Association of extremes of prepregnancy BMI with the clinical presentations of preterm birth

OBJECTIVE: The purpose of this study was to examine associations between the prepregnancy maternal body mass index (BMI) across the 3 clinical presentations of preterm birth (PTB). STUDY DESIGN: We conducted a retrospective cohort study of the records of 11,726 women. The World Health Organization International Classification was used to categorize BMI. The primary outcome of the study was PTB (< 37 weeks' gestation) presenting as spontaneous preterm labor, preterm premature rupture of the membranes, or a medical indication. We used univariable and multivariable logistic regression analysis to analyze the data (P <.05). RESULTS: We found (1) a significant increase in the overall incidence of PTB at the extremes of BMI, (2) a higher risk for PTB from spontaneous preterm labor at the lower extremes (low plus moderate thinness) of BMI (adjusted odds ratio [aOR], 2.4; 95% confidence interval [CI], 1.4-4.2; P= .003), (3) a higher risk for preterm premature rupture of the membranes at the upper extremes (obese class II plus III) of BMI (aOR, 1.6; 95% CI, 1.1-2.3; P=.02), and (4) a higher risk for a medically indicated PTB at the lower (aOR, 2.8; 95% CI, 1.4-5.6; P=.004) and upper (aOR, 1.5; 95% CI, 1.1-2.2; P=.02) extreme of BMI. CONCLUSION: Women at the extremes of prepregnancy BMI are at risk for PTB.