The Natural History of the Normal First Stage of Labor

Abstract

The inability to reduce the climbing rate of cesarean delivery worldwide is due, in part, to an inadequate understanding of the normal labor process, especially the first stage of labor. Improved diagnostic and interventional procedures are needed to manage women who labor slowly. This multicenter, prospective, observational study investigated patterns of labor in a large population, and assessed an alternative diagnostic approach for abnormal labor progression. The first stage of labor was studied using data from the National Collaborative Perinatal Project collected from 1959 to 1966. A total of 26,836 singleton term pregnancies delivered at 20 weeks or later were included in the final analysis. Other inclusion criteria were spontaneous onset of labor, a vertex fetal presentation at admission, and a normal perinatal outcome. To assess labor progression among nulliparous and multiparous pregnancies, a repeated-measures analysis with an eighth-degree polynomial model was used to construct average labor curves. An interval-censored regression assessed the duration of labor stratified by cervical dilation at admission, and the time for progression centimeter by centimeter. As labor progressed in nulliparas, the median time needed to progress from one centimeter to the next became shorter (from 1.2 hours at 3-4 cm to 0.4 hours at 7-8 cm). Labor progressed more rapidly in parous women and differed little among women of different parities. Moreover, although the labor curves of nulliparous women were very gradual with no clear active phase characterized by precipitous dilation, the labor curves of multiparous women were characterized by a clear active phase starting after 5 cm with precipitous dilation of the cervix to 10 cm. In most patients, the occurrence of a deceleration phase in the late phase of active labor was not evident. The investigators conclude from these findings that a graduated threshold based on the level of cervical dilation would be more useful approach for defining labor arrest in nulliparas. They propose this approach as an alternative in managing individual patients based in large part on study data showing: (1) that the use of a 2-hour threshold for diagnosing labor arrest may be too short before 6 cm, whereas use of a 4-hour threshold may be too long after 6 cm; and (2) that the speed of cervical dilation is variable and accelerates as labor advances.