Published: Jun 11, 2013 | Updated: Jun 12, 2013
By Charles Bankhead , Staff Writer, MedPage Today

Action Points

• Note that this large Swedish registry study demonstrated that maternal obesity increased the risk of extremely preterm delivery.
• Be aware that obesity was not associated with more moderately preterm deliveries.

The risk of preterm delivery increased significantly in women who were overweight or obese at conception, a large retrospective cohort study showed.

Obese and severely obese women had a two to three times greater risk of extreme preterm delivery (at 22 to 27 weeks of gestation) as compared with normal-weight women.

Excess weight increased the risk across the range of severities for prematurity but was most pronounced in association with extreme preterm delivery, Sven Cnattingius, MD, PhD, of Karolinska Institute in Stockholm, and co-authors reported in the June 12 issue of JAMA.

“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 wrote. “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 body mass index (BMI).”

In many parts of the world, excess weight has supplanted smoking as the leading preventable risk for adverse pregnancy outcomes. Two recent reviews suggested the risk of moderately and very preterm birth is increased among women with a BMI of 35 or greater (BMJ 2010; 341: c3428; J Matern Fetal Neonatal Med 2009; 22: 957-970).

Studies of associations of preterm delivery and overweight (BMI 25 to 30) or grade 1 obesity (BMI 30 to 35) have been less consistent, the authors noted in their introduction. However, two recent studies (one published this year) did show an increased risk of extreme prematurity (<28 weeks gestational age) among women with a BMI ≥30 (J Matern Fetal Neonatal Med 2010; 23: 1444-1440; PLoS One April 15, 2013).

Obesity increases the likelihood of medically indicated preterm delivery, but the association with spontaneous preterm delivery has been less clear. Moreover, an association might vary according to duration of gestation.

Cnattingius and colleagues sought to clarify the relationship of overweight/obesity with preterm delivery. They analyzed data from the Swedish Medical Birth Register, which contains information on more than 98% of births in the country since 1973.

They limited the study to singleton births from 1992 through 2010. Data on maternal height and weight were used to calculate BMI for each mother, and each preterm birth was classified as medically indicated (induced labor or cesarean section before onset of labor) or spontaneous (preterm contractions or membrane rupture).

Preterm deliveries were categorized as extreme (22 to 27 weeks), very (28 to 31 weeks), and moderate (32 to 36 weeks).

The final analysis comprised 1,599,551 deliveries involving mothers with information on BMI in early pregnancy. The births included 77,034 preterm deliveries: 3,082 extremely preterm, 6,893 very preterm, and 67,059 moderately preterm.

Rates of extremely, very, and moderately preterm delivery all increased with maternal BMI, but the association achieved statistical significance only for extreme prematurity. Normal-weight women (BMI 18 to 25) had a rate of extreme preterm delivery of 0.17%. Corresponding rates and odds ratios associated with increasing BMI were:

• BMI 25 to 30 — 0.21%, OR 1.26
• BMI 30 to 35 — 0.27%, OR 1.58
• BMI 35 to 40 — 0.35%, OR 2.01
• BMI ≥40 — 0.52%, OR 2.99

The odds of extremely preterm delivery increased with BMI among obese women (BMI ≥30), and the odds of medically indicated preterm delivery increased among overweight and obese women.

Many ob/gyns have suspected preterm delivery as one of the potential consequences of obesity in pregnancy, but the Swedish study provides data to back up the suspicions, said Eugene Scioscia, MD, of West Penn Allegheny Health System in Pittsburgh.

“There are a whole slew of problems associated with patients who are obese when pregnant — everything from preterm labor to gestational diabetes to preeclampsia to macrosomia and large babies,” Scioscia told MedPage Today.

“If only these patients would understand the complications of being obese in association with being pregnant, it might motivate them to change their ways,” he added. “Obesity has replaced smoking as the No. 1 complication associated with pregnancy.”