Smoke toxins can pass to the foetus
Smoking during pregnancy significantly increases the risk of having a child with behavioural problems, according to UK and US researchers.
Writing in the Journal of Epidemiology and Community Health, they say the problems can be evident in children as young as three years old.
They believe smoking in pregnancy may damage the developing structure of the baby's brain.
One expert said it was another strong reason for mothers to give up smoking.
The researchers from the universities of York, Hull and Illinois looked at more than 14,000 mother and child pairs who were taking part in the Millennium Cohort Study.
This covers UK children born between 2000 and 2001.
The mothers were categorised as light or heavy smokers depending on how many cigarettes they smoked every day during pregnancy.
They were asked to score their three-year-old children's behaviour using a questionnaire called Strengths and Difficulties, which focuses on behaviour problems and hyperactivity, or attention deficit disorders.
They took into account factors likely to influence the results, including the mother's age at the child's birth, her level of education and socioeconomic status, family stability and problematic parenting.
Mothers who were light smokers were 44% more likely to have boys who had problems with their conduct.
Heavy smokers were 80% more likely to have boys with these problems.
Both heavy and light smokers were also significantly more likely to have boys who were hyperactive or had attention deficit disorders.
For three-year-old girls, light and heavy smoking in pregnancy were significantly associated with conduct problems but not with hyperactivity and attention deficit behaviours.
Professor Kate Pickett, who lead the research, said their findings were consistent with previous research in older age groups.
She said: "Smoking in pregnancy may have direct effects on the foetal development of brain structure and functioning which has been shown in studies of rats.
"Or it may be a marker for the transmission of processes between the generations that are associated with both smoking in pregnancy and behaviour problems in children."
Professor Alan Maryon-Davis, president of the Faculty of Public Health, said: "This is another reason why mothers should make every effort to give up smoking - ideally before they get pregnant.
"There are four thousand toxic substances in cigarette smoke and many of these will pass into the brain of the foetus and it is possible that they could have an effect on how the brain chemistry works."