Overweight, obese pregnant women have higher risk of very early delivery

Women who are overweight and or obese during pregnancy may be at greater risk of preterm-and more notably-extremely preterm delivery, according to results of a new Swedish study published in JAMA.

The purpose of the study was to assess early pregnancy body mass index (BMI) and preterm delivery risk by gestational age and by forerunners of preterm delivery.

The population-based retrospective study focused on women with live singleton births in Sweden from 1992 to 2010. Data pertaining to the mothers as well as pregnancy factors were gathered from the Swedish Medical Birth Register. Preterm deliveries were classified as either extremely preterm (22-27 weeks); very preterm (28-31 weeks); or moderately preterm (32-36 weeks). In addition, the results were listed as either spontaneous (preterm contractions or premature rupture of membranes) or medically indicated preterm delivery (cesarean delivery before onset of labor or induced onset of labor).

Risks were adjusted for the mother’s age, smoking, level of education, height, mother's country of birth, and year of delivery.

The results showed that for 1,599,551 deliveries that had data on early pregnancy BMI, 3082 fell into the “extremely preterm” category, 6893 were “very preterm,” and 67,059 were “moderately preterm.” As BMI increased, so did risks of extremely, very, and moderately preterm deliveries; further, overweight and obesity-specific risks were highest for extremely preterm delivery.

“Considering the high morbidity and mortality among extremely preterm infants, even small absolute differences in risks will have consequences for infant health and survival,” the authors said in a statement. “Even though the obesity epidemic in the U.S. appears to have leveled off, there is a sizable group of women entering pregnancy with very high BMI.”

For women of normal weight (BMI 18.5 to <25), extremely preterm delivery rate was 0.17%. Compared to women of normal weight, rates (%) of extremely preterm delivery were: BMI 25 to <30 (0.21%), BMI 30 to <35 (0.27%), BMI 35 to <40 (0.35%), and BMI of ≥40 (0.52%). For obese women (BMI ≥30), risk of spontaneous extremely preterm delivery was elevated. Risks of medically indicated preterm deliveries also increased with BMI among overweight and obese women.

For more on the risks associated with obesity during pregnancy, see this month’s cover story on page 26.

Cnattingius S, Villamor E, Johansson S, et al. Maternal obesity and risk of preterm delivery. JAMA. 2013;309(22):2362-2370.