Time of day of induction impacts the total duration of labor

BACKGROUND: Spontaneous labor onset peaks during the late evening and early morning hours, indicating a circadian influence on parturition. However, the effect of the time of day of labor induction on labor duration and obstetrical outcomes remains unexplored. We hypothesize that the time of day of labor induction affects induction of labor duration and the risk of cesarean delivery. OBJECTIVE: This study aimed to determine whether the time of day of labor induction impacts induction of labor duration and delivery outcomes in term pregnancies, and whether maternal characteristics such as body mass index and parity modulate this effect. STUDY DESIGN: This retrospective cohort study analyzed 3363 term pregnant participants who underwent induction at a single US hospital between 2019 and 2022. Time of induction was defined as the time of administration of the first cervical ripening agent or synthetic oxytocin (ie, Pitocin). Induction of labor duration was calculated as the time from induction to delivery. Multivariable analyses, survival models, and circadian rhythm analyses were performed to evaluate associations between time of day of labor induction, induction of labor duration, cesarean delivery, and neonatal outcomes. RESULTS: Induction of labor duration followed a significant circadian rhythm (P<.05, Lomb-Scargle), with a gradual lengthening in duration when induction was initiated later in the day, peaking at 11:00 ᴘᴍ (average duration of 21.0 vs 14.8 hours at 5:00 ᴀᴍ; P<.01, Kruskal-Wallis test). Participants induced during the early morning had up to 6 hours shorter labor compared with those induced in the late evening (P<.01). The optimal time of day for initiating labor induction was influenced by body mass index and parity, with significant differences in delivery probability by time of day of labor induction among nulliparous obese (P<.05, 2-way analysis of variance), and parous obese participants (P<.05). Time of induction was associated with reduced cesarean delivery rates and did not impact rates of neonatal intensive care unit admission or adverse neonatal outcomes. CONCLUSION: The time of day when labor induction was initiated influenced induction of labor duration, with the shortest duration observed when induction occurred during early morning hours. No increase in adverse maternal or fetal outcomes was identified after accounting for the time of day of labor induction. The optimal time of day for inducing labor is influenced by body mass index and parity and should be considered when performing this common obstetrical intervention.