More often than not, a woman's first prenatal visit comes with a lot of do's and don'ts.  What to eat.  What not to eat.  When to exercise.  How to exercise.  And most importantly, which over-the-counter medications are safe to take in the event of minor ailments like an upset stomach or a headache.

Until recently, that list banned ibuprofen but approved acetaminophen (the main ingredient in Tylenol) for reducing fevers, and relieving minor aches and pains.  

But a new study may have health experts thinking twice about that list as it found that the children of women who took Tylenol during pregnancy were 40 percent more likely to be diagnosed with attention deficit hyperactivity disorder - or ADHD- than children of mothers who did not.

The likelihood of a child developing ADHD increased the most  - by 63 percent - when the acetaminophen was taken during the last two trimesters of pregnancy.  When it was used in the third trimester alone, researchers found an increased in ADHD of about 28 percent.  The risk was smallest - about nine percent - when the pregnant woman only used the medication during her first trimester of pregnancy.

The study is interesting not just for its findings, but for its methodology.  Researchers could have talked to the mothers of children who had been diagnosed with ADHD to determine whether or not they used Tylenol during pregnancy.  But the potential for bias in memory recall was too great.  Instead, the researchers followed more than 64,000 Danish mothers and their children over a period of about 15 years.  They collected information on the mothers' use of acetaminophen before their children were born and long before they knew that their future children would be diagnosed with ADHD.  In this way, they were able to draw a stronger connection between the cause and effect.

But health experts warn that these are still two potentially unrelated events.  Now that researchers know that a link is possible, they need to figure out why it would exist.

They also warn that an unchecked fever during pregnancy could be more harmful to the developing fetus than the possibility of the future development of ADHD.  

Bottom line: It's best to steer clear of any unnecessary medication during pregnancy.  But if you develop a fever, talk to your doctor before you hit the medicine cabinet.

Related posts on MNN:

The opinions expressed by MNN Bloggers and those providing comments are theirs alone, and do not reflect the opinions of While we have reviewed their content to make sure it complies with our Terms and Conditions, MNN is not responsible for the accuracy of any of their information.