Importance

Preterm birth is a leading cause of infant mortality, morbidity, and long-term disability, and these risks increase with decreasing gestational age. Obesity increases the risk of preterm delivery, but the associations between overweight and obesity and subtypes of preterm delivery are not clear.

Objective

To study the associations between early pregnancy body mass index (BMI) and risk of preterm delivery by gestational age and by precursors of preterm delivery.

Design, setting, and participants

Population-based cohort study of women with live singleton births in Sweden from 1992 through 2010. Maternal and pregnancy characteristics were obtained from the nationwide Swedish Medical Birth Register.

Main outcomes and measures

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). Risk estimates were adjusted for maternal age, parity, smoking, education, height, mother's country of birth, and year of delivery.

Results

Among 1,599,551 deliveries with information on early pregnancy BMI, 3082 were extremely preterm, 6893 were very preterm, and 67,059 were moderately preterm. Risks of extremely, very, and moderately preterm deliveries increased with BMI and the overweight and obesity-related risks were highest for extremely preterm delivery. Among normal-weight women (BMI 18.5-<25), the rate of extremely preterm delivery was 0.17%. As compared with normal-weight women, rates (%) and adjusted odds ratios (ORs [95% CIs]) of extremely preterm delivery were as follows: BMI 25 to less than 30 (0.21%; OR, 1.26; 95% CI, 1.15-1.37), BMI 30 to less than 35 (0.27%; OR, 1.58; 95% CI, 1.39-1.79), BMI 35 to less than 40 (0.35%; OR, 2.01; 95% CI, 1.66-2.45), and BMI of 40 or greater (0.52%; OR, 2.99; 95% CI, 2.28-3.92). 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.

Conclusions and relevance

In Sweden, maternal overweight and obesity during pregnancy were associated with increased risks of preterm delivery, especially extremely preterm delivery. These associations should be assessed in other populations.