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An expectant mother who smokes exposes her fetus to relatively high concentrations of nicotine, which in turn alter receptors for the brain chemical dopamine essential for brain development, said doctors from Aarhus University in Copenhagen.

Writing in the journal Pediatrics, the researchers compared the backgrounds of 170 children diagnosed with hyperactive disorders against 3,800 children matched by age.

Of those mothers with children born with the disorder, 59 percent were smokers. The study found expectant mothers who smoked during pregnancy had a nearly three-fold risk of having a child with hyperkinetic disorders, which involves excessive muscular activity, inattention and impulsive behavior including attention deficit-hyperactivity disorder.

These conditions are the most prevalent mental disorders in child psychiatry.

Animal studies and studies on first-trimester human pregnancies have demonstrated the action of nicotine on developing brains, wrote study author Dr. Karen Linnet.