Over 400 overweight mums-to-be are trialling the drug in the UK and Scotland, and researchers hope that taking it from the second trimester will mean they deliver smaller babies.
The overweight women will take the medication up to three times a day during their pregnancy in an attempt to reduce the food supply to the foetus.
The doctors behind the NHS trial say obesity among pregnant women is reaching epidemic proportions and they need to protect the health of tomorrow's children.
Metformin has been used for decades to treat diabetes, but some experts are angered by the new trial, saying that the mums-to-be should be controlling their weight through diet and exercise. But medics backing the research say that many women are obese when they fall pregnant, and that diet and exercise at that stage would be fruitless.
Professor Jane Norman from Edinburgh University - who is leading the study - said:
"I absolutely support the improvement of diet and encouraging exercise but we are increasingly faced with women who start their pregnancy obese. Saying at that stage to eat less and exercise more is not particularly helpful."
Latest figures suggest that almost half of all women of child-bearing age are overweight or obese, and more than 15 per cent of expectant mums are obese.
What do you think about this? Was there any correlation between your weight and your baby's weight?