Mothers who smoke while pregnant are causing changes to their unborn babies that can lead them to have less of a type of cholesterol known to protect against heart disease, scientists said on Tuesday.

In a study in the European Heart Journal, Australian researchers found that by the age of eight, children born to mothers who smoked in pregnancy had lower levels of HDL cholesterol, at around 1.3 millimoles per liter (mmol/L), than those born to mothers who hadn't smoked, with about 1.5 mmol/L.

High-density lipoprotein (HDL) cholesterol is often referred to as "good" cholesterol and is known to play an important role in protecting against atherosclerosis, where fatty materials collect along the walls of arteries, thickening and eventually blocking them, leading to heart problems and heart attacks.

"Our results suggest that maternal smoking 'imprints' an unhealthy set of characteristics on children while they are developing in the womb, which may well predispose them to later heart attack and stroke," said David Celermajer, a professor of cardiology at the University of Sydney.
"This imprinting seems to last for at least eight years and probably a lot longer," he said, adding that the heart disease risk for smokers' children could be 10 to 15 percent higher.

Smoking during and after pregnancy is already known to be linked to a wide range of childhood health problems, including behavioral and neurocognitive problems and sudden infant death.

Yet the prevalence of smoking while pregnant is still high, at around 15 percent in many Western countries, the researchers said. And until now scientists were not clear how prenatal exposure to cigarette smoke might affect future heart risks.

Celermajer's team analyzed data from 405 healthy eight year olds, born between 1997 and 1999, who had been enrolled before birth into a randomized controlled trial that was investigating asthma and allergic diseases.

The researchers collected data before and after the children were born, including information on mothers' smoking habits before and after pregnancy, exposure to passive smoke, and data on height, weight, waist measurement and blood pressure.

They used ultrasound scans to measure the arterial wall thickness and, in 328 children who agreed, they took blood to measure lipoprotein levels.

Although there was no effect on the thickness of the children's arterial walls, Celermajer's team found there was an effect on levels of HDL cholesterol.

He suggested that lower HDL levels at this age might have a serious health impact in later life, since the children will probably continue to have low levels as they grow up.

"Cholesterol levels tend to track from childhood to adulthood, and studies have shown that for every 0.025mmol/L increase in HDL levels, there is an approximately 2.0 to 3.0 percent reduction in the risk of coronary heart disease," he said in a statement about his research.

"If we extrapolate this, we can suggest that the difference between children of smoking mothers versus non-smoking mothers might result in a 10 to 15 percent higher risk."