Treating obese pregnant women with a diabetes drug does not stop their babies from being born overweight, a study has found.
Doctors had hoped that the treatment would help to reduce obesity rates and lower the number of difficult births.
Heavier babies are more likely to grow into overweight adults. They also have a higher risk of illnesses later in life, such as diabetes and heart disease.
It is thought that the additional weight gain in the womb is caused by exposure to excess blood sugar.
Researchers tested whether treating overweight mothers-to-be with the diabetes drug metformin - which helps to regulate blood sugar - would reduce the weight of their babies.
They treated 226 obese pregnant women with the medication from the second trimester until their babies were born.
There was no difference in the weight of babies born to mothers who received the treatment compared with a group of 223 women who received a dummy pill, they found.
Metformin also had no effect on the number of birth complications, such as miscarriages and still births.
The treatment did help to reduce blood sugar levels in the mothers-to-be. It also helped to lower the levels of other markers that have been linked to pre-eclampsia and premature births.
Researchers will need to follow the babies involved in the study for longer to determine whether the treatment lowers their chances of developing other health problems in later life.
The study was led by the University of Edinburgh in collaboration with the Universities of Warwick and Sheffield, and Liverpool. It is published in The Lancet Diabetes and Endocrinology.
The research was funded by the Efficacy and Mechanism Evaluation Programme, a Medical Research Council and National Institute for Health Research partnership.
Professor Jane Norman, Director of the Tommy's Centre for Maternal and Fetal Health at the University of Edinburgh, said: "The children of obese pregnant women face a lifetime of long term health complications as they grow up. The results of the EMPOWaR study emphasise the importance for women to be of normal weight before pregnancy."
Jane Brewin, Chief Executive of the baby charity Tommy's, said: "This well conducted study illustrates one of the most important health challenges we face. We must find ways to encourage women to manage their weight before they become pregnant to minimise the potential adverse impact on their children. This study shows us there are no easy answers and we must re-double our efforts to find effective ways to help women who are overweight in pregnancy."