Women who enjoy potatoes are at increased risk of suffering diabetes in pregnancy, research suggests.
Those who eat two to four servings of potato a week may be around 27% more likely to suffer diabetes in pregnancy, even when taking into account their weight, a study found.
Experts analysed total potato consumption, including baked, boiled, mashed and fried.
One serving included one baked or boiled potato, 237ml of mashed potatoes or 113g of fries.
Even one serving a week appeared to increase the risk by 20% compared with women eating less than one serving a week, once body mass index (BMI) was taken into account.
Those eating more than five servings a week had a 50% increased risk.
When women substituted two servings a week with other vegetables, pulses such as beans, lentils and peas, and whole grain foods, they had a 9% to 12% lower risk.
In the 10-year study of more than 21,000 pregnancies, published in the British Medical Journal (BMJ), 854 were affected by gestational diabetes.
Experts said higher potato consumption before pregnancy "was significantly associated with an increased risk" of the condition but found no specific link for eating fries alone.
The experts said: "Though potatoes are rich in vitamin C, potassium, dietary fibre and some phytochemicals, unlike other vegetables they can have detrimental effects on glucose metabolism because they contain large amounts of rapidly absorbable starch."
They said high potato consumption had already been associated with insulin resistance and an increased risk of Type 2 diabetes.
The women in the study were asked about potato consumption in the previous year as part of a questionnaire, looking at how often they ate certain foods.
The experts, including from Harvard Medical School in Boston, said: "Higher levels of potato consumption before pregnancy are associated with greater risk of (gestational diabetes), and substitution of potatoes with other vegetables, legumes, or whole grain foods might lower the risk."
The authors stressed that the study did not prove that potatoes caused diabetes.
The NHS estimates that up to 18% of women giving birth in England and Wales are affected by gestational diabetes.
It usually develops in the third trimester (after 28 weeks) and usually disappears after the baby is born.
However, women who develop the condition are more likely to develop Type 2 diabetes later in life.
Gestational diabetes often does not cause any symptoms and women are screened in pregnancy. The condition can increase the risk of stillbirth, miscarriage and premature labour.
Emily Burns, research communications manager at Diabetes UK, said: "This study does not prove that eating potatoes before pregnancy will increase a woman's risk of developing gestational diabetes, but it does highlight a potential association between the two.
"However, 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."
Dr Louis Levy, head of nutrition science at Public Health England, said: "As the authors acknowledge, it is not possible to show cause and effect from this study.
"The evidence tells us that we need to eat more starchy foods, such as potatoes, bread, pasta and rice, as well as fruit and vegetables to increase fibre consumption and protect bowel health.
"Our advice remains the same: base meals around a variety of starchy foods, including potatoes with the skin on, and choose wholegrain varieties where possible."
Professor Judith Buttriss, director general of the British Nutrition Foundation, which receives some funding from food producers and manufacturers, said: "It's important to recognise that the main nutrition-related determinant of gestational diabetes is pre-pregnancy body weight where the risk far exceeds the level of risk reported in the paper from potato consumption."
Janet Fyle, professional policy adviser at the Royal College of Midwives, said: "We cannot draw definitive conclusions from this research nor apply the findings to everyone.
"The key message from the research should be about varying the diet. Potatoes are a healthy food group. We need to tailor our messages to pregnant women so that they are able to have a well-balanced diet and not be put off eating them.
"We would encourage women who are pregnant or are thinking of becoming pregnant to have a healthy, varied diet including fresh fruit and vegetables, alongside taking folic acid supplements."
Dr Michael Heard, a spokesman for the Royal College of Obstetricians and Gynaecologists (RCOG), said: "Most women with gestational diabetes will have healthy pregnancies and healthy babies.
"While this small study found an increased risk of gestational diabetes for women who ate potatoes, there are a number of limitations to the study design, including that potatoes were included as a vegetable rather than a starchy food, and that gestational weight gain was not measured. Importantly it does not show a causal link between eating potatoes and increased risk of gestational diabetes.
"Women should not be alarmed by this study. Potatoes are a valuable carbohydrate and our advice continues to be that women should eat a healthy balanced diet including five portions of fruit and vegetables a day, starchy foods, lean protein and fibre-rich foods."