"Obese women have increased risk of urinary tract infections and vaginal infections," Rabin said. "These increase the risk of preterm delivery."
The study noted that a bacterial infection is considered the most important risk factor for spontaneous extremely preterm delivery.
The American Congress of Obstetricians and Gynecologists has issued a recommendation that it is OK for women who are overweight or obese to not gain weight during pregnancy, or even to lose weight, said Artal, of Saint Louis University.
"I consider pregnancy to be an ideal time for behavioral modification," Artal said. He noted that pregnant women have an improved chance to adopt healthy habits because they have better access to medical care than at any other time in their life, are more prone to comply with recommendations because they have the added responsibility of their unborn child, and are under close medical supervision.
Although the study found an association between maternal obesity and premature birth, it did not establish a cause-and-effect relationship.
For his part, Fothergill said the study cries out for follow-up research that will better elaborate upon the link between obesity and premature birth.
"There needs to some additional research that explains why these women are delivering early," he said. "What is it about obesity that's making these women go into labor early?"
The March of Dimes has more about overweight and obesity during pregnancy.
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