Researchers from the University of Glasgow looked at the records of nearly 4,000 Scottish diabetic mothers.
They found high blood sugar levels in pregnant diabetics was a “risk factor” in stillbirths.
The Body Mass Index (BMI) of diabetic women is also a critical factor, the study found.
Researchers also found that a third of stillbirths in diabetic women happened at full term.
Dr Sharon Mackin, who carried out the study, said: “It is vital that we, as healthcare professionals, find better ways to support women during their fertile ages to optimise weight and blood sugar, so that when entering pregnancy, whether that be planned or unplanned, they are better prepared and their risk of adverse outcomes is reduced.
“It is important that women with diabetes are mindful of this, and are able to access appropriate pre-conceptual counselling, even if not imminently planning a pregnancy.
“Women with diabetes should also make contact with their diabetes clinic as soon as they get a positive pregnancy test so that we can see and support them early on.”
The study ruled that earlier delivery may be considered “an attractive option” but that more research was needed before recommendations for optimal timing were made.
Dr Mackin said the question “has to be asked about whether earlier delivery of all diabetic pregnancies could prevent these term stillbirths”.
She said: “We don’t know the answer to this. The optimal timing of delivery in such pregnancies is not clear.”
The study identified 5,392 babies born to 3,847 mothers with diabetes in Scotland between April 1998 to June 2016.
Mothers with type 1 diabetes were more than three times likely to deliver a stillborn child, while those with type 2 were at least four times likely.
Stillbirth rates were 16.1 per 1,000 births in the women with type 1 diabetes and 22.9 per 1,000 births in type 2 diabetes, compared with 4.9 per 1,000 births in the general population.
Women with type 1 who had stillbirths had higher than average blood sugar levels throughout their pregnancy, while pre-pregnancy levels were a more important predictor of stillbirth in those with type 2.
Babies with the highest and lowest birth weights were most at risk, the study published in the Diabetologia journal, found.
Dr Emily Burns, head of research communications at Diabetes UK, said: “Most women with diabetes have healthy pregnancies and healthy babies, but this research reinforces the importance of supporting women to manage their blood glucose levels if they are planning a pregnancy, in order to reduce their risk of complications as much as possible.
“It also suggests that losing extra weight, for women with type 2 diabetes who are overweight, could help to reduce this risk as well.
“We need research to find better ways of predicting who is most at risk of complications during pregnancy, to ensure support can be provided to those who need it most.”