The Natural History of the Normal First Stage of Labor

Abstract

OBJECTIVE: To examine labor patterns in a large population and to explore an alternative approach for diagnosing abnormal labor progression. METHODS: Data from the National Collaborative Perinatal Project were used. A total of 26,838 parturients were selected who had a singleton term gestation, spontaneous 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 duration 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 parities had very similar curves. Nulliparas may start the active phase after 5 cm of cervical dilation and may 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 graduated approach based on levels of cervical dilation to diagnose labor protraction and arrest is proposed.