Previous studies have shown various harmful effects of smoking during pregnancy on the baby. Recent research has revealed that smoking during pregnancy increases the risk of delivering physically aggressive children.

Universite de Montreal researcher Jean Seguin, one of the lead researchers in the study, said, "What's new about this study is that we can pick up the link (between smoking and physical aggression) in children as early as 17 to 42 months of age."

Researchers studied the behaviour of 1,745 children who are part of a long-term research project called the Quebec longitudinal study.

Study revealed that the children of mothers who smoked at least 10 cigarettes a day had 30 percent higher risk of physical aggression as compared to the offspring of mothers who did not light up during pregnancy. Children of the mothers who had a serious history of anti-social behaviour and smoked heavily had 200 per cent greater risk than the offspring of women who did not smoke and had no anti-social history.

The study also revealed that heavy smokers with annual incomes of less than $40,000 had a 40-per-cent chance of having aggressive children, compared with 25 per cent for mothers who were moderate or non-smokers.