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Association of Cervical Effacement With the Rate of Cervical Change in Labor Among Nulliparous Women

Langen, Elizabeth S. MD; Weiner, Steven J. MS; Bloom, Steven L. MD; Rouse, Dwight J. MD; Varner, Michael W. MD; Reddy, Uma M. MD, MPH; Ramin, Susan M. MD; Caritis, Steve N. MD; Peaceman, Alan M. MD; Sorokin, Yoram MD; Sciscione, Anthony DO; Carpenter, Marshall W. MD; Mercer, Brian M. MD; Thorp, John M. Jr MD; Malone, Fergal D. MD; Iams, Jay D. MD for the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) Maternal-Fetal Medicine Units (MFMU) Network

doi: 10.1097/AOG.0000000000001299
Contents: Original Research
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OBJECTIVE: To assess the association of cervical effacement with the rate of intrapartum cervical change among nulliparous women.

METHODS: We conducted a secondary analysis of a prospective trial of intrapartum fetal pulse oximetry. For women who had vaginal deliveries, interval-censored regression was used to estimate the time to dilate at 1-cm intervals. For each given centimeter of progressive cervical dilation, women were divided into those who had achieved 100% cervical effacement and those who had not. The analysis was performed separately for women in spontaneous labor and those who were given oxytocin.

RESULTS: A total of 3,902 women were included in this analysis, 1,466 (38%) who underwent labor induction, 1,948 (50%) who underwent labor augmentation (combined for the analysis), and 488 (13%) who labored spontaneously. For women in spontaneous labor, the time to dilate 1 cm was shorter for those who were 100% effaced starting at 4 cm of cervical dilation (P=.01 to <.001). For women who received oxytocin, the time to dilate 1 cm was shorter for those who were 100% effaced throughout labor (P<.001).

CONCLUSION: The rate of cervical dilation among nulliparous women is associated with not only the degree of cervical dilation, but also with cervical effacement.

CLINICAL TRIAL REGISTRATION: ClinicalTrials.gov, www.clinicaltrials.gov, NCT00098709.

Achievement of 100% cervical effacement is associated with a shorter median time of dilation among nulliparous women.

Departments of Obstetrics and Gynecology, Stanford University, Stanford, California, the University of Texas Southwestern Medical Center, Dallas, Texas, the University of Alabama at Birmingham, Birmingham, Alabama, the University of Utah Health Sciences Center, Salt Lake City, Utah, the University of Texas Health Science Center at Houston–Children's Memorial Hermann Hospital, Houston, Texas, the University of Pittsburgh, Pittsburgh, Pennsylvania, Northwestern University, Chicago, Illinois, Wayne State University, Detroit, Michigan, Drexel University, Philadelphia, Pennsylvania, Brown University, Providence, Rhode Island, MetroHealth Medical Center, Case Western Reserve University, Cleveland, Ohio, the University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, Columbia University, New York, New York, and The Ohio State University, Columbus, Ohio; the George Washington University Biostatistics Center, Washington, DC; and the Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, Maryland.

Corresponding author: Elizabeth S. Langen, MD, Obstetrics and Gynecology, Floor 9, Room 109 VVWH, 1540 W Hospital Drive, SPC 4264, Ann Arbor, MI 48109-4264; e-mail: elangen@med.umich.edu.

Supported by grants from the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD) (HD21410, HD27860, HD27869, HD27915, HD27917, HD34116, HD34136, HD34208, HD40485, HD40500, HD40512, HD40544, M01 RR00080 (NCRR); HD40545, HD40560, and HD36801).

Comments and views of the authors do not necessarily represent views of the NICHD.

Financial Disclosure The authors did not report any potential conflicts of interest.

Presented at the Society for Maternal-Fetal Medicine annual meeting, February 11–16, 2013, San Francisco, California.

* For a list of other members of the NICHD MFMU Network, see Appendix 1 online at http://links.lww.com/AOG/A781.

The authors thank Allison Todd, MSN, RN, for protocol development and coordination between clinical research centers; Elizabeth Thom, PhD, for protocol development, data management, and statistical analysis; and Kenneth J. Leveno, MD, and Catherine Y. Spong, MD, for protocol development and oversight.

Dr. Rouse, Associate Editor of Obstetrics & Gynecology, was not involved in the review or decision to publish this article.

In recent years, the Consortium on Safe Labor's labor curves have replaced the traditional labor curve proposed by Friedman.1–4 The importance of understanding normal labor progression was highlighted by Rouse et al5 in 1999 when they challenged the idea that arrest of labor could be diagnosed after only 2 hours of inadequate cervical change. Rouse et al found that 60% of women who were given 2 additional hours to demonstrate cervical change went on to deliver vaginally. This observation highlighted that an inappropriate model of normal labor can lead to an overdiagnosis of arrest disorders of labor and subsequently unnecessary cesarean deliveries. The American College of Obstetricians and Gynecologists and the Society for Maternal-Fetal Medicine consensus statement on the Safe Prevention of the Primary Cesarean Delivery recommends using the Consortium on Safe Labor's labor curves to define normal labor progress.6

The Consortium's labor curve emphasizes the notion that active labor may not begin until 6 cm of cervical dilation. It does not specifically address the role of cervical effacement in predicting normal rates of cervical change. Cervical effacement, however, has been used by many authors as a traditional part of the definition of active labor7 and clinical experience would suggest that cervical effacement plays a role in labor progress. The current study assesses the association of cervical effacement with the rate of intrapartum cervical change among nulliparous women.

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MATERIALS AND METHODS

We conducted a secondary analysis of a prospective trial of fetal pulse oximetry conducted by the Eunice Kennedy Shriver National Institute of Child Health and Human Development Maternal-Fetal Medicine Units Network.8 In the original study, women were eligible to participate if they were nulliparous with a singleton, cephalic, living fetus at or beyond 36 weeks of gestation. Exclusion criteria included a planned cesarean delivery, maternal temperature 38°C or greater, maternal human immunodeficiency virus or hepatitis infection, maternal heart or renal disease, and diabetes mellitus requiring insulin. Women were enrolled with cervical dilations between 2 and 6 cm. Labor was managed according to the usual clinical practice at the participating centers. The original study was approved by the institutional review board at each participating Maternal-Fetal Medicine Units Network center and written informed consent was obtained from each participant. Data were collected by trained research nurses.

For the current study, we included all participants who underwent vaginal delivery of a liveborn neonate. There were no stillbirths. A separate analysis included those participants who underwent cesarean delivery for an arrest disorder. We abstracted data on patient race, body mass index (BMI, calculated as weight (kg)/[height (m)]2), use of epidural anesthesia, use of oxytocin for labor induction or augmentation, use of cervical-ripening agents, and details of labor progression including cervical dilation and effacement at each examination.

When comparing demographic and other patient characteristics between those with induced or augmented labor with those with spontaneous labor, the Wilcoxon rank-sum test was used to compare continuous variables, and categorical variables were compared by means of the χ2 or Fisher exact test, as appropriate. Analyses are presented separately for those with induced or augmented labor and those with spontaneous labor. Cervical effacement was recorded as a percentage of effacement (0–100%). Given that measurements of cervical effacement can often vary significantly by observer9,10 as well as the presumed clinical importance of achieving 100% cervical effacement, women were stratified into those who had achieved 100% cervical effacement and those who had not at each cervical examination. We used interval-censored regression to estimate the time to progress from one integer centimeter dilation to the next assuming a log-normal distribution.11 Because cervical examinations are often irregularly spaced, an individual may have progressed several centimeters of dilation from one examination to the next. Therefore, interval censoring allows an estimation of the time between any two 1-cm measurements (eg, from 4 to 5 cm), even when those precise measurements were not observed for all patients. The median, fifth percentile, and 95th percentile were calculated for the time to progress between every two successive dilations, and the times for those at 100% effacement compared with less than 100% effacement were compared with a Wald test using procedure LIFEREG in SAS 9.3. Using these same methods, a model was constructed with the covariates of 100% cervical effacement (yes compared with no), use of regional anesthesia (yes compared with no), maternal BMI, age, and race (white compared with all others). To calculate the cumulative time from 4, 5, and 6 cm to complete cervical dilation, right-censored regression assuming a log-normal distribution was used with effacement assessed at the initial cervical dilation. Among women whose labor was induced, a model was constructed with additional terms for mechanical ripening, medical ripening, and their interactions with 100% cervical effacement. Finally, these analyses were repeated for the women who had cesarean deliveries for arrest disorders.

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RESULTS

The original trial randomized 5,341 women. Of these, 1,439 women underwent cesarean delivery, leaving 3,902 women with vaginal deliveries in this analysis. There were no stillbirths. The current cohort included 1,466 (38%) women who underwent labor induction and 1,948 (50%) who underwent labor augmentation with oxytocin (combined for the analysis) as well as 488 (13%) women who labored spontaneously. Women in spontaneous labor were different from those women who were augmented or induced in most baseline characteristics, although the absolute magnitude of the differences was small (Table 1). Missing information on cervical effacement was rare. Fewer than 1% of cervical examinations were missing an effacement measurement.

Table 1

Table 1

For women who received oxytocin during labor, the time to dilate from each centimeter to the next was significantly shorter for women who were 100% effaced compared with those who were not (Table 2). For women who had spontaneous labor, the time to dilate from 2 cm to 3 cm and from 3 cm to 4 cm did not differ by effacement, whereas the time to dilate from 4 cm to 5 cm, 5 cm to 6 cm, 6 cm to 7 cm, 7 cm to 8 cm, 8 cm to 9 cm, and 9 cm to 10 cm was significantly shorter for those who were 100% effaced compared with those who were not (Table 2). When potential confounders including maternal race, age, BMI at the time of delivery, and use of regional anesthesia were considered in the model, the trend remained unchanged (Table 3).

Table 2

Table 2

Table 3

Table 3

For women who received oxytocin during labor, the time to reach 10 cm of cervical dilation from 4, 5, and 6 cm, respectively, was longer for those women who had not yet achieved 100% cervical effacement at the starting dilation. For women in spontaneous labor, this was true for 4 and 5 cm of cervical dilation. However, the 100% cervical effacement at 6 cm dilation was not associated with a significantly shorter duration to reach 10-cm dilation (Table 4).

Table 4

Table 4

Among women whose labors were induced, 370 (25.2%) were medically ripened and 243 (16.6%) were mechanically ripened. Medical ripening shortened the time to dilate from 6 to 7 cm beyond the effect of 100% effacement (P=.03), but not at other dilations. Mechanical ripening shortened the time to dilate from 9 to 10 cm beyond the effect of 100% effacement (P=.01), but not at other dilations.

From the 1,439 women who underwent cesarean delivery, we performed an analysis of the 985 who delivered by cesarean for an arrest disorder in the first (n=773) or second (n=212) stage of labor despite the use of oxytocin. Those women with 100% effacement had consistently faster rates of cervical dilation compared with those at less than 100% (Tables 5 and 6).

Table 5

Table 5

Table 6

Table 6

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DISCUSSION

Labor is defined as “uterine contractions that bring about demonstrable effacement and dilation of the cervix.”12 Although labor involves both dilation and effacement, existing labor curves demonstrate only the rate of cervical change in relationship to cervical dilation.1–4 We sought to investigate how cervical effacement might influence expectations of the rate of cervical change. In this large cohort of nulliparous women, the rate of cervical dilation in labor was significantly associated with achieving 100% cervical effacement.

Much attention has been paid to when the transition to active labor begins. The Consortium on Safe Labor data suggest that active labor may not begin until 6 cm.1,6 This strict criterion, however, has been challenged by Cohen and Friedman who “discouraged the use of any specific degree of dilation for the identification of the active phase.13 They argue that the timing of active labor depends on assessment of the individual patient but is typically between 3 and 6 cm. Our observations suggest that combining the assessment of cervical dilation with cervical effacement may allow us to better define the beginning of active labor. If our findings are confirmed, future labor guidelines may wish to include the combination of cervical dilation and effacement when defining active labor.

Our analyses consistently demonstrated that achievement of 100% cervical effacement was associated with faster labor progression. Even when the comparisons did not reach statistical significance, the trend was in this direction. The instances that were not statistically significant occurred in those with spontaneous labor, which were a minority of our cohort. In particular, the availability of data at earlier dilations was scarce in this group, likely secondary to fewer women in spontaneous labor having been admitted to labor and delivery before more advanced cervical dilations. Overall, these smaller numbers do limit our ability to comment on this group.

Strengths of this study include the prospective collection of data from a large number of nulliparous women from multiple institutions. However, measuring the rate of labor progress was not the focus of the study, and there was no protocol regarding the frequency of examinations or the experience level of those performing them. The uneven frequency is partially addressed through our statistical methods, but potential observer errors are not. By separating women into those who were 100% effaced compared with not, we hoped to eliminate some of the interobserver variability that is inherent in measuring cervical effacement.9,10 This decision was designed to provide more reproducible results while allowing us to provide insight into how cervical effacement is associated with labor progression.

Although those who undergo cesarean deliveries for arrest disorders have slower labor progression, we wished to address the basic question regarding cervical effacement in this group as well. Therefore, we repeated the analysis in this group and found that the pattern of more rapid cervical dilation was associated with achievement of 100% cervical effacement in these women as well. Although the absolute range of time to dilate from 1 cm to the next may differ between those with a vaginal or cesarean delivery, the association between rate of cervical change and 100% cervical effacement was consistently observed.

Our study is applicable only to those women who share characteristics with the women in the original study. We do not have data on multiparous women or diabetic women. Also, our analysis was limited to those who arrived at the hospital and agreed to participate in the randomized trial before reaching 7 cm dilation. Those missed would include women whose labor was progressing more quickly and therefore without sufficient opportunity to enroll in the trial. The result is an unknown lengthening of the time we report for labor progression. Lastly, those who chose to participate in the randomized trial may be different from those who did not.

Finally, our analysis combined women who had labor inductions with those who had labor augmentations. This was done because the distinction between these groups can be difficult to make and there is likely substantial overlap. The combination of these two groups is supported by the findings by Harper et al14 who analyzed the labor progress of women with augmented and induced labors and found them to be similar.

Labor involves a complex process of both cervical dilation and effacement. As we strive to safely reduce the number of unnecessary cesarean deliveries, we hope that an understanding of how cervical effacement may affect the expected rate of cervical change in labor will allow clinicians to more appropriately diagnose arrest disorders.

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