Background: Induction of labor is increasingly common. In 2007, 20 percent of all pregnancies were induced, a 140 percent increase since 1990. Nulliparous women who are induced are more likely to have a cesarean delivery than women who present in spontaneous labor. Although there may be several reasons for the increased rate of cesarean deliveries, differences in the course of induced labors may result in a premature diagnosis of arrest of dilation. Accurately characterizing the course of induced labors could prevent unnecessary cesarean deliveries. Harper and colleagues compared the progression of induced and augmented labors with those of women who labored spontaneously.
The Study: This retrospective cohort study reviewed all consecutive full-term deliveries of women who reached 10-cm dilation between 2004 and 2008 at Washington University Medical Center in St. Louis, Mo. Women with singleton vertex presentation pregnancies who presented at 37 weeks' gestation or greater were included in the study. Women who delivered preterm, women whose fetuses had congenital anomalies, and women who underwent cesarean delivery before full dilation were excluded.
Labors were categorized as induced, augmented, or spontaneous. Augmentation was defined as patients who were admitted in spontaneous labor but then subsequently required oxytocin (Pitocin). Women in the spontaneous labor group did not require oxytocin. Artificial rupture of membranes, which may be considered an augmentation, was performed in all groups; a secondary analysis measured the effect of artificial rupture of membranes in the spontaneous labor group. The primary outcome was the time required for cervical dilation from 4 to 10 cm in each group and for each 1-cm change. Labor curves were constructed for each group and adjusted for the statistically significant variables of race, macrosomia, maternal obesity, and admission Bishop score greater than 5.
Results: Of the 5,388 women included in the study, 37.5 percent presented in spontaneous labor, 31.9 percent were augmented, and 30.6 percent were induced. Women in the induction group were more likely to be white, 35 years or older, and nulliparous. They were also more likely to have diabetes mellitus, hypertension, obesity, a macrosomic neonate, and a Bishop score less than 5. Nulliparous women who were induced took significantly longer to progress from 4- to 10-cm dilation than those in the spontaneous labor group (median = 5.5 versus 3.8 hours; 95th percentile = 16.8 versus 11.8 hours), and longer for each 1-cm change up to 6 cm. Multiparous women who were induced also took longer to progress from 4 to 10 cm than the spontaneous labor group (median = 4.4 versus 2.4 hours; 95th percentile = 16.2 versus 8.8 hours). Beyond 6 cm, the rate of cervical change in induced women was similar to that of women in spontaneous labor. Augmented labor followed a similar pattern to induced labor up to 6 cm, but then continued more slowly compared with spontaneous labor.
Conclusion: Although women whose labor was induced spent longer in labor, the rate of cervical change after 6 cm was similar to those in spontaneous labor. Arrest of labor should not be diagnosed until induced or augmented patients reach 6-cm dilation.