title: Reassessing the labor curve in nulliparous women.
abstract: OBJECTIVE
  Our purpose was to examine the pattern of labor progression in nulliparous parturients in contemporary obstetric practice.
  
  
  METHODS
  We extracted detailed labor data from 1329 nulliparous parturients with a term, singleton, vertex fetus of normal birth weight after spontaneous onset of labor. Cesarean deliveries were excluded. We used a repeated-measures regression with a 10th-order polynomial function to discover the average labor curve under contemporary practice. With use of an interval-censored regression with a log normal distribution, we also computed the expected time interval of the cervix to reach the next centimeter, the expected rate of cervical dilation at each phase of labor, and the duration of labor for fetal descent at various stations.
  
  
  RESULTS
  Our average labor curve differs markedly from the Friedman curve. The cervix dilated substantially slower in the active phase. It took approximately 5.5 hours from 4 cm to 10 cm, compared with 2.5 hours under the Friedman curve. We observed no deceleration phase. Before 7 cm, no perceivable change in cervical dilation for more than 2 hour was not uncommon. The 5th percentiles of rate of cervical dilation were all below 1 cm per hour. The 95th percentile of time interval for fetal descent from station +1/3 to +2/3 was 3 hours at the second stage.
  
  
  CONCLUSIONS
  Our results suggest that the pattern of labor progression in contemporary practice differs significantly from the Friedman curve. The diagnostic criteria for protraction and arrest disorders of labor may be too stringent in nulliparous women.
authors.affiliation: Division of Epidemiology, Statistics, and Prevention Research, National Institute of Child Health and Human Development/NIH, Building 6100, Room 7B03, Bethesda, MD 20892, USA. jim_zhang@nih.gov
authors.forename: Jun
authors.initials: J
authors.lastname: Zhang
authors.forename: James F
authors.initials: JF
authors.lastname: Troendle
authors.forename: Michael K
authors.initials: MK
authors.lastname: Yancey
mesh_terms: D000328:Adult; D005260:Female; D006801:Humans; D007747:Labor Stage, First; D007748:Labor Stage, Second; D007743:Labor, Obstetric / Q000502:physiology*; D010298:Parity; D011247:Pregnancy; D013997:Time Factors