Transvaginal ultrasonography-measured cervical length versus the modified Bishop score for preinduction cervical assessment at term: A randomised controlled trial.

Introduction: The inducibility of the cervix for labour induction is usually determined by cervical status evaluation. The Bishop score is historically used to forecast the success of induction of labour, although it is subjective, and not reproducible. However, transvaginal ultrasound measurements of cervical length are rarely used for preinduction cervical assessment. The study compared cervical length measured via transvaginal ultrasound and the modified Bishop score for preinduction cervical assessment at term.

Methods: The study involved 72 pregnant, nulliparous women for induction of labour at term. They were randomised into the transvaginal ultrasound group and the modified Bishop score group. The cervix was said to be 'ripe' when the transvaginal ultrasound cervical length (CL) was < 28 mm or the modified Bishop score was ⩾ 6. The cervix was considered 'unripe' when the Bishop score was < 6 or the transvaginal ultrasound was ⩾ 28 mm. Participants with ripe cervices had induction of labour with an oxytocin infusion, while those with unripe cervices had preinduction cervical ripening with misoprostol. The primary outcome measures were the mode of delivery and the total amount of prostaglandins administered for preinduction cervical ripening.

Results: There was no significant difference between the two groups with regard to the mode of delivery (p = 0.795), the total amount of prostaglandins administered for preinduction cervical ripening (105.0 ± 51.04 µg vs 111.90 ± 52.2 µg; p = 0.0671), the proportion of women who were administered prostaglandins due to an unfavourable cervix (41.7% vs 55.6%; p = 0.812), induction-to-the-active phase of the labour interval (11.00 ± 4.2 hours vs 11.82 ± 4.12 hours; p = 0.407) or the induction-delivery interval (20.15 ± 5.7 hours vs 22.66 ± 4.33 hours; p = 0.06) in both groups, respectively. Compared with those in the Bishop score group (Bishop score ⩾ 6), the induction-to-active phase of labour intervals (6.47 ± 0.77 hours vs 7.33 ± 1.21 hours, p = 0.024) and the induction-to-delivery intervals (14.97 ± 1.0 hours vs 18.39 ± 0.85 hours; p = 0.0001) in the transvaginal ultrasound group (cervical length < 28 mm) were significantly shorter, respectively.

Conclusion: Preinduction cervical assessment using transvaginal ultrasound (cervical length < 28 mm) or the modified Bishop score is a successful predictor of the outcome of labour induction A larger multicentre studies are needed to identify optimal cervical length cutoffs and to determine if this could decrease unnecessary prostaglandin use or decrease caesarean section rate.