Pregnant women have long been assured that acetaminophen can treat their aches, pains and fevers without bringing harm to the babies they carry. Now researchers say they have found a strong link between prenatal use of the medication and cases of attention-deficit hyperactivity disorder in children. The results, published ... in the journal JAMA Pediatrics, add to growing evidence that the active ingredient in Tylenol may influence brain development in utero. But they do not provide clear answers for mothers-to-be or their doctors about whether acetaminophen is safe during pregnancy. In analyzing data on more than 64,000 Danish women and their children, researchers found that kids whose mothers took the painkiller at any point during pregnancy were 29% more likely to be diagnosed with ADHD than were kids whose mothers took none. The risk increased the most " by 63% " when acetaminophen was taken during the second and third trimesters, and by 28% when used in the third trimester alone. But when taken only in the first trimester, the added risk was 9%. Members of the research team had long suspected that acetaminophen may behave as an endocrine-disrupting chemical capable of influencing fetal brain development. The drug is known to cross the placental barrier between mother and fetus, and some studies have found higher rates of male babies with undescended testicles born to women who took it during pregnancy.