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Comparison of transvaginal cervical length and modified Bishop’s score as predictors for labor induction in nulliparous women

Objective: To compare the transvaginal cervical length (TVCL) to the modified Bishop’s score for prediction of successful labor induction in nulliparous women. Methods: A total of 210 nulliparous women who were diagnosed as premature rupture of membranes were recruited in this comparative prospective study, which was carried out in the Obstetrics and Gynecology Department of Ain Shams University, Egypt over two years for labor induction. The studied women were examined by trans-vaginal ultrasound for measurement of the cervical length (CL) and vaginally to calculate the modified Bishop’s score, followed by induction of labor. Collected data were analyzed to compare the TVCL to the modified Bishop’s score for prediction of successful labor induction in nulliparous women. The success of induction process was defined as vaginal birth after the induction of labor. Results: One hundred and forty-three women of studied women had CL <28 mm; 122 of them delivered vaginally (P=0.030). One hundred and forty-six women of studied women had modified Bishop’s score >4; 128 of them delivered vaginally (P=0.006). The CL <28 mm was significantly more specific with more positive predictive value as predictor of successful labor induction compared to modified Bishop’s score. Induction to delivery time was significantly shorter in women with CL <28 mm than women with CL ≥28 mm (P=0.02; 95% confidence interval: 4.9-8.4). In addition, induction to delivery time was significantly shorter in women with Bishop’s score >4 than women with Bishop’s score of ≤4 (P=0.01; 95% confidence interval: 1.6-4.5). Conclusions: Both TVCL and the modified Bishop’s score are complementary tools in pre-induction cervical assessment before induction of labor, while the TVCL at <28 mm is significantly more specific with more positive predictive value as predictor of successful induction than the modified Bishop’s score.


Introduction
Induction of labor is defined as the use of surgical or medical means to start uterine contractions before the spontaneous onset of labor [1]. The reported rate of labor induction increased in the past decade [2]. Multipara, non-obese, tall women with estimated fetal weight <3.50 g are favourable factors for vaginal delivery [3,4].
Objective: To compare the transvaginal cervical length (TVCL) to the modified Bishop's score for prediction of successful labor induction in nulliparous women. Methods: A total of 210 nulliparous women who were diagnosed as premature rupture of membranes were recruited in this comparative prospective study, which was carried out in the Obstetrics and Gynecology Department of Ain Shams University, Egypt over two years for labor induction.
The studied women were examined by trans-vaginal ultrasound for measurement of the cervical length (CL) and vaginally to calculate the modified Bishop's score, followed by induction of labor. Collected data were analyzed to compare the TVCL to the modified Bishop's score for prediction of successful labor induction in nulliparous women. The success of induction process was defined as vaginal birth after the induction of labor.
Results: One hundred and forty-three women of studied women had CL <28 mm; 122 of them delivered vaginally (P=0.030). One hundred and forty-six women of studied women had modified Bishop's score >4; 128 of them delivered vaginally (P=0.006). The CL <28 mm was significantly more specific with more positive predictive value as predictor of successful labor induction compared to modified Bishop's score. Induction to delivery time was significantly shorter in women with CL <28 mm than women with CL 曒28 mm (P=0.02; 95% confidence interval: 4.9-8.4). In addition, induction to delivery time was significantly shorter in women with Bishop's score >4 than women with Bishop's score of 曑4 (P=0.01; 95% confidence interval: 1.6-4.5). Conclusions: Both TVCL and the modified Bishop's score are complementary tools in pre-induction cervical assessment before induction of labor, while the TVCL at <28 mm is significantly more specific with more positive predictive value as predictor of successful induction than the modified Bishop's score. In 1964, Dr. Edward Bishop developed his scoring system and stated the success of the process of labor induction depended on the pre-induction cervical condition [5,6]. Dr. Edward Bishop reported no failure with Bishop's score 曒9 and the American College of Obstetricians and Gynecologists generally defines the cervix with Bishop's score 曑6 as unfavourable cervix for the process of labor induction [1].
In an attempt to minimize the subjectivity of Bishop's score, the original system has been modified to replace the cervical effacement with cervical length (CL), which is known as modified Bishop's score [7], and it is commonly used in clinical practice. Another modification to the original Bishop's score was proposed by Elghorori et al in which the ultrasound measurement of the CL was used instead of the clinical assessment of CL [8]. Ben-Harush et al concluded that the CL of <28 mm was significantly associated with shorter delivery time [9].
Ultrasound is available in most of obstetrics centers, as it is safe, accurate and inexpensive. Transvaginal approach is preferred route for ultrasound assessment of the CL. Transvaginal sonography (TVS) for CL assessment was reported in the diagnosis of cervical incompetence, labor induction and prediction of preterm labor [10,11].
This study is designed to compare the transvaginal cervical length (TVCL) to the modified Bishop's score for predicting successful labor induction in nulliparous women.

Method of data collection
Nulliparous women who were diagnosed as PROM, single fetus, cephalic presentation and estimated fetal weight between 2.5 and PROM was defined as rupture of fetal membranes <37 gestational weeks and diagnosed by history of sudden gush of water, positive fern and nitrazine tests. PROM was confirmed by sterile speculum examination and visualization of amniotic fluid leaking from the uterine cervix followed by amniotic fluid index 曑5 cm by tansabdominal ultrasound (TAS) [12]. Gestational age of the studied women was calculated from the first day of the last menstrual and ultrasound imaging was performed <20 weeks' gestation [12].
The studied women were subjected to thorough evaluation and TAS to confirm the fetal lie, fetal life, placental site, absence of congenital anomalies, estimated fetal weight and amount of the amniotic fluid.

Sample size
Required sample size was calculated using G* Power software for sample size calculation (*Heinrich Heine Universität; Düsseldorf; Germany). Sample size 曒202 women needed to produce statistically accepted figure.

Statistical analysis
Statistical Package for Social Sciences (SPSS) version 18 was used for statistical analysis of the collected data. The numerical data were presented as mean ± standard deviation (mean ± SD) and the categorical data were presented as number and percentage [(n(%)].
The Student's t-test and the Chi-square ( 氈 2 ) were used for statistical analysis and P<0.05 was considered statistically significant.

Results
The 210 studied nulliparous women's mean age was (  Induction to delivery time was significantly less in women with CL <28 mm than women with CL 曒28 mm (P=0.02; 95% confidence interval: 4.9-8.4). In addition, induction to delivery time was significantly less with Bishop's score >4 compared to Bishop's score of 曑4 (P=0.01; 95% confidence interval: 1.6-4.5) ( Table 1). Table 1 Duration of labor induction according to CL and modified Bishop's score (mean ± SD).

Discussion
Since the introduction of Bishop's score in 1964, various scoring systems have been suggested to evaluate the cervical condition before induction of labor [7,8]. Verhoeven et al [13] and Ware et al [14] suggested that the cervical shortening by TVS should be equal to cervical effacement detected during the digital vaginal examination. In fact, conflicting results were detected in different studies comparing the TVCL versus the modified Bishop's score as predictors for successful labor induction.
In this study and the study conducted by Abdelazim et al [4], both the modified Bishop's score and the TVCL were good predictors for successful labor induction. In a multicenter study, Pandis et al assessed the pre-induction of modified Bishop's score and the TVCL, and reported that both methods were significantly related to induction to delivery interval [15]. Pandis et al concluded that the TVCL at 28 mm cut-off point and the modified Bishop's score at 3 cut-off point were good predictors for successful induction of labor [15].
Previous studies concluded that both the modified Bishop's score and the TVCL can predict successful labor induction and were associated with successful vaginal delivery [14][15][16]. The systematic review conducted by Crane [17] reported that the successful labor modified Bishop's score) for pre-induction cervical assessment were equal and no one method was superior to the other [5].
Both the TVCL and the modified Bishop's score could be complementary for pre-induction cervical assessment and for prediction of successful induction of labor, while the choice of one particular method for pre-induction cervical assessment depends on the available tools and the environment of practice [5]. Ware and Raynor [14] pointed out that both the TVCL and the modified Bishop's score can predict the duration of labor and the successful induction. Besides, they indicated that the TVCL and modified Bishop's score were good predictors for the delivery mode [14]. In this study, the TVCL <28 mm was statistically more specific with more positive predictive value as predictor for successful induction of labor than the modified Bishop's score.
Tan et al concluded that the TVCL was more tolerated than the digital vaginal assessment for calculation of the modified Bishop's score and both the TVCL and the modified Bishop's score were good predictors for failed induction and cesarean delivery [20]. They also summarized that the TVCL >20 mm was good predictor and associated with high incidence of cesarean delivery after induction labor at term [20]. Uzun et al confirmed that the modified Bishop's score was good predictor for cesarean delivery compared to the TVCL in nulliparous women with prolonged pregnancy [21]. Maitra et al drew a conclusion that that the pre-induction TVCL can independently predict the success of the vaginal delivery. They also concluded that the probability of cesarean section was <30% at TVCL cut-off point of 曑3 cm and the probability of cesarean section increased by 45% with one unit increase in the TVCL [22]. In addition, Park et al expressed that the amount of prostaglandin used during induction of labor reduced by 50% at pre-induction TVCL of ≥曒28 mm [23].
Recently, Papillon-Smith and Abenhaim [24] have said that studies did not demonstrate the superiority of TVCL compared to the modified Bishop's score, and they concluded that the TVCL was useful to plan the labor induction and significantly reduce the amount of prostaglandins needed for cervical ripening.
Women who refused to give consent are the limitation of this study, while the comparative nature and the proper sample size are the strengths of the study. This study concludes that both TVCL and the

Conflict of interest statement
All authors declare that there is no conflict of interest.