Oct. 31, 2008 -- Gaining 40 pounds or more during pregnancy nearly doubles the risk of having a baby who weighs 9 pounds or more, in turn increasing the health risks to mother and baby, according to a new study.

Excessive pregnancy weight gain and big babies have often been linked, says Teresa Hillier, MD, senior investigator at the Kaiser Permanente Center for Health Research, Portland, Ore., and the study's lead author. Researchers have also known that women who develop diabetes during pregnancy, called gestational diabetes, are more likely to deliver heavier babies, Hillier tells WebMD.

But the new study is believed the first to conclude that women who gain excessive weight during pregnancy are even more likely to have heavier babies than women treated for gestational diabetes who don't gain excess weight.

"More than one in five women gain too much weight during pregnancy and only 5% have gestational diabetes," Hillier tells WebMD. The study, she says, points to the need for all women to follow recommendations about not gaining excessive amounts of weight.