OBJECTIVES: The effect of body mass index (BMI) was assessed on unplanned cesarean section (CS) rate and its indications among healthy, nulliparous women without other risk factors for CS. METHOD: A cross sectional study was performed on 1649 healthy, nulliparous women at term who were admitted in spontaneous labor and delivered at Boston Medical Center between Jan 1st 2008 and Dec 31st 2012. The demographics and outcomes were compared by using a logistic regression analyses. RESULT: There were no statistically significant differences in unplanned CS rates between the three BMI groups (19% in normal weight, 24% in overweight, and 21% in obese women, p=0.1). Compared with normal weight women the crude odds ratio for overweight women was 1.34 (95%CI 1.03-1.76) and for obese women 1.04 (95%CI 0.84-1.54). A multivariate logistic regression analysis was used to adjust for maternal age, birth weight, race and augmentation of labor. The adjusted ORs were 1.073 (95%CI 0.781-1.473) for obese and 1.291 (95%CI 0.978-1.705) for overweight women. Obese women had a higher rate of CS for non-reassuring fetal status (56%, p= 0.01) compared to overweight (46.5%) and normal weight women (37%). CONCLUSION: high maternal BMI per se does not appear to be an independent risk factor for unplanned CS in healthy nulliparous women presenting at term with a singleton pregnancy in spontaneous labor.