A time to be born

OBJECTIVES: This study used a large, population-based data set (n = 619,455) to establish reference standards of the timing of spontaneous vaginal births. METHODS: Low-risk births in Massachusetts from 1989 to 1995 were studied. This group comprised 242,276 births that met the following criteria: singleton, vertex, vaginal births with a birthweight of between 2500 and 4000 g; gestation between 37 and 42 weeks; a 5-minute Apgar score greater than 6, and no induction or stimulation. RESULTS: Low-risk births displayed a mild circadian pattern, with a peak between 11:00 AM and 1:00 PM and a trough between 11:00 PM and 1:00 AM. Subgroup analysis showed no clear differences except for parity. CONCLUSIONS: Reference standards should be developed and used as comparative norms for hospital and practice-based perinatal studies of diurnal patterns of birthing.