To estimate the effects of obesity on the duration and progression of the first stage of labor in a predominantly obese population and estimate the dose-effect with increasing classes of obesity. We performed a retrospective cohort study of labor progression among 5,204 consecutive parturients with singleton term pregnancies (37 weeks of gestation or more) and vertex presentation who completed the first stage of labor. Two comparison groups were defined by body mass index (BMI) less than 30 (n=2,413) or 30 or more (n=2,791). Repeated-measures analysis with polynomial modeling was used to construct labor curves. The duration and progression among women with BMIs less than 30 and BMIs of 30 or more were compared in a multivariable interval-censored regression model adjusting for parity, type of labor onset, race, and birth weight more than 4,000 g. The labor curves indicate longer duration and slower progression of the first stage of labor among women with BMIs of 30 or more for both nulliparous and multiparous women. Multivariable interval-censored regression analysis confirmed significantly longer duration (4-10 cm: 4.7 compared with 4.1 hours, P<.01) and slower progression of cervical dilation from 4 to 6 cm (2.2 compared with 1.9 hours, P<.01 with a range of 0.5-10.0 hours) among women with BMIs of 30 or more after adjusting for confounders. The overall duration is longer and progression of the early part of the first stage of labor is slower in obese women. This suggests that obesity should be considered in defining norms for management of labor, particularly in the early part of the first stage. II.