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.