University of Michigan researchers say a woman's weight during pregnancy may have a much bigger impact on her infant than previously understood.
A study that looked at 1.8 million live single births in Sweden found that women who had a BMI of 35 or higher had twice the infant mortality rates of women who were not obese.
"What we saw was that the risk of infant mortality increased as the BMI of the women was higher," says Dr. Eduardo Villamor.
"So women who had a BMI over 35, which is obese, had twice the risk of having an infant death during the first year of life."
It's tempting to attribute those infant deaths to other culprits, like poverty, pre-term births, or health complications related to obesity, like diabetes.
But Villamor says they accounted for those factors in their research, and had to rule them out.
"This study was conducted in Sweden," he says, "and in general the Swedish society is more egalitarian. Health services for women and children are free and universally available.
"So the poverty issue is less relevant than it might be here.
"But despite that, we did account for some factors that are indirect outcomes of poverty, such as maternal education and home country.
"It is possible that in other countries where poverty is more of an issue, especially when it affects health care access, the link between maternal obesity and infant mortality may be strong, we don’t know.
"But I don’t think that was a major driver of the association in the Swedish study."
Even when they excluded women who had experienced illnesses like diabetes or other complications, the findings remained the same.
"Which means not all of this is explained by those complications related to obesity," he says.
So far, it's too early to know what, exactly, causes the correlation of obesity and infant mortality.
"It’s the first step on a research process that we hope will provide us with better answers as we dig further into it," says Villamor.
"So the next step, perhaps, is to try to address those potential mechanisms. We hope to be able to examine, for example, the relation between obesity and illnesses which might lead to death in the children. We might also examine the effects of weight change before and even during pregnancy on infant mortality, and eventually the additional research should provide more light."
Until then, Villamor says it's important women understand that the risks exist.