LABOR DURATION IS DEFINED BY THE TIME OF DAY OF LABOR INDUCTION

ABSTRACT BACKGROUND Spontaneous labor and birth peak during the late evening and early morning hours, indicating an endogenous rhythm in labor onset and birth. We hypothesize that the time-of-day of labor induction will define labor duration and the risk of cesarian section. METHODS In a retrospective study of pregnant women who were induced for labor (n =3,688), time-of- day of labor induction was studied across maternal phenotypes. Survival analysis and Cox Proportional Hazards model were used to identify differences in time-to-birth as a result of inducing labor at a specific time-of-day. RESULTS Labor induction was circadian (p<0.05, Lomb-Scargle test), with a gradual lengthening in labor duration when labor induction was initiated later in the day, peaking at 23:00 hours (average labor duration of 20.72 hours) as compared to induction at 5:00 hours (average labor duration of 14.74 hours, p<0.01, Kruskal-Wallis test). The optimal time-of-day of labor induction was conditioned by maternal phenotype with significant differences in probability of giving birth as a result of the time-of-day labor was induced for nulliparous obese (p<0.05, Two-way ANOVA), and parous obese women (p<0.05). CONCLUSIONS Labor duration in response to induction is circadian, with the shortest labor duration when induced during early morning hours. The optimal time-of-day of labor induction is conditioned by maternal phenotype and should be considered as a labor management practice.