Treating obese pregnant women with a diabetes drug that regulates blood sugar does not prevent their babies from being overweight when they were born.

The drug was chosen by researchers for a recent study based on the theory that exposure to excess blood sugar causes babies to gain more weight while in the womb. Heavier babies are expected to become overweight adults, increasing their risk for diabetes and heart disease later in life.

"We must find ways to encourage women to manage their weight before they become pregnant to minimize the potential adverse impact on their children," said Jane Brewin, chief executive of Tommy's Center for Maternal and Fetal Health, in a press release. "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."

Researchers gave 226 obese pregnant women metformin, a diabetes drug that helps regulate blood sugar, beginning in their second trimester and continuing until they gave birth.

While the drug lowered the women's blood sugar levels and the levels of indicators for pre-eclampsia and premature births, metformin had no effect on the weight of their babies as compared with 223 women who received a placebo.

Researchers plan to follow the babies to see whether the treatment will have any effect on their chances for developing health problems later in life.