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. “Fight like hell not to take it,” he said, warning that if someone did take it, “That, you’ll have to work out with yourself.” Repeatedly, he insisted there was “no downside” to avoiding it.

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.

Decades of studies have shown a clear association between fevers early in pregnancy and a specific set of birth defects, including neural tube defects, some heart defects and cleft palates, said Dr. Eric Benner, a neonatologist at Duke University. “The evidence is really strong,” he said.

Dr. Benner emphasized that most of the time, the children of women who have fevers will be fine; these birth defects are rare. But research has consistently found an increased risk.the defectsby

If pregnant women are scared away from acetaminophen, “you run the risk of seeing increased incidence of those types of birth defects potentially for nothing,” he said.