Women smoking during pregnancy cause genes to switch on or off in their unborn babies, DNA researchers discover.

Pregnant mothers who smoke chemically alter the DNA in their unborn babies, mirroring patterns seen in adult tobacco users, scientists have discovered.

In one of the largest studies of its kind, international researchers analysed the DNA of more than 6000 mothers from around the world and their newborn children.

Mothers were questioned about their smoking habits and the results showed that children of the 13 per cent of women who smoked daily throughout most of their pregnancy had DNA modified in ways that could affect the functioning of genes.

More than 6000 genetic locations were identified where DNA differed from that of newborns whose mothers avoided tobacco.

About half could be associated with specific genes known to play a role in lung and nervous system development, smoking-related cancers and birth defects such as cleft lip and palate.

A separate analysis showed that many of these DNA alterations remained in older children whose mothers had smoked during pregnancy.

Maternal smoking appeared to have an "epigenetic" effect on foetal DNA - a chemical influence that does not change the genetic code as such but determines whether a gene is switched on or off.

Senior author Dr Stephanie London, from the US National Institute of Environmental Health Studies, said: "I find it kind of amazing when we see these epigenetic signals in newborns, from in-utero (in the womb) exposure, lighting up the same genes as an adult's own cigarette smoking. There's a lot of overlap.

"This is a blood-borne exposure to smoking - the foetus isn't breathing it, but many of the same things are going to be passing through the placenta."

The scientists now plan to investigate in more detail how the DNA modifications might influence child development and disease.

"We already knew that smoking is related to cleft lip and palate, but we don't know why," said Dr London.

The research is published in the American Journal of Human Genetics.