Nulliparity and preterm birth in the era of obesity epidemic

Objective. To assess the impact of obesity on preterm birth among nulliparous women. Methods. Retrospective cohort study of nulliparous mothers delivering infants in Florida between 2004 and 2007. Women were classified as non-obese (pre-pregnancy body mass index (BMI) < 30) or obese (BMI >= 30). The main outcomes assessed were preterm birth, very preterm birth and extremely preterm birth. Risk estimates were obtained using logistic regression. Multiparous non-obese mothers were the referent group for all analyses. Results. As compared to multiparous women, nulliparous mothers had an increased risk of very preterm and extremely preterm birth with the highest risk observed for extremely preterm birth (odds ratios (OR) = 1.37, 95% CI = 1.28, 1.47) (p for trend < 0.01). Obese nulliparous mothers had an elevated risk of preterm, very preterm and extremely preterm birth, with the risk of extremely preterm birth being the most pronounced (OR = 1.97, 95% CI = 1.75-2.22) [p for trend < 0.05]. The heightened risk associated with obesity among nulliparous women was observed across all racial/ethnic sub-populations, with black nulliparous obese mothers being at greatest risk of all preterm birth-subtypes. Conclusions. Obesity is a risk marker for preterm, very preterm and extremely preterm birth among first-time mothers and particularly among blacks and Hispanics.