Methods for assessing pre-induction cervical ripening, the ability of the cervix to open in response to spontaneous uterine contractions

In this review, researchers from The Cochrane Collaboration examined a comparison between the Bishop score and any other method for checking pre-induction cervical ripening in women admitted for induction of labour. The Bishop score is the traditional method of determining the readiness of the cervix to open (dilate) before labour induction. It also assesses the position, softening and shortening of the cervix, and the location of the presenting part of the baby. After searching for relevant trials up to 31 March 2015, we included two randomised controlled trials that recruited 234 pregnant women.

What are the methods for pre-induction softening of the neck of womb and why is it important to soften the neck of womb before induction of labour?

Induction of labour is the non-natural process of starting labour in a pregnant woman after the age the baby is more likely to survive following delivery, when there is no clear evidence of serious onset of labour and the membranes covering the baby are unruptured. Induction of labour may be needed because of a problem in the mother, or her baby or both, and is carried out at or before the ninth (last) month of pregnancy. Obstetricians (specialist caring for pregnant women) have long known that for this to be successful, it is important that the uterine cervix (the neck of the womb) has the favourable characteristics that make it ready to go into the labour. The delivery method and total duration of labour are affected by many factors and cervical readiness (ripeness) is just one of these.

What the research says

Moderate quality evidence was available from the two included studies which compared the Bishop score with transvaginal ultrasound (TVUS) (ultrasound done through the vagina). The studies were considered to be at a low risk of bias. The need for misoprostol (a drug) for softening the cervix (cervical ripening) was more common in the TVUS arm. No clear difference was seen between the two methods in terms of vaginal birth, caesarean delivery, admission of the newborn into the neonatal intensive care unit, meconium staining of the amniotic fluid, abnormal heart beat of the baby within the womb whilst the mother was in labour and Apgar score less than seven (difficulty of the baby establishing life and other life movements on its own immediately after childbirth). None of the included studies reported on tears of the womb or death of the baby just before, during or immediately after childbirth. We did not find any studies that compared Bishop score with any other methods such as the presence of vaginal fetal fibronectin or insulin-like growth factor binding protein-1.

Authors conclusions

Although the overall quality of evidence is moderate, there is no difference in outcomes between the two methods (Bishop score and TVUS) apart from the increased need of misoprostol in the TVUS group. Both methods could be useful to each other, or complementary as the Bishop score does not need any special equipment and uses digital examination which is required to induce labour (to insert a cervical ripening agent, rupture the membranes or separate them from the cervix) but TVUS can give additional information that may affect the course and management of the labour. The choice of a particular method may differ depending on the environment and need since TVUS requires training and may not be readily available and affordable in resource-poor countries.

Future research
The two included studies involved a small number of women and further studies are needed. Such studies should include outcomes such as rupture of the womb, perinatal mortality, most appropriate cut-off value for the cervical length and Bishop score to classify women as having ripe or unripe cervices, and cost.

Authors' conclusions: 

Moderate quality evidence from two small RCTs involving 234 women that compared two different methods for assessing pre-induction cervical ripening (Bishop score and TVUS) did not demonstrate superiority of one method over the other in terms of the main outcomes assessed in this review. We did not identify any data relating to perinatal mortality. Whilst use of TVUS was associated with an increased need for misoprostol for cervical ripening, both methods could be complementary.

The choice of a particular method of assessing pre-induction cervical ripening may differ depending on the environment and need where one is practicing since some methods (i.e. TVUS) may not be readily available and affordable in resource-poor settings where the sequelae of labour and its management is prevalent.

The evidence in this review is based on two studies that enrolled a small number of women and there is insufficient evidence to support the use of TVUS over the standard digital vaginal assessment in pre-induction cervical ripening. Further adequately powered RCTs involving TVUS and the Bishop score and including other methods of pre-induction cervical ripening assessment are warranted. Such studies need to address uterine rupture, perinatal mortality, optimal cut-off value of the cervical length and Bishop score to classify women as having favourable or unfavourable cervices and cost should be included as an outcome.

Read the full abstract...
Background: 

Induction of labour is the artificial initiation of labour in a pregnant woman after the age of fetal viability but without any objective evidence of active phase labour and with intact fetal membranes. The need for induction of labour may arise due to a problem in the mother, her fetus or both, and the procedure may be carried out at or before term. Obstetricians have long known that for this to be successful, it is important that the uterine cervix (the neck of the womb) has favourable characteristics in terms of readiness to go into the labour state.

Objectives: 

To compare Bishop score with any other method for assessing pre-induction cervical ripening in women admitted for induction of labour.

Search strategy: 

We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (31 March 2015) and reference lists of retrieved studies to identify randomised controlled trials (RCTs).

Selection criteria: 

All RCTs comparing Bishop score with any other methods of pre-induction cervical assessment in women admitted for induction of labour. Cluster-RCTs were eligible for inclusion but none were identified. Quasi-RCTs and studies using a cross-over design were not eligible for inclusion. Studies published in abstract form were eligible for inclusion if they provided sufficient information.

Comparisons could include the following.
1. Bishop score versus transvaginal ultrasound (TVUS).
2. Bishop score versus Insulin-like growth factor binding protein-1 (IGFBP-1).
3. Bishop score versus vaginal fetal fibronectin (fFN).
However, we only identified data for a comparison of Bishop score versus TVUS.

Data collection and analysis: 

Two review authors independently assessed the trials for inclusion, extracted the data and assessed trial quality. Data were checked for accuracy.

Main results: 

We included two trials that recruited a total of 234 women. The overall risk of bias was low for the two studies. Both studies compared Bishop score withTVUS.

The two included studies did not show any clear difference between the Bishop score and TVUS groups for the following main outcomes: vaginal birth (RR 1.07, 95% CI 0.92 to 1.25, moderate quality evidence), caesarean delivery (RR 0.81, 95% CI 0.49 to 1.34, moderate quality evidence), neonatal admission into neonatal intensive care unit (RR 1.67, 95% CI 0.41 to 6.71, moderate quality evidence). Both studies only provided median data in relation to induction-delivery interval and reported no clear difference between the Bishop and TVUS groups. Perinatal mortality was not reported in the included studies.

For the review's secondary outcomes, the need for misoprostol for cervical ripening was more frequent in the TVUS group compared to the Bishop score group (RR 0.52, 95% CI 0.41 to 0.66, two studies, 234 women, moderate quality evidence). In contrast, there were no clear differences between the Bishop scope and TVUS groups in terms of meconium staining of the amniotic fluid, fetal heart rate abnormality in labour, and Apgar score less than seven. Only one trial reported median data on the induction-delivery interval and induction to active phase interval, the trialist reported no difference between the Bishop group and the TVUS group for this outcome. Neither of the included studies reported on uterine rupture.

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