The cervix is known to undergo histologic changes before the onset of labor.1 Such changes may be reflected ultrasonographically with the cervix shortening before clinical evidence of labor occurs. Correspondingly, a short cervix seen in the second trimester has been associated with preterm birth.2–6
Conversely, a cervix that does not shorten may be an indication that the cardinal changes required to enter and progress effectively through labor have not occurred. One could hypothesize, therefore, that women with longer cervical lengths would be more likely to undergo a cesarean delivery. In fact, there has been some evidence that a longer cervical length at term is associated with an increased risk of postterm pregnancy and cesarean delivery.7 Moreover, in one study of nulliparous women in the United Kingdom, cesarean delivery was found to be more likely even if the longer cervical length was present as early as the second trimester.8
However, the existing data regarding the association between second-trimester cervical length and cesarean delivery are derived from a secondary analysis of a randomized controlled trial and, as such, may not be valid for the general population.8 Furthermore, adjustment previously has not been made for potentially confounding factors such as the use of assisted reproduction or maternal comorbidities such as hypertensive disorders or diabetes mellitus. Finally, the exact reason for the increased frequency of cesarean delivery remains uncertain. We hypothesized that the cervical length measured during routine second-trimester screening will be independently associated with the chance of cesarean delivery as well as cesarean delivery specifically resulting from arrest disorders in the first stage of labor.
MATERIALS AND METHODS
This was a cohort study of nulliparous women who underwent routine transvaginal cervical length measurement during an ultrasonographic examination of fetal anatomy between 16 and 24 weeks of gestation. During the period of study (December 2010 to January 2012), transvaginal ultrasonography was a routine part of the second-trimester examination at the institution where the study occurred. The cervical length images were obtained by staff ultrasonographers, all of whom had been trained to obtain cervical length according to the method of Iams et al with images reviewed by attending perinatologists.2 The records of all women who received a second-trimester examination were identified through a query of the electronic record of data derived from clinical ultrasonographic reports, in which all cervical lengths obtained during the study time period were recorded by the ultrasonographer and confirmed by the sonologist. Women were included in this analysis if they were older than 18 years of age, had a singleton gestation, had no prior pregnancies that progressed beyond 20 weeks of gestation, labored with an intent to delivery vaginally, and delivered at or after 34 weeks of gestation. Women who underwent a termination of pregnancy or experienced a fetal death were excluded.
Once potentially eligible women were identified, their ultrasonographic reports and electronic records were reviewed to ensure inclusion criteria were met. If cervical length was measured on more than one occasion, the measurement closest to 20 weeks of gestation was used in our analysis. Charts were abstracted for demographic information and for obstetric outcomes. Demographic data that were collected included maternal age, race and ethnicity, smoking history, body mass index (calculated as weight (kg)/[height (m)]2) at delivery, and history of any pre-existing comorbidities (eg, chronic hypertension, diabetes mellitus) or cervical excision procedures (ie, cold knife cone or loop electrosurgical excision procedure). Obstetric data abstracted included method of conception, pregnancy complications (eg, preeclampsia), type of labor (induced compared with spontaneous), route of delivery, and neonatal birth weight.
For purposes of analysis, cervical length was categorized into quartiles. Patient characteristics and obstetric outcomes of the population were compared according to these quartiles. The primary outcome was cesarean delivery. Univariable comparisons were performed using the Kruskal-Wallis test or χ2 analysis for continuous and categorical variables, respectively. Trends were examined using the nonparametric test of trend. Additional analyses that assessed the association of cervical length and cesarean delivery after stratification by indication for cesarean delivery (arrest disorder compared with other indication) and type of labor were performed. Lastly, multivariable logistic regression was performed to assess whether cervical length quartile (both categorized as quartiles and as a continuous variable) was independently associated with cesarean delivery. Variables were entered into each multivariable model if they demonstrated an association with cervical length quartile at a P<.1 level in the univariable analysis. Also, a regression with labor induction as an independent term was performed given the reported association between labor induction and cesarean delivery.9–19 Odds ratios and 95% confidence intervals were estimated. All tests were two-tailed and P<.05 was used to define statistical significance. Analyses were performed with Stata 11.1. The institutional review board of Northwestern University approved this study with a waiver of informed consent.
During the study period, 6,993 nulliparous and multiparous women underwent midtrimester cervical length measurement. Of the 3,220 (46.0%) women who met inclusion criteria (Fig. 1), 710 (22.0%) underwent a cesarean delivery. Five hundred eighteen (73.0%) of the cesarean deliveries were the result of an arrest disorder.
Baseline patient characteristics of the study population are shown in Table 1. Women with longer cervical lengths were significantly older, more likely to be white, and less likely to have undergone a prior cervical excisional procedure. In addition, women with longer cervical lengths delivered at significantly later gestational ages and delivered significantly larger neonates.
Results of the analyses regarding delivery characteristics are depicted in Table 2. The frequency of cesarean delivery increased with cervical length quartile. When analyzing only those women whose cesarean delivery was for the indication of an arrest disorder, this same trend was present, although only for women whose arrest disorder occurred in the first stage of labor. In contrast, there was no association between cervical length quartile and cesarean delivery performed for indications other than an arrest disorder.
Lastly, multivariable analyses (Table 3) estimated whether the association between cervical length quartile and cesarean delivery was independent of other potential confounding factors. After adjusting for all other variables associated in univariable analysis with cervical length quartile (maternal age, race, and prior cervical excisional procedure), the increased odds of cesarean delivery associated with increasing cervical length quartile remained significant. This association persisted even after sequentially adding gestational age at delivery and induction of labor into the regression, signifying that the relationship was not merely the result of women with longer cervical lengths delivering at later gestational ages or being induced more frequently. This multivariable analysis also was performed with cervical length as a continuous variable. For every additional centimeter of cervical length, the odds of cesarean delivery increased by 20.3%. This increase was similar in magnitude even after controlling for gestational age at delivery and induction of labor.
In this study, we found that increasing second-trimester cervical length quartile in nulliparous women is associated with a greater likelihood of cesarean delivery. When indication for cesarean delivery was examined, the increase in cesarean delivery frequency was seen only in those experiencing an arrest disorder in the first stage of labor. This relationship between cervical length and cesarean delivery persists even after controlling for potential confounding factors.
Although women with longer cervical lengths were more likely to deliver at a later gestational age, the increased frequency of cesarean delivery was not related solely to the gestational age at delivery. Similarly, despite the reported association between labor induction and cesarean delivery, there was no evidence that the higher frequency of cesarean delivery was related to women undergoing labor induction. In fact, regardless of gestational age or whether labor was induced or spontaneous, the association between cervical length and cesarean delivery remained significant in multivariable analysis. Rather, our analyses have demonstrated that a longer second-trimester cervical length does seem to have an effect on the labor process itself. In particular, it appears that the chance of arrest in the first stage of labor increases as cervical length lengthens.
This study is not without its limitations. Although the demonstrated association between cervical length and cesarean delivery persisted in multivariable analysis, omitted variable bias cannot be excluded. However, this analysis was derived from data abstracted by primary chart review and analyses included factors known to be associated with cesarean delivery. In addition, although this cohort is large and composed of women receiving routine obstetric care, patients were cared for at a single tertiary care institution and thus generalizability to the entire population of the United States cannot be assumed. Mitigating this concern are previously published data from the United Kingdom demonstrating this same association.8 In addition, these cervical length measurements were routinely performed among a largely nonreferral population.
This study corroborates previous research supporting second-trimester cervical length as an independent predictor of cesarean delivery.8 Our study not only confirms these findings, but with the addition of a chart abstraction process by which the indication for cesarean delivery was clearly identified, our analysis substantiates the notion that this association represents an increased frequency of the diagnosis of arrest of dilation. These data do provide further insight into the parturitional process and highlight that processes intimately connected to labor appear to occur months before labor manifests overtly.
This pathway has not only epidemiologic importance, because protraction disorders are the most common indication for cesarean delivery, but also biologic plausibility. These data suggest that the increased frequency of cesarean delivery is not the result of labor occurring at later gestational ages or labor being more frequently induced. Rather, a cervix that remains long may indicate that the remodeling that is crucial for cervical change to occur when labor begins is less likely to have occurred. A longer cervical length, therefore, may indicate that the cervix is less capable of making the changes during labor that are necessary for complete dilation to be reached.
Although an association between cervical length and cesarean delivery has been documented, it should be recognized that even in nulliparous women with the longest cervical length quartile, the majority (72.3%) achieved a vaginal birth. The positive predictive value for cesarean delivery of a cervical length in the highest quartile is only 28%. Thus, even a long cervical length should not be used to dissuade women from attempting a vaginal birth. Further exploration of the predictive capacity of second-trimester cervical length to predict delivery outcomes in populations at high risk for labor complications (ie, those undergoing a trial of labor after cesarean delivery) may augment contemporary clinical counseling.
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