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NEW YORK (Reuters Health) - Pregnant women who are overweight or obese are more likely to give birth prematurely, according to a new study from Sweden.
Researchers found that link was strongest for babies born the earliest - between 22 and 27 weeks - and therefore most at risk of complications.
"This study suggests that there is a direct association between maternal overweight and obesity during pregnancy and the risk of preterm birth," said Dr. Muktar Aliyu, who has studied pregnancy risks at Vanderbilt University School of Medicine in Nashville, Tennessee.
"Women should maintain a healthy weight before and after pregnancy not just for their own health but also for the sake of the unborn child," Aliyu, who wasn't involved in the new research, told Reuters Health in an email.
For their study, Dr. Sven Cnattingius from the Karolinska Institute in Stockholm and his colleagues analyzed Swedish databases of 1.6 million pregnant women and their babies born between 1992 and 2010.
A total of 5 percent of those babies were born early - most "moderately preterm," or between 32 and 36 weeks. Normal gestation is considered 37 to 42 weeks.
As a woman's weight at her first prenatal visit increased beyond a normal body mass index (BMI) - a measure of weight in relation to height - the researchers found her risk of delivering prematurely increased as well.
Just 0.17 percent of normal-weight women - one in 588 - had an extremely premature baby, for example. That increased to 0.21 percent for overweight women and up to 0.52 percent for the most severely obese women, with a BMI of 40 or higher.
A five-foot, six-inch woman has a BMI of 40 at 248 pounds.
Medically-induced early births were more common at all stages of pregnancy with increasing weight.
"That was, we can see, more or less entirely due to the increased risk of obesity-related maternal complications, with preeclampsia as number one, " Cnattingius told Reuters Health.
Preeclampsia occurs when a woman develops high blood pressure and protein in her urine during pregnancy, requiring the baby be delivered right away.
In this study, overweight and obese women were also at higher risk of a spontaneous extremely early birth, Cnattingius and his colleagues reported Tuesday in the Journal of the American Medical Association.
He thinks the obesity-related risks are tied to higher rates of inflammation and certain proteins that make heavy women more vulnerable to minor infections.
How much weight a woman gains during pregnancy - not just her pre-pregnancy size - can also raise her risk of delivering early, Aliyu said. He recommends pregnant women focus on eating a healthy diet and improving their lifestyle.
"For the actual woman, her individual risk increase is not very big," Cnattingius said. However, across a whole population of women - especially one with high rates of obesity, such as in the U.S. - these extra risks can add up, he added.
"I think it's important to focus on this in countries where obesity is prevalent," Cnattingius said.
SOURCE: bit.ly/JjFzqx Journal of the American Medical Association, online June 11, 2013.