The study, published in Child Psychiatry and Human Development, showed that those whose mothers had been exposed to tobacco smoke while pregnant -- either by smoking or by being around smokers -- did not show more symptoms of depression or anxiety than those children whose mothers were not exposed to smoke.

However, University of Washington psychologists Lisa Gatzke-Kopp and Theodore Beauchaine did find that the children whose mothers were exposed to smoke showed more symptoms of ADHD and conduct disorder.

"This is a matter of severity," Gatzke-Kopp said in a statement. "Children with these disorders have a range of behaviors that stretch from problematic to severe. It is a continuum based on the number of symptoms, and children who were exposed to smoke exhibited more symptoms."

The Seattle researchers believe the nicotine in tobacco may affect brain development during pregnancy.

Evidence suggests that the dopamine system in the brain gets over stimulated during pregnancy, according to Beauchaine. Dopamine is a brain chemical that plays an important role in behavior and cognition, among other functions.