The risk of heart damage in unborn babies is doubled if the mother-to-be is an overweight smoker, scientists are warning.
A study found women with both risk factors were more than twice as likely to give birth to a child with congenital heart disease - a general term to describe a range of birth defects that affect the normal workings of the heart.
Researchers believe an imbalance of good and bad cholesterol could be to blame.
Disturbances in blood cholesterol levels are known to be associated with obesity and smoking.
Smoking increases bad cholesterol and decreases the good cholesterol, boosting the risk of heart disease.
While high cholesterol is just one of many consequences of obesity.
Lead researcher Dr Maria Bakker from the University of Groningen in the Netherlands, highlighted that women should take action before trying for a child.
Writing in the journal Heart she 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.'
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.
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.
Those 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.
Congenital heart disease is one of the most common types of birth defect, and affects an estimated 6 in every 1,000 babies born in England., but a likely cause is found in only 15 per cent of cases.
Other risk factors include diabetes, rubella infection and epilepsy.