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 vi 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. Law-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 I: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.