In attacking the use of acetaminophen during pregnancy on Monday, President Trump and Health Secretary Robert F. Kennedy Jr. cited unproven claims that the drug is linked to autism, while glossing over well-documented risks posed by fevers, which pregnant women often use acetaminophen to treat. Mr. Trump said that acetaminophen, the active ingredient in Tylenol, might be justified if a pregnant woman had a severe fever. But he suggested that the bar should be extremely high, and at several other points declared, “Don’t take Tylenol,” with no qualification. But there is a downside: the risks of the fevers that acetaminophen can treat. It is the only drug that obstetricians commonly recommend to combat pain or fever during pregnancy, because options like ibuprofen can harm the fetus.