American journal of obstetrics & gynecology MFMJan 23, 2026

Time of Day When Labor Starts May Affect How Long It Lasts

Circadian Biology Weekly Brief ↗PubMed ↗DOI ↗

Updated Jun 27, 2026

Abstract

Essence

Early-morning labor induction was linked to shorter induction-to-delivery times than late-evening induction.

Evidence

This single-center retrospective cohort of 3,363 term inductions found circadian variation, with average duration of 14.8 hours at 05:00 versus 21.0 hours at 23:00.

Caveat

Induction timing was not randomized, so hospital scheduling patterns and patient-selection factors may explain part of the association.

Simplified

BACKGROUND: Spontaneous labor and birth peak 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 (TOI) on labor duration and obstetric outcomes remains unexplored. We hypothesize that the time-of-day of labor induction will impact induction of labor (IOL) duration and the risk of cesarian section.

OBJECTIVE: To determine whether TOI impacts IOL duration and delivery outcomes in term pregnancies, and whether maternal characteristics such as body mass index (BMI) and parity modulate this effect.

STUDY DESIGN: This retrospective cohort study analyzed 3,363 term pregnant subjects who underwent induction at a single U.S. hospital between 2019 and 2022. Time-of-induction was defined as the time of administration of the first cervical ripening agent or synthetic oxytocin (i.e., Pitocin). IOL duration was calculated as the time from induction to delivery. Multivariable analyses, survival models, and circadian rhythm analyses were performed to evaluate associations between TOI, IOL duration, cesarean delivery, and neonatal outcomes.

RESULTS: IOL duration followed a significant circadian rhythm (p<0.05, Lomb-Scargle), with a gradual lengthening in IOL duration when induction was initiated later in the day, peaking at 23:00 hours (average IOL duration of 21.0 hours) as compared to induction at 05:00 hours (average IOL duration of 14.8 hours, p<0.01, Kruskal-Wallis test). Subjects induced during the early morning had up to 6 hours shorter labor compared to those induced in the late evening (p<0.01). The optimal time-of-day to initiate labor induction was influenced by BMI and parity with significant differences in probability of giving birth resulting from the time-of-day labor was induced for nulliparous obese (p<0.05, Two-way ANOVA), and parous obese subjects (p<0.05). Time-of-induction was associated with reduced cesarean delivery rates and did not impact rates of NICU admissions or adverse neonatal outcomes.

CONCLUSION: The time of day of initiation of labor induction influenced IOL duration, with the shortest duration when induced during early morning hours. No increase in adverse maternal and fetal outcomes was identified when controlling for time-of-day of labor induction. The optimal time-of-day to induce labor is influenced by BMI and parity and should be considered when performing this common obstetric intervention.