The children of mothers who smoke while pregnant may suffer breathing problems because their DNA is altered, experts have suggested.
It is already clear the offspring of women who smoke in pregnancy are more susceptible to wheezing and asthma but the precise reasons for that have not been discovered.
Researchers, who presented their work at the American Thoracic Society conference in Denver, suggest the AXL gene may be altered in the womb.
They found DNA methylation, in which gene expression is changed and the function of cells affected, had happened to that gene in the babies of women who smoke.
The gene has a key role in the body's systems to fight infection and in many cancers. The study found a stronger link between maternal smoking and asthma in girls, than in boys.
Carrie Breton, Assistant Professor of Preventive Medicine from the University of Southern California in Los Angeles, said: "We found that children exposed to maternal smoking in-utero had a 2.3% increase in DNA methylation in AXL. These results confirm results from a previous study and present compelling evidence that environmental exposure to tobacco smoke during pregnancy may alter DNA methylation levels."
Other research found there is a link between grandmothers smoking and the risk of asthma their grandchildren face.