Most women, when they get pregnant, expect to carry the child for 40 weeks ? about nine months. But fully 10 percent of women give birth before completing 37 weeks of pregnancy. These pre-term infants are at higher risk for many kinds of problems, including cerebral palsy, as Rose Hoban reports.

Cerebral palsy is caused by damage to the areas of a baby's brain that control movement and sometimes, to areas controlling speech, communication or cognitive development.

Obstetrician John Thorp from the University of North Carolina says no one knows what causes cerebral palsy. But he says the numbers are consistent.

"For a full term baby, the risk of cerebral palsy is about one in 1000," he says. "For one of these very early pre-term babies at less than 32 weeks, the risk is 50 or 60 in 1000."

Thorp says some previous research had indicated that magnesium sulfate could reduce the incidence of cerebral palsy in pre-term infants. The common drug ? also known as Epsom salts ? is used to stop seizures in pregnant women who develop them.

But the data about any relationship to cerebral palsy was unclear. So Thorp and his colleagues recruited more than 2000 women who were at risk for giving birth prematurely.

"We did a randomized controlled trial? an experiment in which half the women got this medication and half got placebo," Thorp explains. "We followed their children out for two years, which is about the earliest a reliable diagnosis of cerebral palsy can be made."

Thorp and his colleagues found the women who received an infusion of magnesium sulfate as they were giving birth had children who were 50 percent less likely to develop cerebral palsy.

"That's an incredibly powerful effect to reduce risk by 40 or 50 percent with a drug that is safe, cheap, and you can get in any labor and delivery [room] in the world," Thorp says.

The reasons behind this effect are not clear. But Thorp says if scientists could determine how magnesium sulfate works and what dose works best, perhaps they could reduce rates of cerebral palsy even more.

Thorp presented his findings at a meeting of the Society of Maternal-Fetal Medicine, and his results are due to be published later this year. He predicts that magnesium sulfate could soon become a recommended treatment for women who go into labor early.