Pre-eclampsia is a multisystem disorder of pregnancy characterized by high blood pressure and excess protein in the urine. The condition causes complications during labor and delivery that threaten the lives of mothers and infants. Pre-eclampsia can also progress to eclampsia—a more serious and potentially fatal condition in which the mother has convulsions during labor and delivery.

For decades, doctors have used anticonvulsant drugs to stop eclamptic convulsions and prevent their recurrence. Although research has established that magnesium sulfate is the best anticonvulsant for managing eclamptic convulsions, there has been very little research as to its effectiveness in women with pre-eclampsia.

Now, research recently published in The Lancet suggests that magnesium sulfate is also effective in preventing eclampsia in women who have pre-eclampsia. This very large, international trial also suggests that magnesium sulfate may reduce the risk of maternal death during labor and delivery.

About the study

The Magpie Trial Collaborative Group, an international group of researchers, conducted this study in 175 hospitals in 33 countries worldwide. They enrolled 10,141 women with pre-eclampsia—blood pressure of 140/90 or higher and excess protein in urine—who had not given birth yet or had given birth within 24 hours. Women were excluded from the study if they were hypersensitive to magnesium, were in a liver-failure coma and at risk for renal failure, or had myasthenia gravis (chronic muscular weakness).

Women were randomly assigned to receive an injection of either magnesium sulfate or placebo (an inactive salt water solution) during labor. Because magnesium sulfate is already the accepted medication for women with eclampsia, the attending physician or midwife had to decide if a woman could safely be enrolled in the study, or if eclampsia was impending and she definitely needed magnesium sulfate. It would be unethical to enroll a clear candidate for magnesium sulfate into a study in which she might receive a placebo instead.

Researchers compared the number of women who had eclamptic convulsions in the magnesium sulfate group with the number who had eclamptic convulsions in the placebo group. They also compared the rates of maternal and infant deaths between the two groups.

The findings

This study was stopped by the data monitoring board earlier than planned because the results from the first 8483 women showed such a strong benefit of magnesium sulfate over placebo.

Women with pre-eclampsia who received magnesium sulfate were 58% less likely to progress to eclampsia than women who received placebo. In addition, women receiving magnesium sulfate were 45% less likely to die in childbirth than women receiving placebo. However, there was no difference between the two groups in the risk of newborn death.

Side effects, such as flushing, nausea, vomiting, muscle weakness, and headache, were more common among women who were given magnesium sulfate than among their counterparts in the placebo group. Though bothersome, these side effects are not severe or life threatening.

Although these results are interesting, there are limitations to this study. Because women were included in this study at the discretion of the attending physician or midwife, an immediate clinical judgment played a role in determining the study population. If women most likely to progress to eclampsia were deemed by their doctors to be candidates for magnesium sulfate, they would have been excluded from the study.

How does this affect you?

This study adds to mounting evidence that magnesium sulfate is the drug of choice for both eclampsia and pre-eclampsia. The international collaborative that authored this study recommends that obstetricians consider using magnesium sulfate for women with pre-eclampsia who are considered to be at risk of eclampsia.

In the United States, rates of eclampsia and pre-eclampsia are much lower than in developing countries. And currently, American doctors more commonly use magnesium sulfate in women with pre-eclampsia than doctors in other countries. This research confirms the effectiveness of magnesium sulfate for pre-eclampsia and highlights the need to make magnesium sulfate more available in developing countries where eclampsia and pre-eclampsia are more common.