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.

The authors have declared no competing interest.

This work was funded by the USDA National Institute of Food and Agriculture Hatch project (MICL1018024) to HMH, the NIH/National Institute of Environmental Health Sciences project (R01ES035691) to HMH, and a sub-award from the Michigan Diabetes Research Center through the National Institute of Diabetes and Digestive and Kidney Diseases (P30DK020572).

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This project is approved by the Institutional Review Board of the Michigan State University under Study ID: 0007199.

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