More than 15 per cent of pregnant women are obese. This raises their odds of dying in pregnancy, of their baby being stillborn and of a host of pregnancy complications, some of which can be fatal.

Big babies are around twice as likely to grow into overweight adults, suggesting obesity and the lifetime of ill-health it can bring is ‘programmed’ in the womb. The trial involves 400 pregnant women in Liverpool, Coventry, Sheffield and Edinburgh.

They have started taking metformin, which has been safely used by diabetics for decades and is cleared for the treatment of diabetes in pregnancy. It costs just pence per tablet.