Children whose mothers smoked while pregnant are at an increased risk of developing asthma, say scientists.
They have discovered this could be partially because smoking while pregnant can change the structure of the child's DNA, weakening the immune system.
While it is commonly thought that genes are immutably, except if exposed to radiation, more and more evidence is showing that DNA can be changed by more everyday environmental influences. This happens through a normal biological process known as DNA methylation.
Now American medical researchers have found a potential genetic "root cause" of the link between smoking while pregnant and childhood asthma.
They found that the children of women who smoked while pregnant were more likely to have experienced more DNA methylation of the AXL gene, which is crucial to development of the immune system.
Carrie Breton, assistant professor of preventive medicine at the Keck School of Medicine of the University of Southern California USC in Los Angeles, said: "We found that children exposed to maternal smoking in utero had a 2.3pc increase in DNA methylation in AXL."
She explained this was "compelling evidence that environmental exposure to tobacco smoke during pregnancy may alter DNA methylation levels."
There was no significant association with grandmaternal smoking, however.
Exactly what effect these genetic changes have in such children is difficult to determine. They could also suffer from asthma more than others because the smoke affected their physical development in the womb, or simply because they breath in more smoke as children.
The research was presented on Wednesday to the American Thoracic Society in Denver.
Leanne Metcalf, director of research at Asthma UK, said: "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. This is bad news for children, communities and our health service.
"Children whose parents smoke are 1.5 times more likely to develop asthma and Asthma UK needs all parents and expectant parents to avoid exposing children to cigarette smoke to help protect them from developing asthma - once the damage is done, it’s very hard to undo."