Maternal overweight and obesity during pregnancy associated with increased risk of preterm delivery

June 11, 2013

In a study that included more than 1.5 million deliveries in Sweden, maternal overweight and obesity during pregnancy were associated with increased risk for preterm delivery, with the highest risks observed for extremely preterm deliveries, according to a study in the June 12 issue of JAMA.

"Maternal overweight and obesity has, due to the high prevalence and associated risks, replaced smoking as the most important preventable risk factor for adverse pregnancy outcomes in many countries. Preterm birth, defined as a delivery of a liveborn infant before 37 gestational weeks, is the leading cause of infant mortality, neonatal morbidity, and long-term disability among non-malformed infants, and these risks increase with decreasing gestational age," according to background information in the article.

Sven Cnattingius, M.D., Ph.D., of the Karolinska Institutet, Stockholm, Sweden, and colleagues conducted a study to examine the associations between early pregnancy body mass index (BMI) and risk of preterm delivery by gestational age and by precursors of preterm delivery. The study included women with live single births in Sweden from 1992 through 2010. Maternal and pregnancy characteristics were obtained from the nationwide Swedish Medical Birth Register. The primary measured outcomes for the study were the risks of preterm deliveries (extremely, 22-27 weeks; very, 28-31 weeks; and moderately, 32-36 weeks). These outcomes were further characterized as spontaneous (related to preterm contractions or preterm premature rupture of membranes) and medically indicated preterm delivery (cesarean delivery before onset of labor or induced onset of labor).

BMI was calculated from information on height and weight at the first prenatal visit. BMI was used to characterize the women as underweight (BMI<18.5), normal (18.5-<25), overweight (25-<30), obese grade 1 (30-<35), obese grade 2 (35-<40), or obese grade 3 (≥40).

Among 1,599,551 deliveries with information on early pregnancy BMI, 3,082 were extremely preterm, 6,893 were very preterm, and 67,059 were moderately preterm. The researchers found that risks of extremely, very, and moderately preterm deliveries increased with BMI and the overweight and obesity-related risks were highest for extremely preterm delivery. Compared with normal-weight women, women with grade 2 and 3 obesity (BMI≥35) had 0.2 percent to 0.3 percent higher rates of extremely preterm delivery and 0.3 percent to 0.4 percent higher rates of very preterm delivery.

"Risk of spontaneous extremely preterm delivery increased with BMI among obese women (BMI≥30). Risks of medically indicated preterm deliveries increased with BMI among overweight and obese women," the authors write.

"These results can be generalized to other populations with similar or higher rates of maternal obesity or preterm delivery in as much as the underlying pathways linking maternal obesity and preterm delivery are common across populations. In the United States, where preterm delivery rates are twice as high as in Sweden, the majority of women are either overweight (26.0 percent) or obese (27.4 percent) in early pregnancy, and severe obesity (BMI≥35) is much more common than in Sweden. In 2008, extremely (<28 weeks) preterm births accounted for 0.60 percent of all live single births and 25 percent of all U.S. infant deaths among singletons, and extremely preterm birth is also the leading cause of long-term disability."

"Considering the high morbidity and mortality among extremely preterm infants, even small absolute differences in risks will have consequences for infant health and survival. Even though the obesity epidemic in the United States appears to have leveled off, there is a sizeable group of women entering pregnancy with very high BMI. Our results need to be confirmed in other populations given their potential public health relevance. Identifying the pathways through which maternal obesity influences offspring health is also needed to provide critical information to specifically target the women at highest risk of preterm delivery," the researchers conclude.
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(JAMA. 2013;309(22):2362-2370; Available pre-embargo to the media at http://media.jamanetwork.com)

Editor's Note: The study was funded by grants from Karolinska Institutet (Distinguished Professor Award to Dr. Cnattingius). Please see the article for additional information, including other authors, author contributions and affiliations, financial disclosures, etc.

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