Miami: Women who smoke while pregnant may harm their babies by chemically altering the DNA of the developing fetus, a major study including more than 6,000 women and children found Thursday.
Doctors have long warned women to avoid cigarettes while pregnant because smoking can lead to stillbirth, or babies born with cleft palate, lung disease, or neurobehavioral problems.
Despite these warnings, as many as 12 per cent of pregnant women in the United States continue to smoke, exposing their fetuses to chemicals in cigarette smoke that pass through the barrier of the mother's placenta.
Experts have not known much about how these changes to DNA take root in the fetus, so they performed a meta-analysis of 13 prior, smaller studies, some of which had suggested links between smoking and chemical modifications to DNA, also known as methylation.
Of the 6,685 babies in the meta-analysis, 13 per cent were born to mothers who smoked regularly while pregnant.
Another 25 per cent had mothers who smoked occasionally while pregnant or had quit early in pregnancy.
Among the sustained smokers, researchers identified "6,073 places where the DNA was chemically modified differently" than in the newborns of non-smoking moms.
"About half of these locations could be tied to a specific gene," said the study, published in the American Journal of Human Genetics.
"Many signals tied into developmental pathways," said co-author Bonnie Joubert, an epidemiologist at the National Institute of Environmental Health Sciences (NIEHS), noting that changes were seen in genes relating to lung and nervous system development, smoking-related cancers, and birth defects such as cleft lip and cleft palate.
The DNA changes were documented in samples of umbilical cord blood drawn after birth.
Such changes were less apparent in mothers who smoked less frequently during pregnancy.
A separate analysis found that some DNA modifications remained apparent in a group of hundreds of older children -- those with an average age of six -- whose mothers had smoked while pregnant.