The pain reliever in question is acetaminophen; the most common pain reliever expectant moms use, and the main ingredient in Tylenol.
Researchers at UCLA looked at data from a 17-year study that followed 64,000 mothers and children in Denmark.
They saw an increase in ADHD and the more sever form, hyperkinetic disorder.
"I would probably steer clear from it while I'm pregnant, you know, and power through a headache or any bit of pain that I would have," said expectant mother Rebecca Barragan, about the study.
Researchers say more study is needed to confirm the findings.
The research is published in this week's JAMA Pediatrics.