis (a muscle disorder) or muscular dystrophy.