Maternal Obesity Ups Risk for Preterm Delivery

Steven Fox

June 11, 2013

Results from large study carried out in Sweden add to mounting evidence that women who are overweight or obese early in their pregnancies are at increased risk for preterm delivery.

Sven Cnattingius, MD, PhD, from the Karolinska Institutet, Stockholm, Sweden, and colleagues, published the results of the 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," the researchers write.

It has long been known that maternal obesity is linked to preeclampsia, miscarriage, gestational diabetes, and cesarean delivery, but research findings have been mixed on exactly how excess maternal weight affects risks for preterm birth and low birth weight.

Some studies have found a link, while others have not. In fact, a few studies have suggested that maternal obesity may have a protective effect against preterm delivery and low birth weight.

To investigate the question more closely, Dr. Cnattingius and colleagues conducted a population-based cohort study using public health records to assess 1,599,551 single births that took place in Sweden from 1992 to 2010. Body mass index (BMI) was measured at each of the women's initial prenatal clinic visits.

The researchers categorized preterm deliveries as extremely preterm (22 - 27 weeks' gestation), very preterm (28 - 31 weeks), or moderately preterm (32 - 36 weeks). The outcomes were further characterized as either spontaneous or medically indicated. Spontaneous deliveries included those associated with preterm contractions or premature rupture of membranes. Medically indicated deliveries included cesarean delivery before onset of labor or deliveries that were induced.

The researchers report that among the more than 1.5 million deliveries, 3082 were extremely preterm, 6893 were very preterm, and 67,059 were moderately preterm.

Maternal overweight and obesity during pregnancy were linked with heightened risk for preterm delivery, with the highest risks seen for extremely preterm deliveries.

"Specifically, compared with normal-weight women (BMI, 18.5-<25), women with grade 2 and 3 obesity (BMI≥35) had 0.2% to 0.3% higher rates of extremely preterm delivery and 0.3% to 0.4% higher rates of very preterm delivery," the authors write.

They note that risks for spontaneous extremely preterm delivery increased along with BMI among obese women (BMI ≥ 30 kg/m2). In addition, risks for medically indicated preterm deliveries increased with BMI in women who were overweight and obese.

"To our knowledge, this is the first study investigating the effect of overweight and obesity on risks of spontaneous and medically indicated extremely preterm deliveries," they write.

The authors emphasize that their findings need to be confirmed in other populations, given their potential relevance to public health. "Considering the high morbidity and mortality among extremely preterm infants, even small absolute differences in risks will have consequences for infant health and survival," they conclude.

This study was funded by grants from Karolinska Institutet. One coauthor has reported receiving an institutional grant or pending grant from the Swedish Research Council for Working Life and Social Health. The other authors have disclosed no relevant financial relationships.

JAMA. 2013;309:2362-2370.


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