Women with diabetes are five times more likely to have a child with structural heart defects, research suggests.
Heart defects are rare
Researchers at hospitals in north-east England examined data on nearly 200,000 babies born in the region between 1995 and 2000.
Heart defects were found in 3.6% of babies with diabetic mothers, but just 0.74% of those born to healthy women.
Writing in the journal Heart, the authors call for special monitoring of pregnant women with diabetes.
Researcher Dr Ginny Birrell, of Sunderland Royal Hospital, told BBC News Online that the standard of monitoring of diabetic women during pregnancy varied across different units.
She said: "We cannot prevent these defects but close monitoring would enable us to pick them up before birth, so that prompt treatment can be given when the baby is born."
A spokesman for the charity Diabetes UK said: "Diabetes can increase the risk of problems in a pregnancy but the risk is still small.
"This is why it is important for those with diabetes to ensure that their condition is very well controlled throughout their pregnancy.
"Diabetes UK recommends that women with diabetes undergo pre-conception counselling in order to ensure they are ready to address any such risks.
"It is also vital that doctors and nurses look out for signs of gestational diabetes, a condition which can sometimes occur during pregnancy."
Structural abnormalities of the heart are the most common kind of congenital birth defect, affecting around six to eight babies out of every 1,000 born. In most cases the cause is unknown.
The most common structural defect is a hole in the heart, but the latest study found that babies born to diabetic mothers were at a particularly increased risk of a condition known as transposition of the great arteries.
Normally, the major blood vessel known as the aorta emerges from the left-hand side of the heart and feeds blood to the rest of the body, while the pulmonary artery emerges from the right-hand side, and feeds blood to the lungs, where it picks up vital new supplies of oxygen.
Transposition means that the position of the arteries is reversed. This leads to blood being pumped round the body again and again without ever being re-oxygenated.
Babies born with the condition require surgery soon after birth, or they will die.
The reason why diabetes - which is essentially an inability to properly break down sugar - increases the risk of heart defects is not known.
The study was based in the former Northern Health Region comprising the counties of Cumbria, Northumberland, Tyne & Wear, Cleveland and Durham.