New research from the UK has revealed that women who smoke heavily during pregnancy have the most difficult babies.
The researchers say the revelation could be the deciding factor for women struggling to quit smoking during pregnancy.
They say mothers can expect to have a happier baby if they break the habit, as giving up cigarettes not only improves mother and child's physical health, it also makes it more likely the baby will have a sunnier nature, with regular sleeping and eating patterns.
It also appears that the babies of recent quitters were even more easy-going than those born to women who had never smoked or had stopped years earlier.
Dr. Kate Pickett, of York University, says a possible explanation for quitters having better behaved children than non-smokers is that women who are strongly motivated to give up smoking in pregnancy are very positive about the decision and the outcomes for the baby.
Dr. Pickett's research is the first evidence of the impact on babies' behaviour of stopping smoking in early pregnancy, even though the risks to physical health caused by mothers' smoking have been well-documented.
However data on their babies' mental well-being has been scant.
Dr. Pickett's study involved 18,000 British children born between 2000 and 2002 who were taking part in the Millennium Cohort Study.
It found at the age of nine months that women who smoked heavily in pregnancy (more than ten cigarettes a day) had the most difficult babies with the worst moods while mothers who had given up smoking had the best behaved babies.