At the end of September, when President Trump claimed taking Tylenol while pregnant was associated with an increased risk of autism in children, Dr. Shakila Thangaratinam knew she needed to act.

Acetaminophen, the drug marketed under the name Tylenol, is the first line medication for treating fever in pregnant women, and the alternatives, as the obstetrician and professor at the University of Liverpool knew, can be dangerous. Taking non-steroidal antiinflammatory ibuprofen, marketed under the name Advil, interferes with a developing fetus’s cardiovascular system. Letting a fever rage unchecked, on the other hand, means exposing the fetus to harmful substances produced as the mother fights off her infection.....

They found that many studies that show a link between acetaminophen and autism and ADHD do not control well for the fact that these diagnoses often travel in families. When, in a handful of better-designed studies, siblings of children with a diagnosis were included, the link weakened substantially.

In particular, Thangaratinam highlights two Scandinavian studies and a Japanese study, in which large numbers of families were followed. In these studies, researchers compared sibling pairs in which one child had been exposed to acetaminophen in utero and the other had not.

“What we found was consistently in [these studies], the association that was initially present went away when a sibling control analysis was done,” she says. There was no difference in the diagnosis rates between these siblings, despite their different experiences during gestation.