CHICAGO — Magnesium routinely given to pregnant women to treat high blood pressure and premature labor may also sharply reduce the risk of cerebral palsy and retardation in their babies, a government study suggests.

Exactly why this is so is not clear, but researchers speculated that magnesium may somehow prevent fetal brain hemorrhage or block the harmful effects of a diminished oxygen supply to the brain.

The magnesium compound used, magnesium sulfate, is found in leafy green vegetables, nuts and whole grains and is available over the counter in capsules or tablets.

Given intravenously in much higher doses, it is used to treat pre-term labor and pre-eclampsia, a serious condition affecting about 7 percent of pregnant women and characterized chiefly by high blood pressure.

The study, published in Wednesday’s Journal of the American Medical Association, involved only large intravenous doses given to women suffering from pre-term labor or pre-eclampsia.

The study’s chief author, Diana E. Schendel, an epidemiologist at the Centers for Disease Control and Prevention, said the results are not clear enough to recommend that all pregnant women take magnesium supplements to protect their infants.

Women with high blood pressure or premature labor run a higher risk of delivering very low birthweight babies, or those weighing less than 3.3 pounds. Such infants face a significant risk of cerebral palsy and mental retardation.

But the study by Schendel and her colleagues found that very low birthweight babies whose mothers had been given magnesium sulfate in pregnancy were about 90 percent less likely to develop cerebral palsy and 70 percent less likely to be retarded than infants whose mothers didn’t receive the mineral.

The study confirms previous research and also may “open the door to new understanding of brain injury in very premature infants,” Dr. Karin Nelson, a neuroepidemiologist at the National Institutes of Health, wrote in an accompanying editorial.