Mothers who smoke during pregnancy could alter their baby’s DNA, making them more prone to asthma, new research suggests.

Experts already know that children are more vulnerable to asthma and wheezing if their mother smoked, although the exact reasons are not well understood.

Now a new study presented at the American Thoracic Society conference in Denver suggests genes may be altered before a baby is born.

A process called DNA methylation – where gene expression is changed and cell function affected – is thought to occur.

Experts noticed DNA methylation-related changes in the AXL gene in children whose mothers smoked in pregnancy.

The AXL gene plays an important role in many human cancers and in the way the body fights infection.

“We found that children exposed to maternal smoking in utero had a 2.3 per cent increase in DNA methylation in AXL,” said Carrie Breton, assistant professor of preventive medicine from the University of Southern California in Los Angeles.

“These results confirm results from a prior study and present compelling evidence that environmental exposure to tobacco smoke during pregnancy may alter DNA methylation levels.”

A total of 173 children were included in the study, with their mothers and grandmothers answering a detailed questionnaire.

DNA samples were also collected from the cheek cells of mothers and children for examination.

The results showed that AXL changes were linked to a baby’s exposure to mother’s smoke in the womb, but no significant link was found with grandmothers who smoked.

Previous research has shown a link between grandmothers smoking and their grandchild’s risk of asthma.

In the latest study, the link between maternal smoking and asthma was stronger in girls than in boys. Dr Breton called for more research into the way environmental factors impact on babies before they are born.

“Environmental exposures occurring in utero have the potential to affect DNA methylation patterns before birth,” she said.

Leanne Metcalf, director of research at Asthma UK, said children whose parents smoke are 1.5 times more likely to develop asthma.

“Researchers have accumulated a wealth of evidence showing the detrimental effects smoking can have on a child’s lung health, beginning in the womb and continuing through childhood via exposure to second-hand smoke.

“This research reinforces why smoking during pregnancy should be avoided.

“Causing fundamental changes to a child’s DNA which is not easily reversed and exposing that child to an increased chance of developing asthma and a weaker immune system will have extremely serious long-term implications for any child.