Objective: Active management has been shown to shorten the length of labor and reduce the incidence of prolonged labor. We examined the influence of this protocol on the rate of cervical dilatation by calculating a mean partogram to explain how this acceleration is achieved. Study Design: We retrospectively analyzed partograms of cervical dilation in 500 consecutive nulliparous women in spontaneous labor at term with a singleton fetus in a cephalic presentation; cesarean deliveries were not excluded. Mean cervical dilations at admission and mean time intervals to reach 2, 3, 4, 6, 8, and 10 cm of dilatation and delivery were calculated, with 95% confidence intervals, both for the whole cohort and specifically in women with cervical dilatations <3 cm at admission. These data were used to construct mean partograms. Results: The mean duration of labor was 6.1 hours. The mean cervical dilatation at admission was 1.7 cm; all but 2.8% were delivered within 12 hours. The cesarean rate was 5.4%. The mean partogram, with narrow 95% confidence intervals, shows a rapid active phase after a much shorter latent phase than reported in other analyses of labor. Conclusion: Active management reduces the duration of first labor mainly by shortening the latent phase in association with amniotomy at very early cervical dilatations and does not delay the diagnosis of labor until the active phase has begun. (Am J Obstet Gynecol 2000;183:438-43.)