Women who smoke while pregnant risk having hyperactive preschoolers who can't pay attention, a large study from the UK hints.

Although previous studies have demonstrated significant risks for school-aged boys, this is the first time an association has been shown between smoking during pregnancy and problems for girls and for boys as young as 3, the researchers point out.

Dr. Kate E. Pickett, of the University of York, Hull-York Medical School and colleagues looked for ties between smoking in pregnancy and behavior and attention problems in more than 13,000 3-year-old boys and girls in the UK Millennium Cohort Study.

As part of the study, a wide range of information was gathered including family economic status, education level of parents, ethnicity, parents' marital status, financial difficulties and maternal smoking, drinking or drug use.

Questions were also asked about the children to assess behavior and hyperactivity-inattention problems such as how easily their child was distracted or if their child was prone to temper tantrums, fight with or bully other kids, argue with grownups, steal, lie, and/or cheat.

In all, nearly 10 percent of women reported smoking heavily (+10 cigarettes a day) throughout their pregnancy, 12.5 percent were light smokers (less than 10 cigarettes a day), and 12.4 percent tried to quit, the researchers note in a report published this week in the Journal of Epidemiology and Community Health.

Even though no ill-effects of even heavy maternal smoking during pregnancy were recorded for most boys (61.6 percent) and girls (71.7 percent), the risk of developing behavior or attention problems increased with maternal prenatal smoking, the researchers found.

They also found that the effect of smoking during pregnancy was different in boys and girls. Boys exposed to cigarette smoke in the womb were more likely to have behavior and attention deficit problems, while girls with this exposure were more likely to experience behavior problems alone.

Girls whose mothers quit smoking during pregnancy had a lower risk of behavior problems than girls whose mothers never smoked leading the researchers to conclude the mother's the ability to quit may be a characteristic of restraint and easy temperament that the daughters inherit.

Smoking throughout pregnancy carries the highest risk for both boys and girls, according to the data. Overall, the most important factor seems to be smoking continuously throughout pregnancy, more than amount smoked, Pickett said.

Persistent heavy smokers had a higher risk of having a boy with conduct problems than light smokers. For conduct problems in girls, any continuous smoking seemed to matter more than amount. Similarly, for hyperactivity-inattention problems both light and heavy smokers had similarly elevated risks compared to non-smokers, Pickett explained.

SOURCE: First Journal of Epidemiology and Community Health, 2009.

United kingdom