Does Magnesium Sulfate in Pregnancy Prevent CP?

by Dr. Nigel Paneth, M.D., M.S.

Many women are treated with a medicine called magnesium sulfate in their pregnancies. This medicine has two uses: lowering blood pressure, and preventing the onset of premature labor.

Magnesium Sulfate is most commonly used to treat pre-eclampsia, a condition in which pregnant women develop a combination of high blood pressure, swelling (edema) and protein in their urine. This disease puts both the mother and the baby at risk. Magnesium is quite effective in lowering blood pressure in pre-eclamptic women, but sometimes the only way to help those with the disease is by delivering the baby. As a result the babies of some women with pre-eclampsia are delivered prematurely.

Another reason doctors use magnesium is to prevent or delay the onset of labor when labor starts prematurely. In this situation, magnesium is not so clearly effective although it is still widely used for this reason.

Because of the two reasons magnesium is used, it is quite common for babies in the womb to have received magnesium. About ten years ago, papers began to be published showing that babies born to mothers who received magnesium for one reason or another were less likely to get CP than similarly premature babies whose mothers did not get magnesium. These results struck some scientists as quite important, because it is known from laboratory studies that magnesium can block a receptor in the brain that is activated when brain-damaging chemicals are released by the body in some sick babies. However, as so often happens in science, other studies of premature babies didn’t show this protective effect of magnesium.

Scientists think that the most convincing type of study in humans is a randomized trial. When you divide people into two groups by a random process (like tossing a coin), the two groups will be just about the same in every way, and if one group is given a medicine and the other isn’t, the comparison of those two groups is the clearest and best evidence as to whether the medicine works or not. None of the studies described above were randomized trials, and for that reason, we could not know for certain whether magnesium does or does not prevent CP.

Last November, the results of the first randomized trial of magnesium sulfate in premature labor looking at CP were published. In Australia and New Zealand, 1,000 women in premature labor were divided randomly into two groups of 500 women, and only one group was given magnesium sulfate. This study was reasonable to do because doctors are still unsure whether magnesium actually stops, or delays labor. What did the study find? Unfortunately, we did not get a very clear answer. In the magnesium group, 32 children got CP. In the group that didn’t get magnesium, 26 children had CP. This was a small difference, and could easily have emerged by chance. However, looking at the information a bit differently, only about half as many children in the magnesium group were unable to walk at age two, and that difference was large enough that it didn’t seem likely to be due to chance.

Where does this leave us? At the present time, we don’t have firm evidence that magnesium can prevent CP, but it is possible that it could reduce the risk of CP by a small amount. A large US trial that is very similar to the one in Australia will soon be finished. Many doctors are looking forward to seeing the results of that trial.