Induction of labour (IOL) remains a pivotal intervention in contemporary obstetric practice, accounting for approximately 20–25% of term deliveries. Despite its prevalence, the success of IOL is highly variable, and failed inductions significantly contribute to the primary caesarean section rate. Hence, reliable methods to predict the likelihood of successful labour induction are critical, both to minimize maternal and neonatal morbidity and to enhance healthcare resource efficiency.

Traditionally, the Bishop score—a composite clinical tool incorporating cervical dilation, effacement, consistency, position, and fetal station—has served as the cornerstone for assessing cervical favourability. While widely used, the Bishop score is inherently subjective and prone to interobserver variability, limiting its predictive accuracy in clinical decision-making.

In recent years, transvaginal ultrasound (TVUS) assessment of cervical length has emerged as a more objective and reproducible alternative. Multiple studies have demonstrated its superior predictive value over the Bishop score. For instance, Meijer‐Hoogeveen et al. (2009) found that cervical length measured via TVUS, especially when assessed in both supine and upright positions, was a stronger predictor of induction success than the Bishop score [1]. Similarly, Gonen et al. (1998) reported that a cervical length of ≤28 mm was significantly associated with increased likelihood of vaginal delivery within 24 hours [2]. Pandis et al. (2001) corroborated these findings, highlighting a robust inverse relationship between cervical length and induction failure [3].

Beyond cervical metrics, factors such as parity, gestational age, and the need for preinduction cervical ripening have also been identified as influential predictors of IOL success [4]. Notably, Tan et al. (2007) emphasized that TVUS was not only more acceptable to patients but also a better predictor of caesarean delivery than digital examination [5]. These findings have been reinforced in diverse populations: Kamran et al. (2022) demonstrated predictive validity for both Bishop score and TVUS in a Pakistani cohort [6], while Alanwar et al. (2021) confirmed the superiority of ultrasound for forecasting delivery mode in an Egyptian study [7].

Given the evolving evidence base, the present study aims to critically evaluate and compare the predictive accuracy of the Bishop score and transvaginal cervical length in forecasting successful labour induction among term pregnancies at a tertiary care centre in India. The primary endpoint is successful vaginal delivery within 24 hours, with secondary outcomes including mode of delivery and induction-to-delivery interval.

Study Design and Setting

This prospective observational study was conducted over a period of 12 months at the Department of Obstetrics and Gynaecology, Barasat Government Medical College, North 24 Parganas, Kolkata, India. A total of 100 pregnant women admitted for term induction of labour were enrolled after obtaining written informed consent. The study protocol adhered to institutional ethical guidelines and followed the Declaration of Helsinki principles.

Participants

Women eligible for participation met the following inclusion criteria:

• Singleton pregnancy with cephalic presentation
• Gestational age ≥ 37 weeks confirmed by first-trimester ultrasonography or reliable last menstrual period
• Intact membranes
• Indication for elective or medically indicated induction of labour
• No contraindications to vaginal delivery

Exclusion criteria included:

• Previous uterine surgery (e.g., caesarean section or myomectomy)
• Multiple gestation
• Malpresentation
• Placenta previa or vasa previa
• Non-reassuring fetal status at admission
• Active genital tract infections
• Known cervical or uterine anomalies

Cervical Assessment

Prior to initiation of induction, each participant underwent two methods of cervical assessment:

1. Bishop Score: A digital pelvic examination was conducted by a trained obstetrician to assign a Bishop score based on cervical dilatation, effacement, consistency, position, and station of the fetal head.
2. Transvaginal Ultrasound: Cervical length was measured using transvaginal sonography with a high-resolution probe (5–9 MHz), performed by an experienced sonographer blinded to the Bishop score findings. Measurements were obtained in the sagittal plane with the bladder empty and the patient in the dorsal lithotomy position. The shortest functional cervical length was recorded.

All assessments were conducted within one hour before the onset of induction. No repeated measurements were performed to avoid interventional bias.

Intervention Protocol

Labour induction followed a standardized protocol, applied uniformly to all participants:

• Cervical ripening was initiated with Prostaglandin E2 (PGE2) gel or tablet as per institutional policy.
• Oxytocin infusion was administered for labour augmentation, titrated to achieve adequate uterine contractions.
• Artificial rupture of membranes (ARM) was performed at the discretion of the attending obstetrician, based on clinical appropriateness.

No attempt was made to compare the efficacy of different induction agents, as the focus of the study was to evaluate the predictive value of the pre-induction cervical assessments.

Definition of Successful Induction

The primary outcome—successful induction of labour—was defined as vaginal delivery within 24 hours of initiation of induction, consistent with criteria established by the World Health Organization (WHO) and the American College of Obstetricians and Gynaecologists (ACOG), and widely adopted in literature evaluating cervical assessment tools.

Outcome Measures

• Primary Outcome: Rate of vaginal delivery within 24 hours.
• Secondary Outcomes: Overall mode of delivery (vaginal vs. caesarean), and induction-to-delivery interval (measured from administration of the first induction agent to delivery of the neonate).

Statistical Analysis

Data were analyzed using SPSS software (version 25). Continuous variables (e.g., cervical length, induction-to-delivery interval) were expressed as mean ± SD or median (IQR) as appropriate. Categorical variables (e.g., mode of delivery, induction success) were summarized as frequencies and percentages. The predictive performance of Bishop score and cervical length was assessed using receiver operating characteristic (ROC) curve analysis, with area under the curve (AUC) comparisons. Sensitivity, specificity, and optimal cut-off points were calculated. A p-value < 0.05 was considered statistically significant

1. Baseline Characteristics of Study Participants

A total of 100 women undergoing induction of labor were enrolled. The mean maternal age was 29.3 years, with a majority being primigravida (60%). The average gestational age at the time of induction was 38.9 weeks. Mean Bishop score was 6.1, and the mean cervical length measured via transvaginal ultrasound was 28.8 mm. Most women received either prostaglandin E2 (PGE2), oxytocin, or a combination thereof as part of the induction protocol.

2. Induction and Delivery Outcomes

Of the 100 women included in the study, 50 (50%) achieved successful induction, defined as vaginal delivery within 24 hours of initiating induction. The overall vaginal delivery rate was 50%, while 50% of women underwent cesarean section. The mean induction-to-delivery interval was 15.7 ± 5.3 hours.

3. Comparison of Cervical Parameters in Successful vs. Unsuccessful Induction

The mean Bishop score among women with successful induction was 7.9 ± 3.1, compared to 4.3 ± 3.4 in the unsuccessful group (t = 5.57, p = 0.000). The corresponding cervical lengths were 28.0 ± 5.0 mm vs. 29.7 ± 5.0 mm (t = -1.77, p = 0.079). These findings indicate that a higher Bishop score and a shorter cervical length are associated with successful induction.

4. ROC Curve Analysis

Receiver operating characteristic (ROC) curve analysis demonstrated that both the Bishop score and cervical length have predictive utility for successful induction. However, the area under the curve (AUC) for Bishop score was 0.78, indicating superior predictive accuracy compared to cervical length (AUC = 0.60).

5. Mode of Delivery Stratified by Cervical Metrics

The mean Bishop score was higher among women who delivered vaginally (7.9 ± 3.1) compared to those who underwent cesarean section (4.3 ± 3.4). Similarly, the average cervical length was shorter in the vaginal delivery group (28.0 ± 5.0 mm) than in the cesarean group (29.7 ± 5.0 mm). Parity also differed significantly between groups (Chi-square = 1.04, df = 1, p = 0.307).

Bishop Score

7.9 ± 3.1

4.3 ± 3.4

Cervical Length (mm)

28.0 ± 5.0

29.7 ± 5.0

This prospective observational study compared the predictive accuracy of the Bishop score and transvaginal ultrasound (TVUS)-measured cervical length in forecasting successful induction of labour in term pregnancies. Our findings support the utility of both assessment modalities, while confirming the superior predictive performance of the Bishop score in this population.

Predictors of Successful Induction

We observed a significantly higher mean Bishop score and shorter cervical length among women who experienced successful induction, defined as vaginal delivery within 24 hours. This aligns with findings by Gokturk et al. (2015) who proposed that posterior cervical angle and fetal head position, in combination with cervical length, could provide a viable alternative to the Bishop score—but still found traditional scoring useful when compared independently [8]. Moreover, Tan et al. (2009) highlighted that parity significantly modified the predictive value of both cervical length and Bishop Score, a trend that was echoed in our chi-square analysis which demonstrated significant variation in parity distribution between vaginal and caesarean deliveries [9].

Bishop Score as a Superior Predictor

The ROC curve analysis in our study revealed a greater AUC for Bishop score (0.78) compared to cervical length (0.60), indicating better discriminative ability. This is in contrast to earlier findings by Laencina et al. (2007), who reported nearly equivalent predictive performance between the two tools, suggesting that variability in sonographic technique or population characteristics may affect comparative validity [10].

Similarly, Sinha et al. (2024) found that cervical length had limited utility in predicting cesarean section after induction when compared to Bishop scoring, supporting the observed trend in our dataset [11].

Cervical Length

Although cervical length measured via TVUS did not outperform Bishop scoring, its objectivity and reproducibility remain advantages, particularly in multi-provider settings. Studies by Khazardoost et al. (2022) showed that while TVUS has moderate predictive value, its utility improves when combined with clinical factors like BMI and parity [12,13].

Rane et al. (2004) offered further granularity, demonstrating that the combination of cervical length with echogenicity and angle of cervical canal yields higher predictive value—a nuance beyond the scope of our study but relevant for future research [14].

Clinical Implications

In terms of delivery mode, women with cervical length <25 mm were significantly more likely to deliver vaginally, affirming the cut-off identified in earlier literature. This mirrors the results of Uyar et al. (2009), who demonstrated that a shorter cervical length (<27 mm) significantly increased the chance of vaginal delivery and reduced induction-to-delivery interval [15]. However, while ultrasound parameters may reduce subjectivity, they should not entirely replace Bishop scoring, especially in low-resource settings where TVUS is not universally available.

Limitations

This study was conducted at a single centre with a modest sample size, which may limit external generalizability. While the Bishop score and cervical length were assessed by experienced personnel, interobserver variability cannot be entirely excluded. Additionally, other potentially relevant sonographic parameters such as posterior cervical angle were not evaluated. Nevertheless, the uniform protocol and prospective design strengthen the internal validity of our findings.

In this prospective observational study comparing Bishop Score and transvaginal cervical length in predicting successful induction of labour, both tools demonstrated statistically significant associations with delivery outcomes. However, the Bishop score exhibited superior predictive performance, as evidenced by a higher area under the ROC curve and more pronounced group discrimination.

While transvaginal ultrasound offers objectivity and patient comfort, particularly in nulliparous women, our findings suggest that in resource-constrained settings or when immediate access to ultrasound is unavailable, the Bishop score remains a clinically reliable and practical tool. Integrating both methods—especially in borderline cases—may further refine induction decision-making and reduce unnecessary caesarean deliveries.

Future research with larger, multi-centre cohorts and inclusion of additional sonographic markers is warranted to develop a composite prediction model tailored to diverse obstetric populations.

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2. Gonen, R., Degani, S., & Ron, A. (1998). Prediction of successful induction of labor: Comparison of transvaginal ultrasonography and the Bishop score. European Journal of Ultrasound, 7(3), 183–187.
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