About one in every ten babies in the United States is born preterm, meaning that labor starts before week 37 of a woman’s pregnancy. Magnesium sulfate has previously been used as an off-label treatment to slow uterine contractions during preterm labor and to prevent seizure in eclampsia. However, the FDA now states that magnesium sulfate should NOT be given to mothers for more than five to seven days due to the risk of harming the bones of the fetus. The labeling on the medicine will be changed to reflect the risks of using a drug not officially intended for preterm labor. The FDA arrived at their decision after a review of 18 case reports where mothers were given magnesium sulfate for an average of 9.6 weeks at a total dose of 3,700 mg. They observed that babies were born with such abnormalities as fractures in long bones possibly caused by too much magnesium and not enough calcium. Babies exposed to magnesium sulfate for more than a week also had differing levels of phosphorus, osteocalcin, magnesium, and calcium. While there is a lack of follow-up data for their study, they noted that in one investigation 11 babies that showed abnormalities at birth manifested none by the age of three. Despite the risks, women who face a situation where the benefits of delaying labor could outweigh the risks of weakening the bones of the fetus may still receive magnesium sulfate as a treatment.

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