NEW YORK - For at least some moms-to-be, it's extra body fat - and not blood sugar levels - that may be key to their risk of having a big baby, a study published Tuesday suggests.
Researchers found that among 472 pregnant women they followed, those who were heavy before pregnancy or gained too many pregnancy pounds were more likely to have a "large-for-gestational-age" baby.
That meant having a newborn who topped 8 pounds, 13 ounces. Babies that large are more likely to cause birth injuries in the mom or to require a cesarean section.
Experts have long known that women who have gestational (pregnancy-related) diabetes are at increased risk of having a big baby. And the baby's weight is largely blamed on those mothers' high blood sugar levels.
The new findings suggest that for women without gestational diabetes, it's a mom's excess pounds that really matter in her baby's birth size, said lead researcher Dr. Ravi Retnakaran of Mount Sinai Hospital in Toronto.
"If we want to reduce the risk of having a big baby," he told Reuters Health, "the real gain would be in targeting the mother's weight."
Nearly half of pregnant women in the US may start their pregnancies already overweight or obese. Recent research has found, though, that about half a per cent, or 50 out of every 1,000 pregnant women, will develop gestational diabetes.
The new findings, which appear in the Canadian Medical Association Journal, come at a time when experts are debating whether they should lower the "threshold" blood sugar level for diagnosing gestational diabetes.
Canada has yet to adopt that approach. In the US, the American Diabetes Association has endorsed the proposal, but the American College of Obstetricians and Gynecologists is holding off on a decision: it's awaiting results from a conference to be held on the issue in October by the National Institutes of Health.
If the new criteria were adopted, Retnakaran said, about one-fifth of pregnant women in Canada would be "labeled" as having gestational diabetes - more than double the current rate.
But based on the current findings, he said, that label would not do those women any good - because it's weight, not moderately elevated blood sugar levels, that seems to be the "big predictor" of a big baby.
Dr. Edmond A. Ryan, a diabetes researcher at the University of Alberta in Canada, agreed.
For women who do not meet the traditional criteria for gestational diabetes, "obesity itself is actually more important than glucose (blood sugar)," said Ryan, who wrote an editorial published with the study.
So helping women manage their weight - ideally before pregnancy - is the key, according to Ryan. On the other hand, giving women the label of gestational diabetes could do more harm than good.
"I think we need to be cautious about the medicalization of pregnancy," Ryan said.
If more women are diagnosed with gestational diabetes, he noted, that will mean more women being referred to an endocrinologist, more women having to do daily blood sugar checks during pregnancy and more women on diabetes treatment.
Although gestational diabetes is often treated with diet and exercise alone, some women are put on insulin - a synthetic version of the blood-sugar-relating hormone.
The new study's findings are based on 472 pregnant women who were free of gestational diabetes - though almost one-quarter had moderately elevated blood sugar levels.
Overall, 68 of their newborns were large for gestational age. There was no clear connection between elevated blood sugar and the odds of a big baby.
There was, however, a correlation between moms' weight and their baby's birth size.
For every kilogram a woman gained during pregnancy, her odds of having a big baby inched up 12 per cent.
Pre-pregnancy weight also mattered. The researchers measured that in terms of body mass index (BMI), a measure of kilos per meter squared. For every one-point increase in pre-pregnancy BMI, the odds of a big baby rose by 16 per cent.
Both Retnakaran and Ryan said the findings underscore the importance of managing pregnancy weight gain with a healthy diet and regular activity.
No one knows what the "ideal" weight-gain range is for cutting your odds of a big newborn.
But Retnakaran said that current guidelines from the Institute of Medicine (IOM), an advisory panel to the US government, are a "good starting point."
The IOM says that obese women should gain no more than 11 to 20 pounds during pregnancy.
That's less than the 15 to 25 pounds IOM recommends for women who are overweight when their pregnancy begins, and the 25 to 35 pounds recommended for normal-weight women.