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%.

The findings do not establish that prenatal exposure to acetaminophen — which is also an ingredient in Excedrin and is known in Europe and other parts of the world as paracetamol — caused the observed increase in hyperactivity disorders. But they underscore that medications are only "safe" for pregnant women until studies become sensitive enough to detect subtle problems, said Dr. Daniel Kahn, a UCLA obstetrician who was not involved in the research.

"We used to count a baby's 10 fingers and 10 toes and assume that any drug his mother took must have been safe," said Kahn, a specialist in fetal-maternal health. Now observational studies like this are capable of picking up on possible drug effects that are less obvious and harder to measure. As such research moves forward, he said, it's best to follow a "less is better" rule when it comes to taking medications during pregnancy.

"The lowest exposure is always the best, for any agent," Kahn said.

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