The findings are based on over 18,000 UK babies born between 2000 and 2002, who were taking part in the Millennium Cohort Study.

Their mothers were classified as either non-smokers during pregnancy, quitters, light smokers, or those who smoked 10 or more cigarettes a day (heavy smokers).

The infants' temperaments were assessed when they were 9 months old, using a validated scale, designed to pick up positive mood, receptivity to new things, and regular sleep and eating patterns (regularity) in infants.

The results showed that mums to be who kicked their habit had the most easy going infants, compared with non-smokers and smokers.

Their children had the lowest chances of unpredictable behaviour and of becoming distressed when faced with new situations or things.

At the other end of the scale, heavy smokers had the most difficult children. Their children were much more likely to achieve the lowest scores for positive mood.

Previous animal research has also shown that nicotine is a behavioural toxin. And being temperamentally difficult can often be a forerunner of antisocial behaviour, the evidence suggests.

Pregnant smokers not only pass on harmful chemicals, which could affect the development of their unborn offspring, suggest the authors, but also traits and behaviours associated with their continuing to smoke during pregnancy.

Giving up smoking during pregnancy is associated with an urge to protect the baby rather than any intention to quit in the long term. Relapse rates are high after the birth, say the authors.

But it indicates the capacity to adapt to different circumstances and the ability to plan and to delay gratification, characteristics which seem to be missing in those who carry on smoking, they say.

Note: If no author is given, the source is cited instead.

• Smoking
• Pregnancy and Childbirth
• Infant's Health

• Smoking Addiction
• Child Development
• Child Psychology

• Nutrition and pregnancy
• Tobacco smoking
• Postpartum depression
• Maternal bond
• Premature birth
• Fertility