Comparison between subjective and objective assessments of the cervix before induction of labour

The aim of the study was to compare the pre-induction cervical assessment by Bishop's score with the transvaginal ultrasound cervical length as predictors of the induction-delivery interval (IDI) and the success of induction. This prospective study included 104 women with singleton pregnancies who were booked for induction of labour at term over a period of 3 years. Transvaginal ultrasound measurement of the cervical length and Bishop's Score were performed by different operators. Data were collected on parity, gestational age, methods of induction, Bishop's score, ultrasound cervical length measurements, IDI and mode of delivery. A total of 87 women (83.7%) delivered vaginally and 17 (16.3%) delivered by caesarean section. Linear regression models demonstrated that ultrasound cervical length was a better indicator of IDI than Bishop's score. The adjusted R2 for the regression including ultrasound cervical length was 0.87 compared with a value of 0.67 for the model including Bishop's score. Although logistic regression analysis confirmed that cervical effacement was the best component of Bishop's score to predict the mode of delivery, ultrasound cervical length assessment provided better prediction. Receiver operating characteristic curve showed that the optimised cut-off value for prediction of vaginal delivery was < or =3.4 cm for the cervical length and >5 for the Bishop's score. At those optimised cut-off values the cervical length predicted vaginal delivery with sensitivity of 62.1% (95% CI [51%, 72.3%]) and specificity of 100% (95% CI [80.5%, 100%]) while the Bishop's score predicted vaginal delivery with a sensitivity of 23% (95% CI [14.6%, 33.2%]) and specificity of 88.2% (95% CI [63.5%, 98.5%]). Further analysis showed that ultrasound cervical length has a higher sensitivity in prediction of vaginal delivery in multiparous than nulliparous women (85.1% compared with 35%) at a cut-off value of < or =3.4 cm. On the other hand, it has a higher sensitivity in nulliparous comparable with multiparous women (85.3% compared with 30%) in prediction of IDI at a cut-off value of >3.5 cm. In conclusion, transvaginal ultrasound cervical length assessment is better than Bishop's score in predicting the IDI and the success of induction of labour.