Pregnant women who are overweight or obese are at increased risk for preterm delivery, according to a population-based cohort study of women in Sweden.1
It is well known that maternal obesity is associated with an increased risk of preeclampsia, miscarriage, gestational diabetes, and cesarean delivery. It is less well known how maternal weight affects risk of premature birth. Some findings have shown that overweight and obesity increase the risk of premature birth. Conversely, there is some evidence, albeit limited, that maternal obesity may have a protective effect against preterm delivery and low birth weight.
To further explore the associations between body mass index (BMI) in early pregnancy and risk of preterm delivery, researchers in Sweden used the Swedish Medical Birth Register to obtain maternal and pregnancy characteristics of all women with live singleton births in Sweden from 1992 to 2010. A total of 1,599,551 deliveries provided information on early pregnancy BMI for this study.
The study authors identified 3082 extremely preterm births (22 to 27 weeks’ gestation), 6893 very preterm births (28 to 31 weeks’ gestation), and 67,059 moderately preterm births (32 to 36 weeks’ gestation). Compared with normal-weight women (BMI, 18.5 to less than 25 kg/m2), increases in BMI were associated with corresponding increases in both the rates and adjusted odds ratios of extremely preterm delivery (Table).
Overall, women with a BMI of 35 kg/m2 or greater had rates of extremely preterm delivery that were 0.2% to 0.3% higher and rates of very preterm delivery that were 0.3% to 0.4% higher than those of women whose weight was considered normal. In normal-weight women, the rate of extremely preterm delivery was 0.17%. Among obese women (BMI, 30 kg/m2 or greater), the risk of extremely spontaneous preterm delivery increased with BMI, as did the risks of medically indicated preterm deliveries in overweight and obese women.
“Maternal overweight and obesity has, due to the high prevalence and associated risks, replaced smoking as the most important preventable risk factor for adverse pregnancy outcomes in many countries,” the authors wrote.1 These findings should be confirmed in other study populations, especially considering their potential relevance to public health, the study authors explained.
“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 study authors concluded.1
- In women in Sweden, maternal overweight and obesity in early pregnancy were associated with increased risk for preterm delivery, especially for extremely preterm deliveries.
- The risk of spontaneous preterm delivery and the risk of medically indicated preterm delivery increased with BMI among overweight and obese women.
Table. BMI and Its Association With Extremely Preterm Delivery
|BMI, kg/m2||Rate of delivery at 22-27 weeks’ gestation||Adjusted odds ratio*|
|18.5 - <25||0.17%||--|
|25 - <30||0.21%||1.26|
|30 - <35||0.27%||1.58|
|35 - <40||0.35%||2.01|
BMI, body mass index.
*Compared with normal-weight women (BMI, 18.5 - <25 kg/m2).
Data from Cnattingius S et al. JAMA. 2013.1
1. Cnattinguius S, Villamor E, Johansson S, et al. Maternal obesity and risk of preterm delivery. JAMA. 2013;309:2362-2370.