Babies born to mothers who suffer diabetes in pregnancy tend to be fatter, research suggests.
Scientists compared 42 babies from mothers with gestational diabetes with 44 babies from healthy mothers, and used MRI scans to determine levels of body fat.
Readings were taken shortly after birth, and again when the babies were 10-weeks-old.
The research, published in the journal Diabetes Care, found no difference between the groups at birth but, by 10 weeks, babies born to mothers with diabetes had a 16% higher body fat volume than those in the other group.
The team said possible explanations included changes to the baby's metabolism while in the womb, or differences in the composition of breast milk among women with diabetes. Most of the babies in the study were breastfed.
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.
But women who develop the condition are more likely to develop Type 2 diabetes later in life.
Gestational diabetes often does not cause any symptoms but women are at higher risk if they are overweight or obese. The condition can increase the risk of stillbirth, miscarriage and premature labour.
Dr Karen Logan, lead author of the study from the department of medicine at Imperial College London, said: "Gestational diabetes is becoming more and more common, and babies born to these mothers are at increased risk of developing diabetes when they grow up.
"Therefore we need to understand what effects maternal diabetes has on the baby.
"This new study suggests diabetes in the mother can trigger changes in the baby at a very early stage."
She said most women in the study were not overweight and there were other possible causes of diabetes in pregnancy, such as a genetic predisposition.
Professor Neena Modi, from the department of medicine at Imperial and president of the Royal College of Paediatrics and Child Health (RCPCH), added: "We found no differences in body fat at birth. However by 10 weeks of age, the babies born to mums with diabetes had accumulated about 16% more fat even though they were predominantly breastfed.
"The importance of this unexpected finding is that the beginnings of obesity are apparent in early infancy in babies born to mothers with diabetes indicating that research targeted at methods to reduce excessive fat deposition in these babies is urgently needed.
"One in five five-year-olds and one in three in 10-year-olds in the UK is overweight or obese.
"These children are at high risk of multiple obesity related conditions.
"Obesity is the biggest public health threat facing the UK children today."
Lucinda Winckworth, whose son James was in the control group of the study, said: "Before James went into the scanner he was breastfed, wrapped up in a warm blanket, and had mini ear muffs placed over his ears.
"He fell asleep before going into the scanner, and even though I'd been warned the scanner was noisy he slept all the way through it and all the way home.
"It was fascinating for us to see his scan pictures - we've kept them in his memory box, along with his ear muffs."