HealthDay News — Maternal overweight and obesity at conception is associated with increased risks for preterm delivery, especially extreme preterm delivery, results of a large retrospective cohort study suggest.

Although excess weight increased risk across a range of severities for prematurity, the risk was most pronounced for extreme preterm delivery (defined as 22 to 27 weeks gestation). Women who were obese or severely obese were two to three times more likely to experience this outcome compared with normal weight women, Sven Cnattingius, MD, PhD, from the Karolinska University Hospital in Stockholm, and colleagues reported in the Journal of the American Medical Association.

“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 researchers 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 [BMI].” 

Continue Reading

In order to better understand the relationship between overweight, obesity and preterm delivery, Cnattingius and colleagues conducted a population-based study of 1,599,551 women in the Swedish Medical Birth Register, who had live singleton births from 1992 through 2010 and information available about BMI in early pregnancy.

Maternal height and weight information was used to calculate BMI for each mother, and the researchers classified each preterm birth as medically indicated (induced or cesarean section before labor onset) or spontaneous (preterm contractions or membrane rupture). Preterm deliveries were further categorized as extreme (22 to 27 weeks gestation), very (28 to 31 weeks) and moderate (32 to 36 weeks).

There were a total of 77,034 preterm deliveries: 3,082 were extremely preterm, 6,893 were very preterm and 67,059 were moderately preterm.

As BMI increased so did the risk for extreme, very and moderate preterm deliveries, the researchers found, but the association was only statistically significant for extreme prematurity. Among normal-weight women (BMI of 18.5 to less than 25 kg/m²) the rate of extreme preterm delivery was 0.17%, and increased as follows:

  • BMI of 25 to less than 30 kg/m² — 0.21%, OR 1.26
  • BMI of 30 to 35 kg/m²– 0.27%, OR 1.58 
  • BMI of 35 to less than 40 kg/m²  — 0.35%, OR 1.58
  • BMI ≥40 — 0.52%, OR 2.99

Among obese women (BMI of 30 kg/m² or more), the risk of spontaneous extremely preterm delivery increased with BMI. Among overweight and obese women, the risks of medically indicated preterm deliveries increased with BMI.

“These associations should be assessed in other populations,” the researchers concluded.


  1. Cnattingius S, et al “Maternal obesity and risk of preterm delivery” JAMA 2013; 309: 2362-2370.