More than half of all the mothers reported using acetaminophen while pregnant. The researchers found that children whose mothers used acetaminophen during pregnancy were at a 13 percent to 37 percent higher risk of later receiving a hospital diagnosis of hyperkinetic disorder, being treated with ADHD medications or having ADHD-like behaviors at age 7. The longer acetaminophen was taken — that is, into the second and third trimesters — the stronger the associations. The risks for hyperkinetic disorder/ADHD in children were elevated 50 percent or more when the mothers had used the common painkiller for more than 20 weeks in pregnancy.

"It's known from animal data that acetaminophen is a hormone disruptor, and abnormal hormonal exposures in pregnancy may influence fetal brain development," Ritz said. Acetaminophen can cross the placental barrier, Ritz noted, and it is plausible that acetaminophen may interrupt fetal brain development by interfering with maternal hormones or through neurotoxicity, such as the induction of oxidative stress, which can cause the death of neurons. "We need further research to verify these findings, but if these results reflect causal associations, then acetaminophen should no longer be considered a 'safe' drug for use in pregnancy," Olsen said. Other authors of the study included Zeyan Liew (first author) and Pei-Chen Lee of UCLA, and Cristina Rebordosa of the University of Arizona. Funding was provided by the Danish Medical Research Council (09-069178). The UCLA Fielding School of Public Health , founded in 1961, is dedicated to enhancing the public's health by conducting innovative research, training future leaders and health professionals from diverse backgrounds, translating research into policy and practice, and serving our local communities and the communities of the nation and the world. The school has 650 students from more than 35 nations engaged in carrying out the vision of building healthy futures in greater Los Angeles, California, the nation and the world. and follow us on Twitter . For more news, visit the UCLA Newsroom