The U.S. Food and Drug Administration (FDA) advised clinicians on Thursday abstain from giving pregnant women magnesium sulfate for more than five to seven days because of reports that it may harm the developing fetus' bones. Magnesium sulfate is commonly prescribed for pregnant women for preventing preterm labor and seizures during pregnancy.

Both the prevention of preterm labor and preventing seizures in women with preeclampsia are off-label uses of magnesium sulfate. During preterm labor, magnesium sulfate slows uterine contractions. The exact mechanism for how it prevents seizures in eclampsia, the culmination of preeclampsia, a condition marked by high blood pressure in multiple organs during pregnancy, is unknown. However, reports say that its use may lead to lower levels of calcium, and osteopenia or fractures in the baby, MedPage Today reports.

"Pregnant women should discuss with their health care professional the possibility of going into labor before term and the risks and benefits of any treatments that may be used," the FDA advisory warned.

Labeling on the medicine will be changed to emphasize that it isn't indicated for prevention of preterm labor. The teratogenicity category will also change from a category A to a category D, which states that "there is positive evidence of human fetal risk based on adverse reaction ... but potential benefits may warrant use of the drug in pregnant women despite potential risks."

The FDA came to these conclusions after a review of 18 case reports in which women were given the drugs for an average of 9.6 weeks, with an average total dose of 3,700 mg. Some of their babies were born with abnormalities including fractures of the long bones and ribs, possibly caused by fetal hypermagnesemia and then hypocalcemia.

Case reports included instances where there were differences in levels of osteocalcin, magnesium, calcium, and phosphorous in babies that were exposed to magnesium sulfate for more than a week.

According to the FDA's safety announcement, "exposure beyond 5-7 days included radiographic findings of radiolucent transverse metaphyseal bands of long bones such as the humerus."

The FDA says that long-term significance is unclear because of the lack of follow-up data for many of the studies — in one study, they found that 11 babies who had shown abnormalities at birth, showed no abnormalities at one and three years of age.

Preterm labor begins before week 37 of a woman's pregnancy. About one in every 10 babies is born preterm in the United States, according to the National Institutes of Health.