Pregnant women with higher-than-normal blood sugar are more likely than others to have a very large baby, a new study shows. Oversized babies can be difficult to deliver, sometimes need to be delivered ahead of schedule and can cause injury to mother or newborn during birth.

Very high blood sugar during pregnancy constitutes gestational diabetes, a condition that occurs in 3 to 7 percent of pregnancies, says study coauthor Donald Coustan, an obstetric gynecologist at Brown University Medical School in Providence, R.I.

Less clear is the effect during pregnancy of hyperglycemia, excess sugar in the blood that falls short of diabetes. It typically arises from obesity, a lack of exercise, excess stress and less common factors.

Women with gestational diabetes control it by eating healthy foods, avoiding simple sugars and spreading their carbohydrate consumption out over the day. Also, doctors often intervene by counseling the women to exercise and by prescribing insulin or diabetes medication when necessary.

But for hyperglycemic women, the effects of elevated blood sugar on the pregnancy, much less how to treat it, remain unclear.

To test for hyperglycemia, Coustan and colleagues analyzed blood glucose test results obtained from more than 23,000 pregnant women who didn’t have diabetes. During the second half of each pregnancy, each woman underwent a blood sugar test, first on an empty stomach and again after ingesting some glucose. These readings offer a glimpse of how well a person is processing carbohydrates and sugars. The researchers matched these test results with each woman’s birth outcome and with data on the newborns.

Hyperglycemia resulted in significantly more newborns weighing in the top 90th percentile, the scientists report in the May 8 New England Journal of Medicine. Women with elevated blood sugar were also slightly more likely to have cesarean section deliveries, a bit more likely to develop a pregnancy complication called preeclampsia and much more likely to have babies whose umbilical cord blood contained high concentrations of a compound called serum peptide C. High peptide C indicates that a fetus is making a lot of insulin in response to the mother’s high blood sugar.

Insulin is the hormone that regulates how cells use sugar. Hyperglycemia occurs when insulin in the body is overmatched by the amount of digested sugar and other carbohydrates. This mismatch can sometimes result from insulin resistance, a condition in which cells fail to efficiently use insulin to capture the energy of sugars.

Coustan and colleagues have provided “an elegantly designed, very large, international study,” write Jeffrey Ecker and Michael Greene of Harvard Medical School in Boston in the same NEJM issue. The data show a clear continuum of risk to the fetus as the levels of blood sugar rise in the mother, leading to adverse pregnancy outcomes, they note.

The threshold between hyperglycemia in pregnancy and gestational diabetes is poorly defined, Coustan says. “Choosing the right threshold for intervention is going to be fairly arbitrary,” he says.

Armed with this and other research, a panel of scientists sponsored by the American Diabetes Association will tackle this issue later this year, says study coauthor Lynn Lowe, an epidemiologist at Northwestern University Feinberg School of Medicine in Chicago.

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