Women who are overweight or obese at the start of pregnancy are much more likely than healthy-weight women to deliver large babies, according to new research.

That's true, the researchers found, even if the women did not develop gestational diabetes, a known risk factor for delivering large babies.

In the past, researchers have focused on gestational diabetes -- diabetes that develops during pregnancy -- as a risk factor for having a large baby, says researcher Mary Helen Black, PhD, a research scientist at Kaiser Permanente's Southern California Department of Research and Evaluation.

Now it appears that overweight and obesity play at least as important a role as gestational diabetes in the risk of delivering a large baby. "And that is important because infants born too large for gestational age are at all sort of increased risk for other complications, including obesity later in life," Black says.

The new research is published in Diabetes Care.

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Weight, Diabetes, and Risks to Babies

For her study, Black looked at the medical records of nearly 10,000 women who got prenatal care and delivered their babies at a Kaiser Medical Center in Downey, Calif., from 2005 through 2010.

Nearly 60% of the women were overweight or obese, and more than 19% had gestational diabetes.

Of those who had gestational diabetes, more than 76% were either overweight or obese.

Black looked at how weight and diabetes affected the baby's birth weight. A baby is determined to be large based not only on birth weight, but on gender and race, Black says.

Compared to women of healthy weight and those without gestational diabetes, Black found those overweight, obese, or with gestational diabetes all had increased risks of having large babies.

    Overweight women without gestational diabetes had a 65% increased risk of having a large baby.
    Overweight women with gestational diabetes had a 177% increased risk.
    Obese women without gestational diabetes had a 163% increased risk.
    Obese women with gestational diabetes had a 447% increased risk.

Obese women who didn't have gestational diabetes but gained more than 20 pounds (the maximum recommended for obese women) during pregnancy were also more likely to have large babies, Black found.
Weight, Diabetes, and Large Babies: Perspective

The findings echo those published earlier this year by Ravi Retnakaran, MD, a clinician scientist at Mount Sinai Hospital and an associate professor of medicine at the University of Toronto. "This is showing that the predominant determinant of risk of having a large-for-gestational-age baby is maternal obesity and overweight," he says.

The recent research findings, he says, do not mean blood sugar levels aren't important to monitor during pregnancy. "It's not saying don't treat gestational diabetes," he says.

The study adds to growing evidence that excess weight and diabetes during pregnancy increase the risk of having a large baby, says Jennifer Wu, MD, an obstetrician-gynecologist at Lenox Hill Hospital in New York.

Large babies carry risks for  mother and infant, Wu says. "For the babies, there is the risk of the shoulders getting stuck [during delivery]. For the mothers, there are more risks of cesarean section or large lacerations with vaginal delivery." Babies born too large are also at increased risk of having birth defects and breathing problems, and some studies have found links to cancer, diabetes, and heart disease later in life.

What isn't clear from either study, Retnakaran says, is how much excess weight is too much.

For that reason, he says, the best advice for now is this: "Women should be looking at entering pregnancy at their ideal weight." A healthy weight is defined as a body mass index or BMI of 18.5 to 24.9.

A 5-foot-4-inch woman who weighs 120 pounds has a BMI of 20.6, for instance.