Association between induction start time and labor duration in nulliparous women undergoing elective induction of labor.

Abstract

Objective: To examine the association between elective induction of labor (EIOL) start time and labor duration among nulliparous women Methods: The ARRIVE trial was a multi-center randomized controlled trial of induction of labor at 39 weeks 0 days to 39 weeks 4 days versus expectant management in low-risk nulliparous women. In this secondary analysis, we included participants randomized to the induction group who had an EIOL without spontaneous labor or rupture of membranes prior to the induction start. Start time of EIOL was categorized as: early AM (midnight to 5:59 AM), late AM (6 AM-11:59 AM), early PM (noon-5:59 PM), or late PM (6 PM-11:59 PM). The primary outcome was labor duration. Cesarean delivery rates by induction start time were also examined. Multivariable analysis was conducted controlling for age, body mass index, insurance status, and modified Bishop score on admission (< 5 or ≥5).

Results: Of 3,062 women randomized to EIOL, 2,197 were included in this analysis. EIOL occurred in the early AM in 13%, in late AM in 28%, in early PM in 13%, and in late PM in 45%. Participants induced in the late AM had the shortest mean labor durations (21.5 ±11.3 hours) and the highest frequency of delivery at < 24 hours (68%). In adjusted analyses, induction in the late AM (vs. grouped other time periods) remained significantly associated with shorter labor duration (-1.5 hrs, 95% CI -2.5, -0.4, p=0.006), and there was no interaction between Bishop score and time of EIOL. Cesarean delivery rates did not differ by start time.

Conclusions: Induction of labor starting between 6AM and 11:59 AM was associated with shorter labor durations, independent of baseline maternal characteristics including cervical status on admission.