Timing of birth after spontaneous onset of labor.

Objective: To describe naturally occurring birth patterns in low-risk women with singleton gestations and spontaneous onset of labor at term.
Materials and Methods: The timing of birth of women who delivered in the low-risk labor unit at Parkland Hospital, Dallas, Texas, between January 1, 2000, to December 31, 2000, was analyzed. Women admitted to this unit were between 36(0/7) and 41(6/7) weeks of gestation, were in spontaneous labor, and had a singleton gestation. Women with contraindications to labor, significant medical problems, a known fetal anomaly, and stillbirths were excluded from analysis. The frequency of birth was analyzed in relation to the time of day, day of week, and month of the year.
Results: Low-risk women (n = 6608) met the study criteria and were included in the analysis. No association was found between the day of the week and the frequency of births (P =.31). Births were most common between the hours of 1 to 2 pm and least common between the hours of 10:00 to 12:00 hours (Central Standard Time, P =.04). Births were more common in the fall, September through November, and least common in the winter, December through February. Daylight Saving Time did not affect these results.
Conclusion: Birth after the spontaneous onset of labor is most common in the early afternoon, and most births occur in the fall. There is no natural association between spontaneous birth in low-risk women and the day of the week.
Level of Evidence: II-2