ST. LOUIS (UPI) -- In utero exposure to smoking is associated with attention-deficit hyperactivity disorder problems in genetically susceptible children, says a U.S. study.

The study, published in the June 15 issue of Biological Psychiatry, investigated male and female twin pairs, ages 7 to 19 years, to assess the relationship among genetic variations, prenatal substance exposures and ADHD sub-types.

Researchers at Washington University School of Medicine, St. Louis, found that when genetic factors are combined with prenatal cigarette smoke exposure, the ADHD risk rises significantly.

When the child has either or both of two specific forms of dopamine pathway genes and was exposed to cigarette smoking in utero, the risk for having combined type ADHD -- many inattention and hyperactive/impulsive symptoms -- increased three- to nine-fold, according to the study.

"These data highlight a new risk of maternal smoking, increasing the risk for ADHD in their children," Dr. John H. Krystal, editor of Biological Psychiatry, said in a statement.

"ADHD, in turn, increases the risk for substance abuse. Thus, it appears that in utero exposure to nicotine may help to perpetuate a cycle across generations that links addiction and behavioral problems."