Treating obese pregnant women with a diabetes drug does not stop their babies from being born overweight, research has found.
Doctors had hoped that the treatment would help to reduce obesity rates and lower the number of difficult births.
The study, published in the journal The Lancet Diabetes and Endocrinology, emphasises the importance for women to be of normal weight before pregnancy.
"The children of obese pregnant women face a lifetime of long term health complications as they grow up," said one of the researchers, Jane Norman, professor at University of Edinburgh in Britain.
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.
So the 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, the researchers found.
Metformin also had no effect on the number of birth complications, such as miscarriages and still births.
However, 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.
