Does Labor Analgesia Affect Labor Outcome?

Evidence Regarding Timing of Initiation, Length of Labor, and Rate of Cervical Dilatation

Conventional wisdom holds that if started too early in labor (during the latent phase), epidural analgesia may markedly slow or even arrest the progress of labor. Amazingly, this once widely accepted clinical dogma has never been proved in carefully performed studies. Its origin can be traced to early case series of caudal or epidural anesthesia for labor, which probably resulted in dense sacral and lumbar blocks. In these uncontrolled reports, although some women in whom blocks were initiated very early may not have progressed through labor, it is unclear whether they would have progressed more quickly without the block.

Some nonrandomized studies have found an association between earlier epidural placement and dystocia. Thorp and colleagues compared various groups of nulliparous women defined by their early cervical dilation rate, their cervical dilation at the time of initiation of analgesia, and the choice of epidural or alternative analgesia. Among women with dilation less than 5 cm and dilation rate less than 1 cm/hr, epidural analgesia was associated with a sixfold increase in cesarean delivery for dystocia. Other comparisons demonstrated smaller relative risks or no difference. In a secondary analysis of the same group's randomized trial, the increased risk for cesarean delivery was greatest in women requesting analgesia earlier, although women were not randomly assigned to dilation at time of initiation of analgesia. Using a case-control methodology, Malone and colleagues identified epidural initiation at less than 2 cm dilation as a significant risk factor for prolonged nulliparous labor (odds ratio [OR] 42.7). In a sophisticated observational study using a variant of multivariate regression (propensity score analysis) to control for multiple simultaneous confounders, Lieberman and colleagues identified both cervical dilation less than 5 cm and station less than 0 at the time of epidural initiation as strong risk factors for cesarean delivery.

Evidence from RCTs has failed to confirm this finding ( Table 52.1 ). Chestnut and colleagues randomized women requesting epidural analgesia to early or late groups (approximately 4 and 5 cm dilation). No differences in labor outcome were seen in either spontaneous labors or induced labors. The early and late groups in these studies, however, were not markedly different in their cervical dilation at the time of epidural placement. Several more recent trials randomized women to early epidural placement or opioids until later in labor ^{, , ,} or to intrathecal opioids followed by later epidural initiation. ^{, ,} In each case, progress through the first stage of labor was either equivalent or faster in the early group than in the later group. No differences in second-stage duration or mode of delivery were found in any of the trials. Two meta-analyses of the RCTs, one performed before and two after the extremely large trial by Wang et al., found no difference in the mode of delivery between early and later epidural initiation. The difference between the RCTs and the retrospective studies may be because of selection bias, in that women requesting analgesia earlier in labor may be experiencing pain because of anatomic or physiologic factors predisposing them to dystocia. Interestingly, a recent prospective cohort study found that early epidural initiation (placed < 6 cm) was associated with lower fetal station during the active phase compared with later (> 6 cm) placement and that the early epidural group was also associated with a decreased risk for a prolonged second stage.

The effect of epidural analgesia on cervical dilation in established labor, which corresponds to the length of the first stage of labor, is probably minimal. Some earlier retrospective studies finding slower cervical dilation were probably hampered by selection bias. A few older meta-analyses of randomized trials of epidural analgesia versus opioid analgesia have concluded that the first stage of labor is not prolonged by epidural analgesia. A recent systemic review did show that both the first (mean difference [MD] 32.28 minutes, 95% confidence interval [CI] 18.34 to 46.22; 2259 women; studies = 9) and second (MD 15.38 minutes, 95% CI 8.97–21.79; 4979 women; studies = 16; P < .00001) stages of labor were longer for epidural versus opioid analgesia. Nevertheless, heterogeneity was seen in the results, and the authors suggested publication bias may be possible. Curiously, this meta-analysis included slightly fewer studies examining length of first stage of labor than earlier analyses by the same group, which found no first stage differences ( Table 52.3 ), while including more studies of second stage duration. This study concluded that epidural initiation need not be delayed because early initiation does not increase incidence of instrumental births or cesarean delivery in nulliparous women.