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Keywords:

  • cervical length;
  • induction of labor;
  • mode of delivery;
  • prognosis;
  • systematic review;
  • ultrasonography

ABSTRACT

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. REFERENCES

Objective

To perform a systematic review and meta-analysis to assess the predictive capacity of transvaginal sonographic assessment of the cervix for the outcome of induction of labor.

Methods

We searched MEDLINE, EMBASE and the Cochrane Library, and manually searched reference lists of review articles and eligible primary articles. Studies in all languages were eligible if published in full. Two reviewers independently selected studies and extracted data on study characteristics, quality and test accuracy. We then calculated pooled sensitivities and specificities (with 95% CIs) and summary receiver–operating characteristics (sROC) curves. Outcome measures were test accuracy of sonographically measured cervical length and cervical wedging for Cesarean section, not achieving vaginal delivery within 24 h and not achieving active labor.

Results

We included 31 studies reporting on both cervical length and outcome of delivery. The quality of the included studies was mediocre. Sensitivity of cervical length in the prediction of Cesarean delivery ranged from 0.14 to 0.92 and specificity ranged from 0.35 to 1.00. The estimated sROC curve for cervical length indicated a limited predictive capacity in the prediction of Cesarean delivery. Summary estimates of sensitivity/specificity combinations of cervical length at different cut-offs for Cesarean delivery were 0.82/0.34, 0.64/0.74 and 0.13/0.95 for 20, 30 and 40 mm, respectively. For cervical wedging in the prediction of failed induction of labor summary point estimates of sensitivity/specificity were 0.37/0.80.

Conclusions

Cervical length and cervical wedging as measured sonographically at or near term have moderate capacity to predict the outcome of delivery after induction of labor. Copyright © 2013 ISUOG. Published by John Wiley & Sons Ltd.


INTRODUCTION

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. REFERENCES

Induction of labor for medical or elective indications is a common procedure in current obstetrics. In the USA, the rate of labor induction increased gradually from 10 to 21% for all term births between 1990 and 2002[1]. In the UK in 2005, one in five deliveries was induced[2].

Induction of labor is believed to be associated with an increase in the need for Cesarean delivery for both nulliparous and parous women[3]. Cesarean delivery not only carries operative risks in the index pregnancy, but also increases risks for future pregnancies[4]. The continuous rise in rates of Cesarean section gives cause for concern to both obstetricians and policy makers[5].

Recent randomized comparisons have shown that the effect of induction of labor on the risk for Cesarean section is limited; in pregnancies in which either the mother or the fetus is at risk for complications, induction of labor is the preferred policy over expectant management[6-9].

To date, the Bishop score remains the standard method for predicting the duration and safety of induced labor[10]. However, a recent systematic review showed that the Bishop score was a poor predictor of the outcome of labor in women scheduled for induction[11]. Over the past decade, pre-induction cervical length, as determined by transvaginal sonography, has been proposed as a predictor for cervical ripeness. Initial changes at the internal os of the cervix can be observed by transvaginal sonography, even in the absence of cervical dilatation. It has been suggested that measurement of cervical length before induction of labor can be used to assess the risk for Cesarean section.

Boozarjomehri et al.[12] evaluated the association between transvaginal ultrasound assessment of the cervix and the outcome of labor induction. They concluded that ultrasound assessment of cervical factors, such as wedging, may be helpful in identifying patients who will have a successful labor induction despite an unfavorable digital examination. Since the publication of this article, many studies have been published on the subject.

Although transvaginal ultrasonographic cervical measurement is quantitative, reproducible and easy to learn[13-16], studies demonstrate conflicting results, as some have reported cervical length to be predictive of successful labor induction[12, 17-19], while others have not found this association[20-22]. The aim of this study was to systematically review the literature on the prognostic capacity of cervical length for the outcome of induction of labor in pregnant women at term.

METHODS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. REFERENCES

Study identification and selection

We carried out an electronic search of MEDLINE and EMBASE from inception to January 2012 using ‘labor, induced’, ‘ultrasonography’ and ‘cervix uteri’ as search terms, and we also searched the Cochrane Database of Systematic Reviews. We checked reference lists of review articles and eligible primary studies to identify cited articles not captured by the electronic searches. Language restrictions were not applied.

We considered controlled trials, cohort studies and case–control studies. A study was considered relevant if it was in full manuscript form, included women in whom labor was induced and provided data on ultrasonographically measured cervical length and/or cervical wedging and outcome of delivery. For multiple publications of one study we included the most recent or most complete study. Non-English articles with English abstracts were also included. Two investigators independently reviewed random titles and abstracts to establish reliable, reproducible inclusion criteria. Once reliability had been established, the primary investigator reviewed the remaining titles and abstracts.

We obtained full manuscripts of all citations that were selected by at least one of the reviewers. Final decisions on inclusion/exclusion were made after independent and duplicate examination of the full manuscripts of selected citations. We included studies if they reported numerical data allowing construction of a 2 × 2 table or if these data could be acquired after contacting the authors.

Two reviewers independently assessed full articles for inclusion in this review and extracted data on clinical and methodological study characteristics and on test accuracy. Study characteristics consisted of: study design, recruitment of patients, inclusion and exclusion criteria, details about the test (ultrasound), quality assessment using QUADAS (Quality Assessment of Diagnostic Accuracy Studies) criteria[23], patient characteristics, reasons for induction of labor, methods of induction and results (mode of delivery). Any disagreements were resolved by consensus or by a third reviewer.

Analysis

For each study we independently constructed a 2 × 2 table cross-classifying cervical length or cervical wedging and the outcome of labor induction, using the cut-off values for cervical length mentioned in the primary article. In cases in which a primary article mentioned more than one cut-off value for cervical length, more 2 × 2 tables were constructed. To visualize data we plotted for each model combinations of sensitivity and specificity in receiver–operating characteristics (ROC) plots. A bivariate meta-regression model was used to calculate pooled estimates of sensitivity and specificity for risk score cut-off values and to fit a summary ROC (sROC) curve. The bivariate model preserves the two-dimensional nature of diagnostic data in a single model, rather than using a single outcome measure for each study such as the diagnostic odds ratio, and takes into account the negative correlation between sensitivity and specificity. This method has been extensively described and is recommended for the meta-analysis of diagnostic tests[24]. From the estimated bivariate model parameters, we calculated likelihood ratios (LRs) and derived the corresponding sROCs[25].

In order to evaluate the overall accuracy over the whole range of possible thresholds, we did not limit our analysis to a single threshold value, but included all reported threshold values, thus assuming that the shift in accuracy (higher sensitivity and lower specificity) due to different thresholds is accounted for by the correlation term in the bivariate model. Bootstrap averaged estimates were based on repeated analyses of stratified bootstrap samples to account for multiple accuracy points reported by the same study.

RESULTS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. REFERENCES

Figure 1 summarizes the literature identification and selection process. The computerized MEDLINE, EMBASE and Cochrane search detected 608 publications. After reading titles and abstracts, 502 of them were excluded, leaving 106 articles for detailed reading. From the cross-references, three more studies were identified and selected for further reading, giving 109 studies. Of these, 78 had to be excluded for various reasons. The main reasons for exclusion of studies were lack of information to calculate 2 × 2 tables (n = 11), no measurement of cervical length (n = 17) and a different spectrum of patients (n = 11). For nine pairs of papers reporting on the same cohort we included the paper with most complete data. We contacted the authors of 14 articles with insufficient data to construct a 2 × 2 table. Three of them (Park[43], Meijer-Hoogeveen et al.[39], Rozenberg et al.[22]) provided us with the required data, resulting in an overall inclusion of 31 articles in the meta-analysis[17-19, 21, 22, 26-51].

image

Figure 1. Flow-chart showing results of electronic search of MEDLINE, EMBASE and Cochrane Library of Systematic Reviews.

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Of these 31 studies, 30 were cohort studies and one study was a randomized controlled trial. All studies were prospectively designed. Eight of the 31 articles[21, 32, 36, 39, 43, 45, 46, 50] provided data both on cervical length before induction of labor and on the presence of cervical wedging in relation to successful induction of labor, whereas the other 23 only provided data on cervical length. The studies reported on a total of 5029 women undergoing induction of labor, of whom 1153 (23%) were delivered by Cesarean section. The Cesarean section rate in the studies varied from 11 to 60%, and the number of women analyzed in the cohort studies ranged from 43 to 460.

Figure 2 summarizes the results of the study quality assessment. In two studies, labor was induced in some of the women before 37 weeks' gestation, whereas 29 studies explicitly reported inclusion of singleton pregnancies in the term period (37–42 weeks) with cephalic presentation. Indications for induction were described in almost all studies. Blinding to the results of the ultrasound was described in 23 (74%) of the studies. No study reported on uninterpretable results.

image

Figure 2. Review authors' judgment of results of the Quality Assessment of Diagnostic Accuracy Studies tool for articles included in the present analysis. image, Yes (high quality); image, no (low quality); image, unclear.

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Several definitions for the outcome of successful induction were used in the studies. The occurrence of Cesarean delivery was used as the primary endpoint of most studies. Other frequently used endpoints were ‘not achieving vaginal delivery within 24 h after the start of induction’ and ‘no achievement of the active phase of labor’.

Only one study described a prespecified cut-off value for cervical length. Most studies reported several cut-off values within a range of 10 to 46 mm, allowing for multiple 2 × 2 tables to be extracted from one study.

From the 31 studies, we were able to construct 69 2 × 2 tables for the endpoints ‘Cesarean delivery’ (43 tables reporting on 22 studies and 3932 women), ‘not achieving vaginal delivery within 24 h’ (17 tables reporting on seven studies and 886 women) and ‘not achieving active labor’ (nine tables reporting on two studies and 211 women) assessing cervical length, and eight assessing cervical wedging (77 2 × 2 tables in total).

Taking into account the heterogeneity in endpoints, we analyzed data per specific outcome. Figure 3 shows the sensitivities and specificities of cervical length of the included studies for these different endpoints. For the prediction of Cesarean delivery, sensitivity ranged from 0.14 to 0.92 and specificity from 0.35 to 1.00. For the endpoint ‘no vaginal delivery within 24 h’, sensitivity ranged from 0.17 to 0.87 and specificity from 0.57 to 0.98.

image

Figure 3. Forest plots showing accuracy of cervical length measurement for prediction of: (a) Cesarean section, (b) no vaginal delivery within 24 h and (c) not achieving active phase of labor. Only the first author of each study is given. Sensitivity and specificity given with CIs. FN, false negative; FP, false positive; TN, true negative; TP, true positive.

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For the prediction of Cesarean delivery, the summary point estimates of sensitivity and specificity combinations were 0.82 (95% CI, 0.73–0.88) and 0.34 (95% CI, 0.24–0.45) for a cervical length of 20 mm, 0.64 (95% CI, 0.47–0.78) and 0.74 (95% CI, 0.63–0.82) for a cervical length of 30 mm and 0.13 (95% CI, 0.07–0.24) and 0.95 (95% CI, 0.89–0.98) for a cervical length of 40 mm, respectively (Figure 4). Corresponding positive (LR+) and negative (LR−) LRs were 1.2 and 0.53 for a cervical length of 20 mm, 2.5 and 0.49 for a cervical length of 30 mm, and 2.6 and 0.92 for a cervical length of 40 mm.

image

Figure 4. (a) Summary receiver–operating characteristics (sROC) curve (image) and pooled sensitivity and specificity (image) of cervical length for prediction of Cesarean delivery overall. image, Observed accuracy. (b) sROC curves (lines), pooled sensitivity and specificity (solid symbols) and observed accuracy (open symbols) of cervical length for prediction of Cesarean delivery using different cut-offs: 20-mm cut-off (image, image, image); 30-mm cut-off (image, image, image); and 40-mm cut-off (image, image, image). 95% confidence regions for estimated pooled sensitivity/specificity are indicated by ellipses.

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For the prediction of vaginal delivery not occurring within 24 h the summary point estimates of sensitivity and specificity combinations were 0.58 (95% CI, 0.46–0.69) and 0.80 (95% CI, 0.70–0.87) for a cervical length of 25 mm and 0.84 (95% CI, 0.69–0.92) and 0.60 (95% CI, 0.47–0.71) for a cervical length of 32 mm (Figure 5). Corresponding LR+ and LR− were 2.9 and 0.53 for a cervical length of 25 mm and 2.1 and 0.27 for a cervical length of 40 mm.

image

Figure 5. (a) Summary receiver–operating characteristics (sROC) curve (image) and pooled sensitivity and specificity (image) of cervical length for no vaginal delivery within 24 h overall. image, Observed accuracy. (b) sROC curves (lines), pooled sensitivity and specificity (solid symbols) and observed accuracy (open symbols) of cervical length for no vaginal delivery within 24 h using different cut-offs; 25-mm cut-off (image, image, image); and 32-mm cut-off (image, image, image). 95% confidence regions for estimated pooled sensitivity/specificity are indicated by ellipses.

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Only two studies reported on the outcome ‘not achieving the active phase of labor’[45, 50]. Roman et al.[45] reported results for different cut-offs for cervical length, whereas Yang et al.[50] reported results only for a cut-off of 31 mm. In the study of Roman et al., the sensitivity and specificity combination for a cut-off of 30 mm was 0.56 and 0.66 and for the study of Yang et al. this was 0.83 and 0.75. We did not calculate summary point estimates of sensitivity and specificity for the outcome ‘not achieving the active phase of labor’.

Eight studies (involving 1139 participants) reported on the presence or absence of wedging before induction of labor. A plot of sensitivity–specificity points for the presence of wedging for failed induction of labor is shown in Figure 6. Sensitivity varied from 0.12 to 0.61 and specificity from 0.63 to 0.91. For the prediction of failed labor induction, summary point estimates of sensitivity and specificity were 0.37 (95% CI, 0.26–0.49) and 0.80 (95% CI, 0.71–0.87), respectively (Figure 7). The likelihood ratio of a positive test was 1.9 (95% CI, 1.2–2.85) and the likelihood ratio of a negative test was 0.79 (95% CI, 0.65–0.96).

image

Figure 6. Forest plot showing accuracy of presence of wedging for prediction of failed induction of labor. Only the first author of each study is given. Sensitivity and specificity given with CIs. FN, false negative; FP, false positive; TN, true negative; TP, true positive.

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image

Figure 7. Summary receiver–operating characteristics curve (image) and pooled sensitivity and specificity (image) for prediction of failed induction of labor by wedging. image, Observed accuracy. 95% confidence region for estimated pooled sensitivity/specificity is indicated by ellipse.

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Five studies included only nulliparous women. We performed a subgroup analysis of this small group. In nulliparous women, sensitivity and specificity for a cervical length with a cut-off of 30 mm were 0.70 (95% CI, 0.61–0.78) and 0.74 (95% CI, 0.69–0.79), respectively, for the prediction of Cesarean delivery. Corresponding LRs were 2.7 for a positive test result and 0.40 for a negative test result.

DISCUSSION

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. REFERENCES

This systematic review assessed whether transvaginal ultrasonographic assessment of the cervix can be used as a predictor for successful induction of labor. We included 31 studies reporting on 5029 women. The sensitivity and specificity combinations of cervical length in the prediction of outcome of induction of labor showed that a long cervix and absence of wedging in general double the odds of failed induction, whereas a short cervix and wedging decrease the odds of failed induction by approximately 50%.

We assessed the quality of the included studies as mediocre. This was mainly owing to the fact that almost no study reported withdrawals and no study reported uninterpretable results of the cervical assessment. Since measuring cervical length is easy to perform and easy to learn, even for inexperienced investigators, a clear image of the cervix can be obtained in nearly 100 % of cases[16]. This could be an explanation for not reporting withdrawals or uninterpretable results, but unfortunately, most studies did not mention these issues explicitly.

Our review has some limitations. Most of the studies provided data on parity, but these data could not be correlated to the outcome of induction. The latter was only possible for the five studies that included only nulliparous women. The same applies to the method of induction and to the indications for Cesarean delivery. Despite this, we chose to include all studies, despite heterogeneity caused by indications for induction of labor or mode of induction.

We are aware of one other review that analyzed cervical length as a predictor of the outcome of induction of labor in patients at or beyond term[52]. This review, conducted by Hatfield et al.[52], included studies published before October 2006. In our review, we included an additional 12 articles published after 2006, which seems quite a lot after the publication of a systematic review on the subject. Our results for cervical length are in line with that study. However, unlike Hatfield et al.[52] and Boozarjomehri et al.[12], we did not find that cervical wedging is a good predictor of the outcome of induction, mainly owing to its low sensitivity.

In nulliparous women, it seems that cervical length with a cut-off of 30 mm best classifies women at high risk for Cesarean delivery, with a sensitivity of 0.70 and a specificity of 0.74, and corresponding likelihood ratios of 2.7 for a positive test and 0.40 for a negative test result.

No consensus has been reached regarding the diagnosis of failed labor induction. A variety of endpoints have been suggested, including Cesarean delivery, not achieving a vaginal delivery within a specified time (such as 12 or 24 h), not achieving active labor within a specified time, or failure to achieve the active phase of labor. Despite the heterogeneity in these outcome measures, we decided to include all studies providing at least one of these outcomes; however, we also decided to analyze data by endpoint.

The challenge with induction of labor is to identify patients in whom induction will be successful and patients in whom induction will fail. Identification of women at high risk for Cesarean delivery following induction of labor is of the utmost clinical importance.

A systematic review to assess the ability of the Bishop score to predict the mode of delivery in women scheduled for induction of labor showed poor accuracy of a Bishop score lower than 6 in the prediction of Cesarean delivery (LR+, 1.35 and LR–, 0.18)[11]. With a sensitivity of 0.78 and a specificity of 0.44, this test performs virtually as well as cervical length (sensitivity and specificity for the prediction of Cesarean delivery, 0.82 and 0.34, respectively).

The use of predictive tests in the decision to induce or not also depends on the need for delivery in a short time. For example, in women with pre-eclampsia at term, delivery is needed either along the vaginal route after induction of labor or, when this fails, by Cesarean section[8]. However, when the indication for delivery is less strong, for example in women with a healthy pregnancy at 41 weeks, this balance might be different. Also, one should realize that an unripe cervix, as indicated by a long cervix or absence of wedging, indicates not only a higher risk for failed induction, but also a longer time to the onset of spontaneous labor. Thus, when the need for immediate delivery driven by the condition of the mother or child is more urgent, there is greater emphasis on the correct interpretation of tests to predict failed induction. In fact, analysis of the importance of cervical length measurement in the HYPITAT trial indicated that in women with hypertensive disease and an unripe cervix, induction of labor is more desirable than in women with a ripe cervix, since the first group may be harmed by expectant management[53].

A limitation of the present meta-analysis is that we could only assess the accuracy of cervical-length measurement and wedging in a univariate way. As studies in which the additional value of cervical length are lacking, we cannot at present answer the question as to whether sonographically measured cervical length adds to the information provided by the Bishop score. Studies with a multivariable approach are needed to combine factors such as method of induction (prostaglandins vs oxytocin vs Foley catheter or double balloon), gestational age or parity in a prediction model. Individual patient data meta-analysis is a next step that will allow assessment of the relative contribution of each of these factors.

In summary, measuring cervical length and assessing cervical wedging before induction of labor, which are easily performed, have limited value in predicting the outcome of labor. They might help to shift the decision in an individual patient on induction or not, but in general their test accuracy is too limited to justify routine use in clinical practice.

REFERENCES

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. REFERENCES
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