THERE was once a poignant health education poster featuring a man offering a cigarette to a heavily pregnant woman.

"Yes, please," says the woman's speech bubble, oblivious to the smaller "No thanks" speech bubble emanating from her bump. Of course, unborn babies do not have a choice in the matter if their mothers choose to continue puffing through pregnancy, despite the known risks.

Moking among pregnant women and mothers is thought to account for around half of all cot deaths in Scotland and a quarter of miscarriages and still births, as well as reduced birthweights and more preterm deliveries. Yet it remains more common in Scotland than England, especially in Glasgow and among teenage mums.

A common response to questions from health professionals is: "My mum smoked when she was expecting me and I turned out fine." It is hard to change behaviour that is so culturally ingrained, especially when friends and family are heavy smokers. It may take literally generations. At best public policy can nudge citizens in the right direction. So we should welcome a study published today from Professor Jill Pell of Glasgow University that suggests the 2006 Scottish ban on smoking in public places has resulted in a reduction in the number of preterm births and low birthweight babies. In view of the increased risk of such babies developing long-term health problems, any reduction in those figures is highly desirable.

This is a major study. It considers all single births, nearly three-quarters of a million, between 1996 and 2009. The results imply that the publicity surrounding the ban about the risks of smoking and the increased support for smoking cessation may have been as important as the ban itself. The percentage of mothers smoking while pregnant fell from more than 25% to less than 19%.

Equally significant perhaps is the recently reported drop in children being admitted to hospital with breathing difficulties such as asthma. That suggests parents have not simply swopped smoking in public places for lighting up in their own living rooms, as some opponents of the ban predicted.

There is still a long furrow to plough on this issue. Nearly two-thirds of teenage mums in Scotland continue to smoke during pregnancy, as do one in four pregnant women in Glasgow, where 600 women are being recruited for a trial to see if offering £400 of supermarket vouchers can help stiffen their resolve to quit the habit. It may smack of desperation but if far more women stop smoking and stay stopped with the incentive than without it, public policy will have given a big nudge in the right direction and the scheme should be adopted more generally.

The argument about drinking in pregnancy is more complex but that too constitutes an avoidable risk to unborn children, bolstering the case for minimum alcohol pricing. It is tragic that having got rid of polio and diphtheria, the big childhood killers of the past, avoidable causes of child harm and death persist in the 21st century.