The Natural History of the Normal First Stage of Labor

OBJECTIVE: To examine labor patterns in a large popu-lation and to explore an alternative approach for diag-nosing abnormal labor progression. METHODS: Data from the National Collaborative Peri-natal Project were used. A total of 26,838 parturients were selected who had a singleton term gestation, spon-taneous onset of labor, vertex presentation, and a normal perinatal outcome. A repeated-measures analysis was used to construct average labor curves by parity. An interval-censored regression was used to estimate dura-tion of labor stratified by cervical dilation at admission and centimeter by centimeter. RESULTS: The median time needed to progress from one centimeter to the next became shorter as labor advanced (eg, from 1.2 hours at 3–4 cm to 0.4 hours at 7–8 cm in nulliparas). Nulliparous women had the longest and most gradual labor curve; multiparous women of different pari-ties had very similar curves. Nulliparas may start the active phase after 5 cm of cervical dilation andmay not necessarily have a clear active phase characterized by precipitous dilation. The deceleration phase in the late active phase of labor may be an artifact in many cases. CONCLUSION: The active phase of labor may not start until 5 cm of cervical dilation in multiparas and even later in nulliparas. A 2-hour threshold for diagnosing labor arrest may be too short before 6 cm of dilation, whereas a 4-hour limit may be too long after 6 cm. Given that cervical dilation accelerates as labor advances, a gradu-ated approach based on levels of cervical dilation to diagnose labor protraction and arrest is proposed