Sitting on the couch a few weeks ago watching television, Jonathan Gardner saw something that made him text his mother immediately. “Weird question,” he typed. “Did you take Tylenol at all when you were pregnant with me?”

He had seen a news report about a new scientific review finding a possible correlation — though not a causal link — between the use of acetaminophen during pregnancy and a higher incidence of neurodevelopmental disorders. The finding remains inconclusive, and other studies have found the opposite.

Mr. Gardner, a local disability advocate in East Bridgewater, Mass., was diagnosed with autism before he turned 2. His mother, Nancy Gardner, replied that she didn’t take acetaminophen — the active ingredient in the painkiller Tylenol — during pregnancy.

And now she worried that a focus on the decisions of mothers would create unnecessary guilt for parents. “It’s no one’s fault” when a child has autism, she said in an interview.

That’s something that many people with autism and their families were repeating on Monday, as President Trump and his health secretary, Robert F. Kennedy Jr., launched a broad offensive against the mainstream understanding of the condition, including telling pregnant women to resist using Tylenol, despite a lack of proof it causes autism.

“Don’t take it,” Mr. Trump said in a White House briefing where he also delivered flawed medical remarks about vaccines and other science on autism, including urging women to “tough it out” when in pain rather than using acetaminophen. “Fight like hell not to take it,” he said.