We compared the duration of labor among nulliparous women with varying body mass index (BMI). Laboring nulliparous women at >37 weeks were included. First visit BMI was used to categorize weight as normal (≤24), overweight (25 to 29.9), or obese (≥30 kg/m(2)). Chi-square, one-way analysis of variance, and Bonferroni multiple comparisons tests were used. During 15 months, 375 women met the inclusion criteria, and 38% were obese. Duration of first stage of labor was significantly longer for obese versus normal-weight women (26.76 ± 0.77 versus 23.87 ± 0.66 hours; p = 0.024) but not between normal versus overweight women (p = 1.00) or overweight versus obese women (p = 0.114). The cesarean delivery rate was significantly different in the three groups (p = 0.0001), highest among obese (47%) and lowest in normal-weight women (24%). When adjusted for age, hypertension, and induction, the likelihood of completing stage I was significantly less among obese nulliparous than those with BMI < 24 kg/m(2) (hazard ratio 0.73, 95% confidence intervals 0.54, 0.99). Compared with those with BMI < 24, the duration of stage I is significantly longer among obese women, even when adjusted for maternal age, induction, and hypertension.