Around 30% of the women in the study smoked before getting pregnant
A gene can explain why some women find it more difficult to stop smoking during pregnancy, say UK researchers.
In a study of 2,500 women who smoked before falling pregnant, the "addictive gene" was associated with a lower chance of quitting once pregnant.
Overall, 28% of pregnant smokers quit in the first three months compared with 21% of those with two copies of the gene and 31% without the gene.
The findings appear in the journal Human Molecular Genetics.
The researchers from Peninsula Medical School say there is no reason why the finding would not also be true in other groups of smokers attempting to quit but they wanted to look at pregnant women because they have a particularly strong incentive to break the habit.
They used data on a total of 7,845 women of European descent from the South West of England who took part in the Avon Longitudinal Study of Parents and Children.
A specific genetic variant associated with the nicotine receptor was analysed because it had previously been found to be associated with how much people smoke once they start.
By the final three months of pregnancy, 47% of women with two copies of the non-addictive gene said they had stopped smoking, compared with only 34% of women with two copies of the smoking addiction gene.
The researchers said pregnant women are under considerable and social pressure to stop smoking but there are a number of factors which influence whether they quit, including age, education, and whether or not their partners smoke.
"However, we were keen to investigate whether the genetic variant that influences increased cigarette consumption also had a role to play as an extra hurdle to quitting smoking during pregnancy, and our study suggests that it does," says researcher Dr Rachel Freathy.
Co-author Professor Tim Frayling, an expert in human genetics, said: "There are parallels with the obesity gene in that people think it's matter of self control but it's more complicated than that.
"It's clear that some women with two copies of the addictive gene can give up, it just means it's more difficult for some people than others."
The team are now planning to investigate the same genetic variant in people with Chrohn's disease who are also strongly advised to stop smoking as it makes their condition worse.
Dr Alex Bobak, a GP in Wandsworth, south London, has a special interest in smoking cessation - he said genes had been implicated in nicotine addiction but that there are many other factors which have an impact, including social circumstances.
"The positive thing is we can look at this sub-group and see if they benefit more from certain kinds of treatment to help them stop smoking."
He added that NHS support was available for pregnant women who are able to take all forms of nicotine replacement therapy.