The Food and Drug Administration is now requiring stronger safety warnings for a popular treatment to prevent pregnant women from prematurely giving birth. Women should not be given injections of the drug terbutaline for more than three days "because of the potential for serious maternal heart problems and death," the FDA said Thursday. It is now requiring a boxed warning — the FDA's most serious type of warning — be added to the drug's label. The FDA also warned doctors against prescribing a pill form of the drug for "any treatment of preterm labor" because it has not been shown to be effective and carries similar risks.