Women who smoke during pregnancy are nearly twice as likely to have children with attention-deficit/hyperactivity disorder (ADHD), a new study finds.

"This study serves to underline the fact that women who are pregnant should stop smoking," said lead researcher Dr. Karen Markussen Linnet, a pediatrician at Aarhus University Hospital in Denmark . "We are not able to make a certain conclusion, but this study points out that there is an association between smoking and ADHD."

Reporting in the August issue of Pediatrics , Linnet and her colleagues used extensive data available from the Danish government's longitudinal registers on nearly 4,000 children born between 1991 and 1994. They compared the smoking habits of the mothers of 170 children who were later diagnosed with ADHD -- or hyperkinetic disorder, as it's called in Europe -- with 3,765 mothers whose children were not hyperactive.

After controlling for low birth weight of babies, poor newborn health status, young maternal age and low socioeconomic status, smoking mothers were nearly twice as likely to have children with ADHD than mothers who didn't smoke while pregnant. Fifty-nine percent of the mothers of ADHD children smoked during pregnancy, compared to 35 percent of the mothers whose children did not have the disorder.

One important factor the researchers could not assess was whether pregnant smokers had ADHD themselves, which might make them more likely to smoke, and could be the reason for the higher number of children in this group who were diagnosed with hyperkinetic disorders.

"Nicotine acts as a stimulant, and young adults with hyperkinetic disorders who are pregnant could be using nicotine as a stimulant," Linnet said. "But it is very difficult to adjust for this factor, and it is important that we not make final conclusions."

Linnet said one strength of her study is that it is longitudinal, meaning that women reported their smoking habits before their children were diagnosed with ADHD. This avoids what is called "recall bias," when women are asked about their smoking habits after they learn that their children have ADHD and when their responses could be skewed by their feelings about their children.

"This is a very interesting study. The authors have controlled for a lot of confounding factors, and it is certainly biologically plausible that smoking could damage the fetus -- less oxygen is going to the brain, nicotine itself is a poison, and lots of other studies have found associations between smoking and poor cognitive development," said Gary Giovino, a senior research scientist in the department of cancer prevention, epidemiology, and biostatistics at Roswell Park Cancer Institute in Buffalo, N.Y.