BAKERSFIELD, Calif. -- Pregnant women often joke that they are "eating for two," but in reality it takes only a few hundred extra calories per day to nourish a growing baby.

Doctors advise average-weight women to gain up to 25 pounds during pregnancy, but one in five packs on 40 pounds or more. This added weight places both the baby and the mother at risk.

In a new study of 1,100 pregnant women, doctors from Kaiser Permanente found that those who gained more than the recommended amount of weight increased their risk for pregnancy-related diabetes by 50 percent.

Even adding a few extra pounds in the first trimester, before women typically even show, increased diabetes risk.

Gestational diabetes in turn raises the odds for complications -- including cesarean-section delivery, abnormally-large body size in the baby, and the mother's later risk for type 2 diabetes.

But doctors don't recommend dieting for pregnant women either; they suggest women work with their doctors to design a healthy diet that will best nourish them and their growing babies.