title: A time to be born.
abstract: OBJECTIVE
  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.
authors.affiliation: Office of Statistics and Evaluation, Bureau of Family and Community Health, Massachusetts Department of Public Health, Boston 02108-4619, USA. marlene.anderka@state.ma.us
authors.forename: M
authors.initials: M
authors.lastname: Anderka
authors.forename: E R
authors.initials: ER
authors.lastname: Declercq
authors.forename: W
authors.initials: W
authors.lastname: Smith
mesh_terms: D002940:Circadian Rhythm; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D007743:Labor, Obstetric / Q000502:physiology*; D008404:Massachusetts; D010298:Parity; D011247:Pregnancy; D012016:Reference Values