Historically, the course of labor has been managed by using labor curves first generated by Friedman in the 1950s.1–3 Although Friedman's labor curves have been a mainstay of labor management, recent studies have questioned their appropriateness in modern obstetric practice.4–8 In particular, modern obstetrics has resulted in more than a doubling in the rate of labor induction.9 It has been hypothesized that increasing induction rates may be associated, in part, with a rise in elective inductions, in which labor is initiated for convenience purposes and without a medical or obstetric indication.10–12 Recently, our research team showed that nulliparous women who had an elective induction with an unfavorable cervix at term had a significantly longer latent and early active phase of labor and a 2- to 3-fold increased risk of cesarean delivery compared with those with a spontaneous onset of labor.8 However, the pattern of labor progression and risk of cesarean delivery among multiparas is less certain. Although some observational studies suggest that the rate of cesarean delivery in multiparous women with an elective induction is similar to that in those women with a spontaneous onset of labor,13–16 others report an increased risk for those who were electively induced.17,18 Moreover, the majority of these descriptive studies do not distinguish between those who received preinduction cervical ripening from those who did not. In addition, it remains unclear how labor progresses in multiparous women when it is induced. Thus, we examined the pattern of labor progression in women undergoing elective induction with or without preinduction cervical ripening among low-risk multiparous women with term pregnancies.
MATERIALS AND METHODS
This study is a retrospective analysis of obstetric data from Christiana Hospital, a tertiary care teaching hospital that serves residents of the state of Delaware and of neighboring regions of Pennsylvania, Maryland, and New Jersey. In 1998, this institution established an electronic obstetric records system. This database consists of contemporaneously collected labor and delivery information and manually abstracted information from the prenatal record. When a woman is admitted to labor and delivery, the nursing staff creates an entry for her in the database. Information such as obstetric interventions performed as well as the details of each vaginal examination, including the time of the examination and cervical dilation, are noted in real time. Full-time trained abstractors extract and input the information from the prenatal and neonatal record within a month after the delivery.
Between January 2002 and March 2004, 9,447 women delivered singleton infants at this institution, and 5,056 of these women were multiparas. Among this group of 5,056 multiparous women, 4,252 delivered between 37+0 and 40+6 weeks of gestation. Further excluded from this analysis were 1,571 women with one or more of the following maternal or fetal conditions during pregnancy or at the time of admission: diabetes, hypertension, maternal medical history of cardiovascular, infectious, pulmonary, renal, mental, or thyroid disorders, multiple gestation, intrauterine growth restriction, uterine bleeding, oligohydramnios, breech presentation, elective cesarean delivery without a trial of labor, prior cesarean delivery, or clinically indicated induction. Thus, our final sample consisted of 2,681 low-risk multiparous women with singleton term pregnancies. The study protocol was reviewed and approved by the institutional review board at Christiana Care Health System and has been exempted by the National Institutes of Health.
Our exposure of interest was the elective induction of labor, which was defined as an induction without a medical or obstetric indication, or induction for “postdates” before 41 weeks of gestation. “Elective” and “postdates” were two of the options under “reason for induction” in the obstetric database that physicians were free to select. Our induction population was further subdivided into 2 subgroups: those who had preinduction cervical ripening followed by oxytocin induction (n = 61) and those who had oxytocin induction without preinduction cervical ripening (n = 735). Although information on the patient's Bishop score at the time of admission was not recorded in the obstetric database, preinduction cervical ripening was generally applied if the Bishop score was less than 6. Our comparison group was composed of 1,885 multiparous women who had spontaneous onset of labor. Labor status was determined at the time of admission by an attending or resident physician. Onset of labor was routinely defined as the onset of regular contractions with cervical change, rupture of the membranes, or complete effacement. We define the active phase of labor as that period during which the cervix is dilated 4 centimeters or more, with regular contractions.
In our study population, induction of labor with preinduction cervical ripening was universally performed by placing a 30-mL Foley Catheter supracervically using a speculum. This was then allowed to spontaneously extrude, at which point oxytocin administration would be initiated at 1–2 milli-International Units per minute and increased every 20 minutes until adequate labor was achieved. No additional measures, such as extraamniotic saline, prostaglandins, or traction to the Foley catheter were performed. For the purpose of analysis, the cervical examination at the time of instituting oxytocin in this group was used as the initial cervical examination. Women who were induced without preinduction cervical ripening were induced with an identical regimen of oxytocin. Amniotomy was performed at the discretion of the attending physician or resident but is generally performed at our institution when the head is engaged. Epidural analgesia was routinely instituted upon patient request with a standard bupivacaine-fentanyl infusion.
To assess the effect of labor status on labor progression, we compared various baseline characteristics by univariable analysis. For continuous variables with a normal distribution, mean and standard deviation values were calculated and either Student t test or analysis of variance was used. For continuous variables with a skewed distribution, median and the values at the 10th and 90th percentiles were calculated, and the Wilcoxon rank-sum test was performed. For categorical variables, percentages were calculated and the χ2 test was used. Variables that were significantly different among the 3 groups were selected as potential confounders to be incorporated into our model of labor progression with multivariable analysis.
Using a censored data methodology, we quantified the duration of the first stage of labor progression and calculated the median time elapsed for women to proceed from 1 cm of cervical dilatation to the next for each group. Because continuous monitoring of cervical dilatation was not performed, it is impossible to know exactly when an individual first reaches a given level of dilatation. However, by considering these times as censored, the median duration of labor can be estimated. Detailed statistical methods have been described previously.8
Briefly, for each interval of cervical dilatation (eg, from 3 cm to 4 cm), a lower and upper possible time range was computed. Each individual contributed an interval-censored value at a given level of dilatation. This value was subsequently used to model the underlying distribution of this duration of labor interval and to assess its dependence on exposure variables. It is well established that the duration of labor is skewed to the right. This distribution generally fits a log-normal distribution. Thus, a natural assumption for the data making up the time interval is that they are log-normally distributed, which was consistent with our data.
We fitted a parametric model with a log-normal distribution and interval-censored time-to-event data to assess the median duration of time elapsed in hours for each centimeter of cervical dilation during labor using PROC LIFEREG of SAS (SAS Institute, Cary, NC). The model was adjusted for all baseline characteristics that had statistical differences between the different labor statuses. Median values in each group were then estimated numerically by finding the time for which the average fitted probability of the event over the observed population covariate pattern equaled 0.5. All analyses were performed with SAS 8.2.
Table 1 presents the baseline sociodemographic and intrapartum characteristics of the overall study population, stratified by labor onset status. Women with elective inductions were slightly older and more often white, compared with those admitted in spontaneous labor. As expected, women with spontaneous labor were admitted at a more advanced stage of cervical dilatation than those in the induction groups. In regard to obstetric interventions, 41% of women with a spontaneous onset of labor received oxytocin for labor augmentation, and epidural analgesia was commonly administered.
Women whose labor was induced without cervical ripening had a slightly higher rate of cesarean delivery compared with those in spontaneous labor (3.9% compared with 2.3%, P < .05; crude odds ratio 1.76, 95% confidence interval [CI] 1.09–2.84). This association increased slightly after adjustment for maternal race, gestational age at delivery, and birth weight (adjusted odds ratio 1.95, 95% CI 1.19–3.19). One cesarean delivery was performed in the elective induction with cervical ripening cohort, which precluded us from comparing cesarean delivery rates in this population with women in other groups. Nonetheless, instrumental delivery was significantly more common in the latter group than in other groups.
The median progression of labor for the 3 groups is presented in Table 2. This table demonstrates that women who were induced without cervical ripening had a faster active phase than women who presented in spontaneous labor. We chose to look at the time interval at 5–10 cm instead of 4–10 cm because a large proportion of women with spontaneous labor were admitted at 5 cm or later. Thus, the time intervals before 5 cm of cervical dilatation in spontaneous labor may not be representative of that in all women with spontaneous labor in our study. Women induced after cervical ripening experienced a marked prolongation of labor from 3 to 4 cm. This may suggest that women who undergo preinduction cervical ripening are slower to enter the active phase and have a slower course of labor than those who are induced without cervical ripening agents. No significant differences were seen between the 2 groups in the second stage of labor.
Almost 60% of cesarean deliveries performed on multiparas in the United States are based on an indication of dystocia or failure to progress during the first stage of labor.19 Although individual practices vary, the decision about when to perform a cesarean delivery has been based on the work of Friedman,1–3 who differentiated between the course of labor only among multiparous and nulliparous patients. Our findings suggest that the progression of labor correlates with whether or not a multiparous woman is induced and receives preinduction cervical ripening. Moreover, it provides additional information about the specific points during labor in which induction and preinduction cervical ripening impact the course of labor. Multiparous women who were electively induced actually had a faster labor progression during the active phase than women who were admitted in spontaneous labor. In contrast, women who underwent preinduction cervical ripening were slower to enter the active phase than women who presented with spontaneous labor. Despite the universal use of oxytocin, women with preinduction cervical ripening were unable to achieve the rapid labor curve that women with a favorable cervix displayed. The implications of these findings are magnified by the rising rate of induction in the United States; currently 1 woman in 5 is artificially induced into labor.11 Together, this information suggests that these variables must be considered throughout the progression of labor.
Our findings about the progression of labor among multiparous women who had an elective induction are consistent with those from 2 previous studies that showed that multiparous women whose labors are electively induced have a shorter first stage of labor than women admitted in spontaneous labor.13,14 In a retrospective analysis of obstetric data from the Royal Victoria Hospital in Canada, Smith and colleagues14 reported a shorter mean duration of labor in induced multiparas without cervical ripening (n = 256) compared with those in spontaneous labor (n = 1,560; 6.1 versus 7.6 hours). Results of statistical significance tests between the 2 groups were not documented. In a smaller retrospective study, Macer and colleagues13 reported a faster first stage of labor in induced multiparas (n = 154) compared with those in spontaneous labor (n = 154; 5.7 verus 6.4 hours; P > .05). However, the induced group included a mixture of multiparas who may or may not have received preinduction cervical ripening, limiting the applicability of their findings to our study. Our findings on the increased risk of cesarean delivery among multiparas who were electively induced without cervical ripening are consistent with previous studies that documented a higher cesarean delivery rate among electively induced multiparas with a favorable cervix.13–15,17,18
Our study used advanced statistical methods to simultaneously examine labor progression centimeter by centimeter while adjusting for potential confounding factors. Only oxytocin and Foley catheter were used for induction in this institution, which gave us a unique opportunity to study induction issues without confounding by various induction methods. Previous studies have compared the efficacy of prostaglandin agents with the Foley catheter and found that they were similarly effective for the purpose of cervical ripening.20–23 No differences in cervical ripening time or in total time from the start of cervical ripening to delivery were noted between the 2 methods.
Nonetheless, the findings of this study must be interpreted with recognition of its limitations. First, the measurement of cervical dilatation was subjective. Continuous monitoring of cervical dilatation was not performed, and the measurements were based on vaginal examinations performed by multiple providers. This potential bias would likely be equally distributed among the studied populations and would thus bias our results toward the null. Second, our sample included a limited cohort of multiparous women who were induced after cervical ripening, which prevented us from performing some multivariable analyses and potentially influenced our ability to detect significant findings. Third, limited information was available on physician and patient factors that may influence labor progression and increase the risk of cesarean delivery. For example, we were unable to examine whether the patient had a rapid delivery in her prior pregnancy, and this was a factor in her being electively induced. Thus, the presence of residual confounding is possible. Lastly, our sample consisted of low-risk multiparous women from one hospital; thus, our findings may not be generalizable to other populations of pregnant women.
Despite these limitations, our data indicate that the pattern of labor progression differs for multiparous women in electively induced labor compared with those in spontaneous labor. Physicians should be aware of such differences as they care for these women.
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