Maternal overweight and obesity are the most important preventable risk factors for adverse pregnancy outcomes worldwide. In this population-based cohort study, the Swedish Medical Birth Register was used to assess risks of spontaneous and medically indicated preterm deliveries among women with overweight and different grades of obesity, based on body mass index (BMI).
The study included women with live singleton births in 1992 to 2010. Maternal and obstetric characteristics were obtained from the Birth Register, and BMI was calculated using the standard ranges for underweight, normal, overweight, and obesity grades 1 to 3. Gestational age was determined by ultrasound, date of last menstrual period, or postnatal assessment. Delivery was categorized as extremely, very, and moderately preterm (22–27, 28–31, and 32–36 weeks, respectively). Preterm deliveries were characterized as spontaneous or medically indicated. Spontaneous preterm deliveries were stratified into preterm premature rupture of the membranes (PROM) or preterm labor. Logistic regression analysis was used to estimate the associations between BMI in early pregnancy and risks of preterm delivery. Odds ratios (ORs) with 95% confidence intervals (CIs) were calculated. Two-tailed P < 0.05 indicated statistical significance. All statistical analyses were performed with SAS software version 9.3.
The final study population included 1,857,822 deliveries. Information about early pregnancy BMI was available in 86% (1,599,551) of deliveries. During the study period, 93,419 (5.03%) preterm deliveries were reported. Overall rates of extremely, very, and moderately preterm deliveries were 0.23%, 0.47%, and 4.36%, respectively. Preterm delivery rates increased with BMI among overweight and obese women and also among short women (<155 cm). Rates of overweight and obesity were substantially increased in women with hypertensive or diabetic conditions. Compared with women with normal BMIs, risks of extremely, very, and moderately preterm deliveries increased with BMI. The overweight and obesity-related risks were highest for extremely preterm delivery and lowest for moderately preterm delivery. Among normal-weight women, the rate of extremely preterm delivery was 0.17%. Compared with normal-weight women, rates and adjusted ORs (aORs) of extremely preterm delivery were 0.21% (aOR, 1.23; 95% CI, 1.13–1.35), 0.27% (aOR, 1.53; 95% CI, 1.35–1.74), 0.35% (aOR, 1.97; 95% CI, 1.62–2.40), and 0.52% (aOR, 2.91; 95% CI, 2.21–3.83) for BMIs of 25 to less than 30, 30 to less than 35, 35 to less than 40, and 40 or greater, respectively. Risks of spontaneous extremely preterm delivery increased with increasing obesity; however risks of spontaneous very or moderately preterm delivery were generally not associated with obesity. Women with grade 2 or 3 obesity had increased risks of extremely preterm delivery because of PROM and spontaneous labor. Risks of moderately preterm delivery due to PROM modestly increased with BMI, but no associations were found between BMI and risks of very or moderately spontaneous preterm labor. Risks of medically indicated extremely, very, and moderately preterm deliveries increased with BMI.
Maternal overweight and obesity during pregnancy were associated with increased risk for preterm delivery with the highest risks for extremely preterm deliveries. These results should be confirmed in other populations given their public health importance. Identifying the pathways by which maternal obesity influences offspring health is also necessary to provide critical information to specifically target the women at highest risk of preterm delivery.
Clinical Epidemiology Unit, Department of Medicine Solna, Karolinska University Hospital and Department of Clinical Science, Intervention, and Technology, Karolinska Institutet, Stockholm, Sweden; Department of Epidemiology, University of Michigan School of Public Health, Ann Arbor, MI; and Department of Women’s and Children’s Health, Uppsala University, Uppsala, Sweden