The new study is the first to look at young children whose mothers took Tylenol while pregnant.
"Our findings suggest that (acetaminophen) might not be as harmless as we think," Ragnhild Eek Brandlistuen said. She led the study at the School of Pharmacy at the University of Oslo in Norway.
She and her co-authors studied 48,000 Norwegian children whose mothers answered survey questions about their medication use at weeks 17 and 30 of pregnancy, and again six months after giving birth.
Mothers filled out a follow-up questionnaire about their child's developmental milestones three years later.
Close to 4 percent of women took Tylenol for at least 28 days total during pregnancy.
Their children seemed to have poorer motor skills than kids whose mothers had taken the drug fewer times or not at all. Tylenol-exposed kids also tended to start walking later, have poorer communication and language skills and more behavior problems.
It's difficult to define risks for pregnant women and their children, since rigorous tests and controlled studies of drug exposure aren't ethical, Brandlistuen said. All researchers can do is closely observe women in the real world.
But this study, published in the International Journal of Epidemiology, involved a large number of women, and researchers also looked for any link to ibuprofen, a pain-relief alternative without acetaminophen.
They found no development problems tied to ibuprofen.
"Long-term use of (acetaminophen) increased the risk of behavior problems by 70 percent at age three," Brandlistuen said. "That is considerable."
Heavy users most often reported taking the drug for five to seven days in a row a few times during pregnancy, she said.
Image: Pregnant woman taking medication, via Shuttestock