Obese women are being given an anti-diabetes drug during pregnancy to stop their babies being born fat.
The trial - the first of its kind in the world - has been launched following evidence of a sharp rise in the number of so-called 'sumo babies'.
Large babies are twice as likely to become overweight adults.
Researchers at the University of Edinburgh are giving 400 obese women the diabetes drug metformin during pregnancy. The drug should reduce the amount of energy reaching the baby in the womb.
Dr Fiona Denison of the Queen's Medical Research Institute in Edinburgh told Sky News: "Metformin is a drug already taken by women during pregnancy if they have diabetes, so we know that it is safe.
"It works by reducing the mother's blood glucose to more normal levels. We are hoping that will lead to a healthier outcome for mother and baby."
More than 15% of pregnant women are now clinically obese.
They are more likely to develop diabetes and pre-eclampsia in pregnancy - and their babies are more prone to complications in the womb and to being stillborn.
Figures from the NHS Information Centre show that last year there were 1,218 babies born weighing 5kg (11lbs) or more - a 30% rise in three years.
Dr Denison added: "A good diet and exercise are paramount for any women in pregnancy.
"We are not saying that taking this pill removes the need for following that advice. It's an adjunct to that."
Nine hospitals around the UK are taking part in the trial. Results are not expected for at least two years.