Objectives: Our purpose was to evaluate in women the diurnal (24 hour) rhythms of the onset of labor and time of delivery and their modulation by parity and seasons.
Study design: A retrospective analysis was performed on 15,910 at term deliveries of single fetuses with noninduced labor.
Results: A diurnal rhythm of labor onset, with peak frequency at 8 PM and 9 AM was observed in nulliparous women but not in multiparous women. No diurnal rhythm of labor length was detectable. A diurnal rhythm of delivery, with peak values at 2 PM was evident in nulliparous but not in multiparous women. In nulliparous women the two acrophases of the diurnal rhythm of labor onset were closer in summer (10 hours) than in winter (13 hours). The acrophase of the diurnal rhythm of delivery progressively shifted to earlier hours from winter (5 PM) to summer (1 PM).
Conclusions: These data show the attenuation of the diurnal rhythms of labor and delivery in multiparous women and their modulation by seasons.