Spontaneous onset of labor, Singletons, cephalic n = 98,169 Excluded: 23,280 cesarean births Vaginal delivery n = 74,889 women
TABLE 23-3. Comparison of Study Populations Analyzed to Define Normal Labor Curves
Year of data collection Early 1950s 1992-1996 Caudal/epidural analgesia (%) 8 48 Oxytocin augmentation (%) 9 50 aTripler Army Hospital. Data from Friedman, 1955; Zhang, 2002.
TABLE 23-4. Rate of Change at Each Stage of Cervical Dilation
Cervical Dilation Rate of Cervical Dilation 2 0.3 (0.1, 1.8) 3 0.4 (0.1,s1.8) 4 0.6 (0.2, 1 .8) 5 1.2 (0.3, 5.0) 6 1.7 (0.5, 6.3) 7 2.2 (0.7, 7.1) 8 2.4 (0.8, 7.7) 9 2.4 (0.7, 8.3) aMedian (5th and 95th percentiles). Data from Zhang, 2002.
Another caveat notes that the eicacy of these recommendations to achieve their primary goal is limited. In support, one retrospective cohort study of 200 women undergoing induction or augmentation before and 200 ater guideline changes found a cesarean delivery rate drop from 35 to 25 percent (WilsonLeedy, 2016). However, although the study was not powered to assess adverse neonatal outcomes, nonsigniicant but higher rates of umbilical artery gases with pH <7 and base deficit > 12 mmollL were noted in the postguideline group (Marte, 2016). In another evaluation of outcomes before and ater guideline implementation, cesarean delivery rates were unchanged. Overall, there were no associations between length of arrest and maternal or neonatal morbidity, but neonatal respiratory morbidity rates rose in newborns of women with longer periods of arrested dilation (Rosenbloom, 2017). Thus, additional studies are needed to define risks and beneits of the new guidelines.
Background for the 6-cm Rule. Review of selected publications helps explain the Consensus Committee (2016) evolution to a 6-cm rule. First, Zhang and coworkers (2002) compared study populations from one report by Friedman (1955) with one from 1992 to 1996 at TripIer Army Hospital, Hawaii (Table 23-3). The Friedman labor curves relected women in spontaneous labor with infrequent use of neuraxial labor analgesia or �: 6.2 0).;;
Duration of labor (hours)
FIGURE 23-3 Cervical dilation curves. (From Friedman (1955) and Zhang (2002).) oxytocin augmentation. In contrast, in the TripIer cohort, approximately 50 percent of women had neuraxial analgesia or augmentation. The rate of cervical change for the ensuing hour at each cervical dilation between 2 and 9 cm in the T ripIer group is shown in Table 23-4. Progress is slow between 4 and 6 cm but accelerates thereafter. his could reasonably be interpreted as the active phase beginning at 6 cm. Shown in Figure 23-3 are the labor curves for Friedman (1955) compared to the TripIer cohort. These difer in a flattening of the active phase beginning at 3 to 4 cm in the TripIer group. his is consistent with the labor results obtained in the Safe Labor Consortium study (Zhang, 2010). Namely, the 6-cm rule for active labor derives from a slowing of the rate or lattening of the slope of cervical change in irst-stage labor.