Why Take Tylenol During Pregnancy? The Risks of Fevers.

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.

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.

There are also indications that fevers later in pregnancy may be linked to preterm birth. That is less strongly established than the link to birth defects, but there is an “increasing body of evidence” for it, said Dr. Scott Sullivan, the director of maternal-fetal medicine at Inova Health. That evidence fits with studies showing that heat in general, including from heat waves, may increase the risk of preterm birth, he added.

Studies of the effects of fever during pregnancy generally have a similar limitation to studies of the effects of acetaminophen: They are observational rather than randomized, meaning they can only show an association between two things, not that one caused the other. But experts said they considered the evidence of risk from fever significantly stronger than the evidence of risk from acetaminophen, on which studies have been inconsistent.

“The conditions people use acetaminophen to treat during pregnancy are far more dangerous than any theoretical risks and can create severe morbidity and mortality for the pregnant person and the fetus,” Dr. Steven J. Fleischman, the president of the American College of Obstetricians and Gynecologists, said in a statement on Monday, an assessment he reiterated in an interview.

Acetaminophen, like all drugs, can have side effects, and medical professionals weigh risks versus benefits of any medication. That risk-benefit analysis has led doctors to recommend acetaminophen to many pregnant women experiencing pain and fever, while advising them to use it at the lowest effective dose for the shortest effective amount of time.

Experts expressed fear that, in setting a higher bar for use or abandoning acetaminophen, women and their fetuses could be left at more risk, not less.

Dr. Judette Louis, the chair of the publications committee for the Society for Maternal-Fetal Medicine, a professional group for specialists in high-risk pregnancies, said Mr. Trump’s remarks were deeply upsetting to her.

Because clinical trials commonly exclude pregnant people on ethical grounds — researchers tend to avoid purposely giving pregnant women something experimental — doctors often lack conclusive data for what is and isn’t safe, and Dr. Louis said many of them erred on the side of offering no drugs at all.

That is potentially harmful to both women and fetuses, and Dr. Louis has spent her career trying to get doctors more comfortable with treating pregnant women for illnesses and symptoms they experience. Tylenol has long been one of the few options they were confident in.

“With this announcement, it is going to make it a lot harder for women to get the appropriate treatment,” Dr. Louis said. “Because now physicians will be scared to provide it, and even patients who do understand the science may start second guessing.”

Dani Blum contributed reporting.