The study of more than 21,000 pregnancies suggests that starch in potatoes can trigger a sharp rise in blood sugar levels but UK experts say proof is lacking and lots of people need to eat more starchy foods for fibre, as well as fresh fruit and veg.
UK dietary advice says starchy foods (carbohydrates) such as potatoes should make up about a third of the food people eat. There is no official limit on how much carbohydrate people should consume each week.
National Institutes of Health in Maryland's study focussed specifically on pregnancy diabetes - gestational diabetes affects about 3 in every 100 pregnancies in the UK and, if not managed properly, could lead to premature birth or miscarriage.
Lead author Cuilin Zhang said: "Gestational diabetes can mean women develop pre-eclampsia during pregnancy and hypertension. This can adversely affect the foetus, and in the long term the mother may be at high risk of type-2 diabetes."
Over the 10-year study, there were 21,693 pregnancies and 854 of these were affected by gestational diabetes. Taking into account other risk factors including overall diet and family history, the researchers found a 27% increased risk of diabetes during pregnancy for those who typically ate two to four 100g (3.5oz) servings of boiled, mashed, baked potatoes or chips a week.
In those who ate more than five portions of potatoes or chips a week, the risk went up by 50%. The researchers estimate that if women swap their potatoes for vegetables or whole grains at least twice a week, they would lower their diabetes risk by 9-12%.
Dr Emily Burns, of Diabetes UK, called for more research as the current findings do not prove cause and effect.
"This study does not prove that eating potatoes before pregnancy will increase a woman's risk developing gestational diabetes, but it does highlight a potential association between the two," she said.
"As the researchers acknowledge, these results need to be investigated in a controlled trial setting before we can know more. What we do know is that women can significantly reduce their risk of developing gestational diabetes by managing their weight through eating a healthy, balanced diet and keeping active."
Read the study results in full at www.bmj.com/content/352/bmj.h6898