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ORIGINAL RESEARCH |
From the Departments of 1Obstetrics and Gynaecology and 2Statistical Science, University College London Hospitals, London, United Kingdom.
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ABSTRACT |
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METHODS: Maternal age, height, body mass index, parity, gestational age, Bishop score, ultrasonic amniotic fluid volume, fetal head position, estimated fetal weight, and transvaginal cervical length were studied prospectively in 267 women at 36 or more weeks of gestation immediately before induction of labor. Logistic regression analysis was used to determine which factors best predicted the risk of cesarean delivery. Receiver operating characteristic curves and a resampling technique were used to evaluate the model’s performance.
RESULTS: Eighty (30%) of these 267 women had cesarean delivery. Logistic regression was performed and a final model chosen, which included parity (odds ratio [OR] 20.56, 95% confidence interval [CI] 7.97–53.05, P < .001), body mass index (OR 6.17, 95% CI 2.10–18.13, P < .001), height (OR 0.94, 95% CI 0.89–0.98, P = .005), and ultrasonic transvaginal cervical length (OR 1.07, 95% CI 1.04–1.11, P < .001) as the best predictors of cesarean delivery. A risk score was calculated containing these 4 parameters, which predicted reasonably accurately the risk of cesarean delivery.
CONCLUSION: Parity, body mass index, height, and ultrasonic transvaginal cervical length are the most accurate parameters in predicting the risk of cesarean delivery after induction of labor. A predictive model using these would allow more accurate counseling and better informed consent in the decision-making process regarding induction of labor
LEVEL OF EVIDENCE: II-2
At present there is little information about how to calculate this risk and consent women for induction of labor. The current standard for predicting the outcome of induction of labor remains the Bishop score, and an unfavorable cervix (Bishop score 
5) is the predominant clinical risk factor.5 The advantage of this score is that it is simple and easy to perform, but it is subjective and has a high intra- and interobserver variation. Several studies have shown that it is a poor predictor of outcome but may help to predict the length of the latent phase.
More recently several authors have evaluated the possible role of transvaginal ultrasound examination of the cervix as a predictor of outcome of induction of labor. The potential advantages of transvaginal ultrasonography are that it visualizes the whole length of the cervix, it can assess effacement and the internal os in a closed cervix, and it is more comfortable and less invasive than digital vaginal examination.11 Transvaginal ultrasonography is likely to be more objective and has been shown to have good inter- and intraobserver variability.12 Several studies have also shown a correlation between Bishop score and transvaginal ultrasonography, whereas others have not. Overall there has been wide variation in the prediction of success of induction of labor with both the Bishop score and transvaginal ultrasonography.11,13–27
As well as assessment of the cervix, many other individual factors have also been shown to help predict the mode of delivery in spontaneous and induced labor. These include maternal age, parity,11,31 body mass index,26,32,33 maternal height,34,35 gestational age at delivery,36 fetal head position,37,38 amniotic fluid volume,39,40 and ultrasonic estimated fetal weight.39 The majority of prediction studies have used transvaginal ultrasonography of the cervical length as the single ultrasound predictor with or without nonultrasound predictors. The main objective of this study was to determine whether a combination of factors in the maternal history and ultrasound parameters could improve the prediction of the risk of cesarean delivery after induction of labor based on the Bishop score or a single factor alone. This would allow more accurate counseling and better informed consent in the decision-making process regarding induction of labor.
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MATERIALS AND METHODS |
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Data were obtained from the maternal notes on maternal age, body mass index at booking, height, parity, and gestational age calculated by using the last menstrual period combined with a first-trimester scan. Parity was categorized into nulliparous and multiparous. Multiparous women were defined as women who had had a previous delivery at more than 23 weeks of gestation or any live birth. Body mass index (BMI) was categorized into 30 or greater and less than 30 kg/m2. An ultrasound scan was then performed on all women with an Aloka 1700 ultrasound machine (Aloka Company Ltd, Mitaka-shi, Tokyo, Japan). Fetal head position, amniotic fluid volume, and sonographic estimated fetal weight were obtained by using a 3.5-MHz abdominal probe. The fetal head position was obtained by visualizing the orbits in a transverse view of the head and was categorized into occipitoposterior and non-occipitoposterior. Occipitoposterior position on ultrasonography was defined as within 75° either side of the vertical posteriorly as previously described.26 Liquor volume was calculated as the amniotic fluid index (AFI) by measuring in the standard way the deepest pool of liquor in the 4 quadrants of the maternal abdomen. A sonographic fetal weight was obtained with the formula described by Shepard et al,41 including the biparietal diameter and abdominal circumference. A transvaginal ultrasound scan was then performed on all women with a 6-MHz probe and the cervix visualized. Three measurements of the closed cervical length were obtained from the internal to external os and the mean calculated. All scans were performed immediately before labor induction by the same operator (E.P.), who was experienced in the use of ultrasonography. The women and clinicians were blinded to the ultrasound findings. After the ultrasound examination, a digital vaginal examination was performed as usual by the midwife in charge of the woman’s care to obtain the modified Bishop score.42
The labor induction was then commenced—the method depending on the Bishop score in accordance with the hospital protocol and the guidelines of The Royal College of Obstetricians and Gynaecologists.43 In a nulliparous woman with a Bishop score of less than 4, 2 mg of prostaglandin E2 gel was inserted in the posterior fornix of the vagina. If the Bishop score was greater than 6 and the cervix was favorable for artificial rupture of membranes, then this was performed. All other women (nulliparous women with a Bishop score of 4–6 and all multiparous women) received 1 mg of vaginal prostaglandin E2 gel. Examinations during the induction were performed every 6 hours, further prostaglandin E2 gel was given if required, and artificial rupture of membranes was performed when possible. In accordance with national guidelines, oxytocin infusion was used if labor did not commence within 2 hours after artificial rupture of membranes.43 Once in labor, the women were managed according to the hospital protocol, and decisions were made by the attending clinicians, with vaginal examinations every 4 hours and the use of oxytocin infusion for slow progress according to the partogram. Cesarean delivery was performed as necessary for failure to progress in labor, fetal distress on the cardiotocograph, or failed induction of labor (failure of the cervix to reach 4 cm dilated).
The outcomes of the labor, delivery, and neonates were obtained from the women’s notes after delivery. The primary outcome was mode of delivery. The term emergency cesarean delivery is used throughout to describe all intrapartum cesarean delivery. A logistic regression model44 was used to develop the risk model to predict cesarean delivery. Assuming a cesarean delivery prevalence of 30% (the cesarean delivery rate for induction of labor in the unit at the time), it was estimated that a sample size of 267 women would be required for this study to estimate 8 regression coefficients with adequate precision.45 Three separate models, with 8 predictors each out of the 10 predictors collected, were considered. Each model contained either maternal age, BMI, or height in addition to the 7 other predictors. The predictors for the final risk model were selected by using a backwards elimination strategy with a statistical significance level of P < .05.44 Likelihood ratio tests were used to assess the statistical significance of the predictors. It was decided to treat the variables measured on a continuous scale as continuous unless the assumption of a linear relationship with the outcome was violated. The assumption of nonlinearity for the continuous predictors was assessed by inclusion of quadratic terms and fractional polynomials in the model.46
The coefficients from the final logistic model were converted to integer scores for ease of use in practice. This was done by performing a grid search for a conversion factor so that the correlation between the predicted log-odds from the original and risk score model was greater than 0.9. An internal validation procedure based on a resampling technique44 was used to assess the model’s performance. The method of Miller et al47 was used to assess how well the model predicted cesarean delivery, and the receiver operating characteristic curve was used as a measure of how well the model discriminated between women at a high risk and those at low risk of having a cesarean delivery.48 All statistical analyses were carried out with Stata 9 statistical software (StataCorp, College Station, TX).
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RESULTS |
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A total of 80 cesarean deliveries (30%) were performed. During this time the overall cesarean delivery rate in our unit was 30% (14% of which were emergency cesarean deliveries), and the rate of induction of labor was 14%. The indications for cesarean delivery are shown in Table 2. The characteristics of the women by mode of delivery are presented in Table 3. Table 4 shows the results from 2 logistic regression models. Model 1 contains height and the other 7 predictors, and Model 2 contains BMI and the other 7 predictors. Maternal age within the third model did not statistically significantly predict cesarean delivery, and therefore this model is not shown. Maternal height, BMI, parity, and transvaginal ultrasound length of cervix showed statistically significant associations with the chance of having a cesarean delivery. There was no evidence of nonlinear association between the continuous predictors and outcome. There were no statistically significant interactions between Bishop score and parity and Bishop score and transvaginal ultrasound cervical length.
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The final model chosen included maternal height (OR 0.94, 95% CI 0.89–0.98, P = .005), BMI (OR 6.17, 95% CI 2.1–18.13, P < .001), parity (OR 20.56, 95% CI 7.97–53.05, P < .001), and transvaginal ultrasound cervical length (OR 1.07, 95% CI 1.04–1.113, P < .001) as the predictors of cesarean delivery. We converted the coefficients estimated by this model to integer scores using a scaling factor of 14.1 found by the grid search. The risk score for nulliparity was 43, for BMI of 30 or more was 26, for transvaginal ultrasound cervical length was 1, and for height was –1. Therefore, to calculate the total risk score, 43 should be added for nulliparity, 26 for BMI of 30 or more, and 1 for each 1-mm increase in transvaginal ultrasound cervical length above zero, and 1 should be subtracted for each 1-cm increase in height above zero. For example, the risk for a nulliparous woman with a BMI of 25, a transvaginal ultrasound cervical length of 25 mm, and height of 160 cm would be 43 + 0 + 25 – 160 = –92. The risk scores are converted into probabilities of having a cesarean delivery using the following relationship:
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where the risk score for each woman is calculated. The risk scores and corresponding probabilities of having a cesarean delivery are presented in Table 5.
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The method of Miller et al47 for assessing the predictive accuracy of the model produced a slope of 0.96 (95% CI 0.68 to 1.23) and intercept of –0.012 (95% CI –0.36 to 0.33), which are not significantly different from their ideal values of 1 and 0 respectively. The results suggest that the model predicts cesarean delivery accurately. The receiver operating characteristic area estimated by this method is 0.83 (95% CI 0.78 to 0.88), again suggesting the reasonably good discriminatory ability of the model.
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DISCUSSION |
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Several other studies have examined the impact of transvaginal ultrasonography of the cervix before induction of labor on the outcome. Some authors have found results similar to ours11,14,15,18,19,22,23,26,27 and concluded that transvaginal ultrasonography is a better predictor of outcome than Bishop score. Some of these also demonstrated that parity and BMI are important predictors.11,14,23,26 However, many studies have also yielded conflicting results.13,17,20,21,24,25,49 The variation in results is likely due to the different populations (gestational age, reasons for induction of labor, parity) and end points used in each study and the fact that some of the studies were multicentered and therefore using a combination of different management protocols for induction of labor within the same study. One other study has combined sonographic cervical length, posterior cervical angle, and fetal head position with factors in the maternal history as predictors of cesarean delivery and found that all the ultrasound parameters predicted the risk of cesarean delivery.26 Our study uses estimated fetal weight and amniotic fluid volume on ultrasonography in combination with height and other factors in the maternal history.
Standardizing care during induction and in labor for obstetric research is difficult, but in this study all scans were performed by one operator, thereby reducing interobserver variability, and all women delivered in one obstetric unit, thereby providing consistent care. The model derived from the results is simple and easy to use. To be brought into clinical practice, the model would require the addition of a transvaginal scan to be performed before induction of labor to measure the cervical length, and this is a procedure that is within the capabilities of most obstetric departments.
The success of induction of labor can be measured in different ways. Vaginal delivery, vaginal delivery within 24 hours, failed induction of labor, delivery within 24 hours, length of latent phase and of labor, and induction-to-delivery interval have all been used as primary outcome measures. We chose to use mode of delivery as the primary outcome because emergency cesarean delivery is the most frequently occurring adverse outcome associated with maternal and fetal morbidity. The high risk of cesarean delivery after induction of labor compared with spontaneous labor is well known,4 but women are not always informed of this risk. This is partly due to the difficulty in predicting which particular woman is more likely to require a cesarean delivery and which are the important risk factors, as well as in accurately quantifying the risk. Recently several authors have attempted to quantify this risk.11,26,50 Smith et al50 described a model based on maternal age, height, gestational age, and fetal sex, which was based on a retrospective study of nulliparous women in Scotland. This study suggested that a better prediction of risk may be achieved by incorporating ultrasonic cervical length into the model.
It has been proposed that, because the optimal outcome for the baby occurs after a spontaneous vaginal delivery or cesarean delivery before labor and the maternal risks between the two are finely balanced, elective prophylactic cesarean delivery should be offered to all women.51 The difficulty lies in the fact that we have been unable to predict which women will have a spontaneous vaginal delivery. Delivery by intrapartum cesarean delivery definitely carries a higher risk of maternal morbidity than elective cesarean delivery.6 In fact, in the Term Breech Trial, there was little difference in the risk of maternal complications between planned elective cesarean delivery and planned vaginal birth.52 There must be a level of risk of emergency cesarean delivery at which elective cesarean delivery for all would be associated with a lower overall risk of maternal complications than planned vaginal delivery. The exact level of this risk is not known, but it would seem sensible that women at very high risk of cesarean delivery after induction should be offered an elective cesarean delivery as an alternative to reduce morbidity and cost and improve satisfaction. At the very least, women should be given accurate information about the risks of any intervention, including induction of labor, so that they can make informed choices about their care. The next step is to validate this proposed model with data before induction of labor in a multicenter study with different operators measuring ultrasonic cervical length. If validated, a prospective trial is needed that would offer women this information before induction of labor, with follow-up of the management and outcomes.
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Footnotes |
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doi:10.1097/01.AOG.0000196508.11431.c0
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REFERENCES |
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