Researchers from the UK and the US say that smoking during pregnancy significantly increases the risk of having a child with behavioral problems, BBC News reported.

Such problems can be evident in children as young as three years old, according to the study published in the Journal of Epidemiology and Community Health.

Smoking during pregnancy may damage the developing structure of the baby’s brain; something one expert said was another strong reason for mothers to kick the habit.

Researchers from the universities of York, Hull and Illinois carefully analyzed data from 14,000 mother and child pairs from the Millennium Cohort Study -– which covers UK children born between 2000 and 2001.

Based on how many cigarettes mothers smoked every day during pregnancy, they were classified as light or heavy smokers.

Each mother was asked to score their three-year-old children’s behavior using a questionnaire called Strengths and Difficulties, which focuses on behavior problems and hyperactivity, or attention deficit disorders.

The mother’s age at the child’s birth, her level of education and socioeconomic status, family stability and problematic parenting were all factors that could likely influence the results.

Mothers who were heavy smokers were 80 percent more likely to have boys with these problems, while lighter smokers were 44 percent more likely to have boys who had problems with their conduct.

However, both heavy and light smokers were also significantly more likely to have boys who were hyperactive or had attention deficit disorders.

Light and heavy smoking during pregnancy were also significantly associated with conduct problems for three-year-old girls, but not with hyperactivity and attention deficit behaviors.

The findings were consistent with previous research in older age groups, according to Professor Kate Pickett, who lead the research.

“Smoking in pregnancy may have direct effects on the fetal development of brain structure and functioning which has been shown in studies of rats,” she said.

She explained that it could also be a marker for the transmission of processes between the generations that are associated with both smoking in pregnancy and behavior problems in children.

The study was another reason why mothers should make every effort to give up smoking — ideally before they get pregnant, according to Professor Alan Maryon-Davis, president of the Faculty of Public Health.

“There are four thousand toxic substances in cigarette smoke and many of these will pass into the brain of the fetus and it is possible that they could have an effect on how the brain chemistry works,” Maryon-Davis said.

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