OBJECTIVE: To analyze the labor curves of nulliparous and multiparous women between 23.0 and 34.0 weeks of gestation who underwent induction of labor and achieved vaginal delivery. METHODS: This is a retrospective cohort study of all live singletons delivered vaginally after medically indicated induction of labor between 23.0 and 34.0 weeks of gestation from 2011 through 2014 at our institution. We excluded those with one or no cervical examinations available during labor. Prior cesarean delivery, 5-minute Apgar score less than 5, and arterial cord pH less than 7.0 were exclusions. The course of cervical dilation was modeled using repeated measures analysis, and smoothed curves for nulliparous and parous women were generated separately. Estimates of the median (5th-95th percentile) traverse times between two dilations were computed using interval censored regression. Traverse times (ie, the elapsed time between two given dilation measures) were compared between nulliparous and parous women. RESULTS: Sixty-seven nulliparous and 69 multiparous women were included. Each group exhibited similar rates of change from 1 to 3 cm of dilation (median 3.6 hours nulliparous and 3.4 hours multiparous, P=.90). Nulliparous women progressed from 3 to 6 cm more slowly than multiparous women (median 10 hours vs 4.4 hours, P<.001). After 6 cm, both groups rapidly progressed to 10 cm (median 0.3 hours vs 0.3 hours, P=.64). Although the 95th percentile traverse time from 6 to 10 cm was about 2 hours in each group, progression from 1 to 6 cm at the 95th percentile was much longer (64.0 vs 42.2 hours). CONCLUSION: Early preterm labor induction takes less time in multiparous women owing to more rapid progression from 3 to 6 cm. At the 95th percentile, both nulliparous and multiparous women delivered vaginally, even with latent labor lasting well longer than 24 hours.