Obesity in Pregnancy Strongest Predictor of Large Babies

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Obesity during pregnancy is the strongest predictor of whether a mother will give birth to a large infant, a new study from Canada suggests.

In the study, the more the woman weighed before pregnancy, and the more weight she gained during pregnancy, the greater her chance of giving birth to a large baby, which can create health problems for the infant and the mother. In general, babies that weigh about nine pounds or more are considered large babies.

At the same time, researchers found no association between slightly elevated blood glucose levels — below the level needed to classify a woman as having gestational diabetes — and an increased chance of having a large baby, once researchers took women's weight into account.

That finding was significant, since the American Diabetes Association recently lowered the criteria for gestational diabetes so that many more women would be classified as having the condition, in an effort to reach more women at risk of having a large baby. The new criteria have not yet been adopted by U.S. health officials, however; nor have the standards changed in Canada, where the study took place.

If weight, rather than glucose levels, turns out to be the most important factor in having a big baby, "lowering the criteria might not be targeting the appropriate problem," said study researcher Dr. Ravi Retnakaran of Mount Sinai Hospital in Ontario. Instead, getting to a healthy weight before pregnancy, and avoiding excessive weight gain in pregnancy, might be more effective ways to reduce the risk, Retnakaran said.

"The rate of obesity has increased so much that maybe glucose levels aren’t as big a factor as they once were," Retnakaran said.

However, Retnakaran said, for women with gestational diabetes, glucose control is very important. The study did not include women with full-blown gestational diabetes.

Risks of large babies

The health risks babies face when they are born too small have long been familiar, but large babies, too, face health complications, including broken bones during delivery and an increased risk for obesity in childhood and adolescence. Mothers who have a large baby are more likely to suffer labor complications and torn tissue, and are more likely to need a Caesarian section.  

Gestational diabetes is known to increase the risk of having a large baby, but recent research has shown women without diabetes are also at risk for this complication. To find out what contributes to the risk in these women, Retnakaran and colleagues analyzed information from 472 pregnant Canadians, 368 of them with normal glucose levels, and the other 104 with slightly elevated glucose levels.

Researchers considered babies to be large if their weight put them at the 90th percentile or higher among all newborns of their race and gender. The medical term for a large baby is macrosomia.

Sixty-eight of the babies born to women in the study were considered large.

Women who were overweight before getting pregnant, or who gained excess weight during their pregnancy, were 12 to 16 percent more likely to have large infants.

Glucose levels, and levels of fatty acids in the blood, did not affect a woman's risk of having a large infant, the researchers said.

Should criteria for gestational diabetes be changed?

In 2010, an International Association of Diabetes in Pregnancy study group proposed lowering the criteria for diagnosing gestational diabetes as a way to reduce more women's risk of having a large for gestational-age infant.

However, lowering the criteria would mean that many more pregnant women would receive a diagnosis of gestational diabetes. In the United States, the percentage diagnosed with gestational diabetes would increase to 18 percent from 7 percent, according to the American College of Obstetrician and Gynecologists, which has not recommended adoption of the new criteria.

The National Institutes of Health is planning a conference on the topic in October.

Until we have more research on the cause of large babies, reducing weight and weight gain in pregnant women is a sound recommendation, Dr. Edmond Ryan of the University of Alberta wrote in an editorial accompanying the study.

The editorial and the study will be published today (May 22) in the Canadian Medical Association Journal.

Pass it on:  Weight should be controlled in pregnancy to prevent babies from becoming too big before they are born.

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Rachael Rettner

Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.