The normal progress of labor has received increasing attention in recent years. Modern statistical methods have been used to evaluate the pattern of cervical dilation in normal labor, creating a new paradigm to replace the Friedman curve.1–4 For instance, the rapid cervical dilation that characterizes the active phase occurs later in labor than described by Friedman. Also, the transition between the latent and active stages is more gradual in nulliparous women. Cervical dilation is essential to normal labor, but fetal descent within the pelvis is also required to achieve vaginal delivery. However, little attention has been paid to the expected progression of fetal descent in labor.
Therefore, we aimed to describe expected norms for fetal descent in labor and estimate the expected station for a given cervical dilation during the first stage.
MATERIALS AND METHODS
A retrospective cohort study was conducted at a single academic teaching hospital including all consecutive-term, vertex singletons who delivered vaginally from 2004 to 2008. Operative vaginal deliveries were excluded. This study received approval from the Washington University School of Medicine Human Research Protection Office.
Term gestation was defined as greater than or equal to 37 weeks 0 day of gestation. Diagnosis of spontaneous labor was made by the admitting attending physician. Known fetal anomalies were excluded. Obstetric history, labor progress, and delivery and neonatal outcomes were collected from medical records. Complete data on all cervical examinations were extracted, including the time of the examination, the cervical dilation in centimeters, the effacement, and the station (on a −3 to +3 scale). Resident physicians provided the majority of labor and delivery services, including performing cervical examinations at regular intervals.
Baseline characteristics of the study sample were tabulated, comparing parturients by parity and by spontaneous compared with induced or augmented (nonspontaneous) labor. Because prior published data suggest similar labor progress in women with induced and augmented labors, the two groups were combined for the purpose of this analysis.5 The χ2 test or Fisher’s exact test was used for categorical variables, as appropriate. The Shapiro-Francia test was used to test normality of continuous variables, and the Student's t test and the Mann-Whitney U test were used to compare these variables as appropriate.
A repeated-measures analysis with a ninth-degree polynomial model was used to construct average dilation and descent curves stratified by parity. A repeated-measures analysis is required to account for the correlation between cervical examinations in the same woman. Labor progression (cervical dilation and fetal descent) data are left-censored, because the progression of labor before women present to the hospital is unknown. For dilation curves, the starting point was set as the time when dilation reached 10 cm, known for all patients, and time was calculated backward. For descent curves, the starting point was chosen as the time the station was +3. The curves were reversed after construction to resemble a traditional labor curve with time increasing from left to right.1–4,6
Interval-censored regression was used to estimate median time to descend from one level of station to the next (traverse time). The median station at a given cervical dilation was also estimated. The data were further stratified by parity and whether labor was spontaneous or nonspontaneous and compared with the Mann-Whitney U test. Labor progression data are also interval-censored, because cervical examinations are performed at intervals rather than continuously. Thus, one knows that the progression from 4 to 5 cm dilation or −1 to 0 station occurred at some time between two cervical examinations, but not exactly when. The distribution of these time intervals was right-skewed (as a result of some long labors, creating a long right tail of the distribution) and was fitted to a log-normal distribution. Potentially confounding factors identified in univariable and stratified analyses were considered using backward, stepwise regression. The final models adjusted for maternal body mass index (BMI, calculated as weight (kg)/[height (m)]2), birth weight greater than 4,000 g, and induction of labor. Other factors considered in the regression, including age, gravidity, race, history of cesarean delivery, regional anesthesia, and diabetes, were not significant and were not included in the final model.
All analyses were performed using Stata 10.0 Special Edition and SAS 9.2.
Of 4,618 parturients, 1,526 (33%) were nulliparous. Most women underwent induction or augmentation of labor; only 39% of women had spontaneous labor without augmentation.
Baseline characteristics were significantly different between nulliparous and multiparous women (Table 1). Multiparous women were older, more likely to African American, have diabetes, and be obese. Their neonates were more likely to be weigh more than 4,000 g. In terms of their labor management, multiparous women more often had spontaneous labor without augmentation and were less likely to receive regional anesthesia.
There were also significant baseline differences between those undergoing spontaneous labor compared with those with augmentation or induction of labor (Table 2). Women who labored spontaneously were younger, of higher gravidity, and more likely to be African American. These women were also less likely to be obese or have diabetes and had fewer neonates weighing more than 4,000 g. They also used regional anesthesia less frequently. Multiparous women had faster fetal descent than nulliparous women between all stations except +2 to +3. This pattern was statistically significant in both unadjusted (not shown) and adjusted analyses (adjusted for nonspontaneous labor, maternal BMI, and birth weight greater than 4,000 g; Table 3). Similarly, women in spontaneous labor had quicker fetal descent at all stations in both unadjusted (not shown) and adjusted analyses (adjusted for parity, maternal BMI, and birth weight greater than 4,000 g; Table 4).
The median times to descend one level of station were all less than 2 hours and less than 1 hour after achieving 0 station. However, the 95% confidence intervals (CIs) included up to 12.5 hours between stations −2 and −1 in nulliparous women, all of whom achieved spontaneous vaginal delivery.
The median stations at a given dilations were stratified by parity (Table 5). Multiparous women tended to have a higher station than nulliparous women until late in the first stage. By 6 cm of dilation, median station was 0 (95% CI −2 to 1) for nulliparous and −1 (95% CI −3 to 0) for multiparous women. At 8 cm, 95% of nulliparous women were at −1 station or lower. Both nulliparous and multiparous women had a median station of 0 after active labor was established (greater than 6 cm dilation) and 95% of all women were 0 station or lower at complete cervical dilation. When stratified by labor type (Table 6), those in spontaneous labor had lower station at 1–3 cm of dilation. However, between 7 and 9 cm of dilation, these women had higher station than those being induced or augmented.
Labor curves are presented in Figure 1 (stratified by parity) and Figure 2 (stratified by induction type). Nulliparous women and those with induced or augmented labor had a gradual rate of descent, although the rate increased later in labor. In comparison, there was an inflection point in the descent curve for multiparous women and those in spontaneous labor with rapid fetal descent occurring after −1 station.
For women in labor at term, the median time for descent between station intervals was short, less than 2 hours in any group and less than 1 hour for most intervals. Multiparous women and women in spontaneous labor had faster descent compared with nulliparous women or those with induced or augmented labors. However, the 95% CIs were wide at higher stations (over 12 hours at the same station in nulliparous women or 10 hours in those who were multiparous) in women who achieved vaginal delivery. Overall, obstetricians should expect wide variation in length of time spent at a higher station (above 0 station), particularly among nulliparous women and those being induced or augmented.
Similarly, there is wide variation in the expected station by increments of dilation among women laboring spontaneously. However, 95% of nulliparous women entering active labor (6-cm cervical dilation) had a fetal station of −2 or lower. At complete cervical dilation, 95% of all women were 0 station or lower.
Few investigators have examined patterns of fetal descent in labor. Friedman, well known for his classic description of labor curves, also created descent curves. In a 1965 series of articles, he created descent curves using the same techniques he previously used for dilation curves.7–9 He described an exponential descent curve, in contrast to the sigmoidal cervical dilation curve, and found that cervical dilation and fetal descent were interrelated. The median station at the active phase of labor was 0 regardless of parity. Higher stations at the onset of each phase of labor predicted dystocia. However, application of these results to a contemporary population is limited given the high rate of midforceps deliveries altering the progression of the second stage in that era. Also, the contemporary parturient is heavier and more likely to undergo induction and augmentation of labor.
Gurewitsch et al10 used created descent curves stratified by parity with emphasis on the descent patterns in grandmultiparous women. The authors used ordinal logistic regression models with generalized estimating equations for robust variance estimation. They observed that grandmultiparous women tended to remain at a high station for a longer time before delivery compared with nulliparous women. A similar trend can be seen in Figure 1.
In an analysis of nulliparous labor, Zhang et al4 focused on cervical dilation curves but presented descent data as well. The authors used interval censored regression and a repeated-measures analysis for their analysis as we have presented here. Zhang also found that nulliparous women can spend several hours at a low station and still achieve vaginal delivery. For example, the 95% CIs included 3 hours from stations +1 to +2 among women at 10 cm of dilation.
A limitation of our study is that it is retrospective, limiting how cervical examinations were performed. For instance, the frequency of cervical examinations was not standardized, and examiners at our institution typically record fetal station using the −3 to +3 scale rather than the 5-cm scale.11 However, this represents the typical management of labor at our institution and can be generalized to similar institutions. There was also a high rate of induction and augmentation of labor (62%). Because we excluded women who underwent cesarean or operative vaginal delivery, we cannot comment on typical patterns of descent for these labors. This exclusion was necessary to be able to show the slowest rates of descent that still resulted in spontaneous vaginal delivery. Several other factors may affect descent in labor that could not be assessed in this study. These include interexaminer variability, molding of the fetal head, head position, maternal pushing effort, and the use of regional anesthesia.
Fetal descent is an essential component of labor progression. There is wide variation in descent rates and expected station early in labor, although at fetal station above 0 is very unusual at complete cervical dilation for both nulliparous and multiparous women. Multiparous women and women in spontaneous labor should be expected to have faster fetal descent.
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