UNBORN babies' hearts can be damaged if their mothers are overweight and smoke, a study has shown.
Scientists studied data on 797 live and stillborn babies and aborted foetuses with congenital heart problems.
They were compared with 322 babies and foetuses having chromosomal abnormalities but no heart defects.
Birth abnormalities affecting the heart were much more common among children, and foetuses, whose mothers both smoked and were overweight.
Women with both risk factors were more than twice as likely to give birth to a baby with a congenital heart defect as those with just one.
Mothers defined as "overweight" had a Body Mass Index (BMI) of 25 or more.
On the BMI scale, which relates height and weight, "overweight" is classified as between 25 to 29.9 and "obese" as 30 and above.
The risk of specific abnormalities which reduce the flow of blood from the heart's ventricle pumping chambers was more than tripled in mothers who smoked and were overweight.
Writing in the journal Heart, the authors, led by Dr Maria Bakker from the University of Groningen in the Netherlands, said: "Maternal overweight and smoking may have a synergistic adverse effect on the development of the foetal heart.
"Overweight women who wish to become pregnant should be strongly encouraged to stop smoking and to lose weight."
Around eight in 1,000 babies are affected by congenital heart abnormalities, but a likely cause is found in only 15% of cases.
Scientists believe the new findings point to a poor balance between levels of "good" and "bad" cholesterol.
Disturbances in blood cholesterol levels are known to be associated with obesity and smoking.
Other risk factors for congenital heart abnormalities include diabetes, rubella infection and epilepsy.