Researchers are claiming diabetic pregnant women have a four-fold risk of their babies being born with birth defects.
A new study looked at data collected from more than 400,000 pregnant women in the north-east of England and found that congenital heart disease and spina bifida were increased in those with diabetes.
Diabetes is already known to cause problems in pregnancy, such as birth defects, miscarriage and the baby being overweight due to too much sugar, but medics are now concerned that the rise in diabetes could make the situation worse.
Type 1 diabetes tends to present itself in childhood, whilst Type 2 occurs largely as a result of diet.
The study was undertaken at Newcastle University and analysed data from 401,149 pregnancies between 1996 and 2008. Over 1,600 of the women had diabetes. The risk of birth defects went from 19 in every 1,000 births for women without pre-existing diabetes to 72 in every 1,000 births for women with diabetes.
The researchers concluded that a woman's sugar levels in the run-up to conception were the "most important" risk factor which could be controlled.
Guidelines from the National Institute of Health and Clinical Excellence suggest women should reduce their blood sugar levels to below 6.1% before trying to fall pregnant.
The charity Diabetes UK funded the study and said that it needed to get the message out to women that they should tell their diabetes healthcare team if they were planning to get pregnant. Dr Iain Frame from the charity said: "We need to get the message out to women with diabetes that if they are considering becoming pregnant, then they should tell their diabetes healthcare team, who will make sure they are aware of planning and what next steps they should be taking.
"It also highlights the importance of using contraception if you are a woman with diabetes who is sexually active but not planning to become pregnant."
The study's findings are published in the journal Diabetologia.