the bishop score as a determinant of labour induction success: a systematic review and meta-analysis

REPRODUCTIVE MEDICINE

The Bishop Score as a determinant of labour induction success:a systematic review and meta-analysis

Cristina Teixeira • Nuno Lunet • Teresa Rodrigues •

Henrique Barros

Received: 24 October 2011 / Accepted: 16 April 2012 / Published online: 1 May 2012

� Springer-Verlag 2012

Abstract

Purpose To evaluate the association between the Bishop

Score and successful induction.

Study strategy and selection criteria We searched the

PubMed and the lists of references of relevant studies to

identify reports on the association between Bishop Score

and achieving active phase of labour or vaginal delivery.

Data collection and analysis We abstracted crude or

adjusted measures of association from studies. Summary

odds ratio (OR) and summary hazard ratio (HR), and 95 %

confidence interval (95 % CI) were obtained by random

effects meta-analysis. Study heterogeneity was assessed

using the I2 test.

Results Fifty-nine studies met the inclusion criteria.

Analyses with crude ORs showed that women with higher

versus lower Bishop Score were more likely to achieve

vaginal delivery either with no time limit for this to occur, or

within a certain time interval; the summary ORs according

to the Bishop Score cutoff ranged from 1.98 (95 % CI:

1.58–2.48; I2 = 36.6 %) to 5.48 (95 % CI: 1.67–17.96;

I2 = 0.0 %) and from 2.15 (95 % CI: 1.36–3.40;

I2 = 0.0 %) to 4.22 (95 % CI: 2.48–7.17; I2 = 11.0 %),

respectively. Summary estimates per unit increase in the

Bishop Score, based on adjusted ORs, showed a

positive association with achieving vaginal delivery,

either with ;no time limit (ORsummary = 1.33; 95 % CI:

1.13–1.56; I2 = 66.1 %) or within a certain time interval

(ORsummary = 1.52; 95 % CI: 1.37–1.70; I2 = 42.4 %).

Summary HRs per unit increase in Bishop Score showed an

association with induction to vaginal delivery (HRsum-

mary = 1.28; 95 % CI: 1.21–1.36; I2 = 0.0 %), but not with

induction to active phase (HRsummary = 1.21; 95 % CI:

0.88–1.68; I2 = 70.7 %) time interval.

Conclusions Bishop Score seems be a determinant of

achieving vaginal delivery and is associated with induc-

tion-to-vaginal delivery time interval.

Keywords Bishop Score � Labour induction �Pre-induction cervical status � Successful labour induction

Introduction

Induction of labour should be considered when the benefits

of prompt delivery outweigh the risk of awaiting sponta-

neous onset of labour [1]. Overall, one-fifth of all deliveries

are preceded by labour induction methods [2, 3]. Though

clinical indications for induction of labour are well estab-

lished in order to improve foetal and maternal health out-

comes [2], induced labour with no identifiable clinical

indication is also likely to account for the prevalence of

this obstetric practice [4, 5].

In some circumstances, induction of labour may lead to

an ineffective and long labour, requiring operative vaginal

delivery [5] and is associated with an increased risk of

caesarean section [5–8]. This is particularly important

when providers choose to induce labour when medical

indications for that are not clearly identified. Thus, the

C. Teixeira � N. Lunet � T. Rodrigues � H. Barros

Department of Clinical Epidemiology, Predictive Medicine

and Public Health, University of Porto Medical School,

Porto, Portugal

C. Teixeira (&) � N. Lunet � T. Rodrigues � H. Barros

Instituto de Saude Publica da Universidade do Porto (ISPUP),

Rua das Taipas n8 135, 4050-600 Porto, Portugal

e-mail: [email protected]

C. Teixeira

Polytechnic Institute of Braganca, Braganca, Portugal

123

Arch Gynecol Obstet (2012) 286:739–753

DOI 10.1007/s00404-012-2341-3

challenge of labour induction is to recognize indications

that could predict the likelihood of successful induction.

It is known that labour induction is more likely to be

successful in multiparous women [4], among younger and in

those with lower body mass index (BMI) [1]. Additionally,

higher birth weight increases the risk of failed induction [7,

9]. Nonetheless, the prelabour cervical status has been rec-

ognized as the most important predictor of induction success

[1, 5, 10]. Based on cervical status, several scoring systems

have been developed to select which women will have

successful labour induction [1, 11]. The Bishop Score, based

on cervical characteristics (dilatation, position, effacement

and consistency) and on the station of foetal presentation, is

easily determined [12] and it has become the most fre-

quently used cervical scoring system to predict the outcome

of induced labour [1, 5, 11, 13].

However, controversy remains regarding the value of the

Bishop Score as determinant of successful induction.

Although two recent reviews [1, 4] have described the Bishop

Score as a factor positively associated with the success of

induced labour, several authors considered it a poor predictor,

particularly if other maternal and obstetric factors are taken

into account [14–22]. Given the importance of pre-induction

cervical assessment to select women for induction and the

wide use of the Bishop Score to do that, it would be important

for clinical practice to clarify the value of this cervical scoring

system in predicting the likelihood of induction success.

An important factor when we address this issue is the

inconsistency across studies regarding the definition of

labour induction success [4]. Several authors define suc-

cessful induction as achieving vaginal delivery with no time

limit for this to occur [9, 14, 23, 24]. Other authors [18, 25–

28] include the factor time in the outcome, considering

induction success as the vaginal delivery achieved within a

specific time interval. Since the purpose of labour induction

is to cause a pregnant woman to go into labour, a definition

of successful induction would be achieving the active phase

of labour [4], and this definition is taken into consideration

by some authors [29–31]. These differences in the definition

should be taken into account when evaluating the influence

of cervical status on labour induction success.

This study assesses the association between Bishop Score

and the labour induction success, using a systematic review

and meta-analysis and taking into account differences across

studies regarding the definition of successful induction.

Methods

Literature search and selection of studies

Published studies assessing the relation between the Bishop

Score and the outcome of induced labour were identified

through PubMed (http://www.ncbi.nlm.nih.gov/entrez/).

Searches covered the period from January 1966 to July

2010 under the following expression:

[(‘‘Bishop score’’ AND induction) AND (‘‘relative

risk’’ OR ‘‘odds ratio’’ OR ‘‘hazard ratio’’ OR prog-

nosis OR sensitivity OR specificity OR ‘‘predictive

value’’ OR ‘‘likelihood ratio’’ OR ‘‘false positive’’

OR ‘‘false negative’’ OR ‘‘accuracy’’)]

Articles in English, French, Spanish, or Portuguese

published as a full paper were considered in the review.

All references obtained by electronic search had the titles

and abstracts scrutinized and the full text of potentially eli-

gible citations was obtained for further assessment. Abstracts

reporting results about outcomes of induced labour according

to the Bishop Score were considered potentially eligible

citations. Studies were eligible if they presented results

regarding measures of association [odds ratios (OR) relative

risks (RR) or hazard ratios (HR)] between the Bishop Score

at admission (exposure) and labour induction success (out-

come), or with data that allowed to compute them. Only

studies referring to the Bishop Score based on digital

assessment of the five components (dilatation, station, posi-

tion, effacement and consistency) and assessing outcomes

such as achievement of active phase of labour or achievement

of vaginal delivery were considered in the review. To avoid

inconsistency across studies, when successful induction was

defined as achieving vaginal delivery, caesarean sections

performed based on other indication than failure of induction

were not included in the group of successful induction, and

vaginal deliveries included all no operative and operative

(forceps or vacuum) vaginal deliveries.

Studies were excluded if: they reported Bishop Score

evaluated taking into account only part of its components

or its components separately; they reported outcomes other

than the achievement of active phase of labour or vaginal

delivery; those included spontaneous and induced labour in

the same analyses; those included twin pregnancies or

foetal deaths for the majority of participants, because of the

expectable differences in caesarean section rates in com-

parison with singleton live pregnancies.

Additionally, the reference lists provided by the papers

selected by electronic search were screened, to identify

further relevant studies using the same inclusion criteria.

The full paper of each study included in two systematic

reviews about this issue was scrutinized to identify reports

on the association between the Bishop Score and our out-

comes of interest.

Data extraction

Each study was characterized according to: year of publi-

cation; country of origin; sample size; proportion of

740 Arch Gynecol Obstet (2012) 286:739–753

123

http://www.ncbi.nlm.nih.gov/entrez/

nulliparous women; maternal age; gestational age; pro-

portion of women according to indications for labour

induction (post-date pregnancy, hypertensive disorders,

premature rupture of membranes (PROM), other medical

indication or no medical indication); percentage of women

submitted to cervical ripening and the method used for this;

the outcome analysed and the percentage of participants

achieving that.

From each study, we extracted adjusted and unadjusted

OR and HR. Alternatively, we extracted the data to com-

pute crude OR using the Bishop Score as a dichotomous

variable. A correction factor of 0.5 was added to each cell

of the 2 9 2 table to be able to compute OR for studies

showing zero events in a single cell [32].

When a study provided measures of association with

different degrees of adjustment for confounders, the one

adjusted for the largest number of possible confounding

variables was selected.

Meta-analysis

Different meta-analyses were conducted, considering the

Bishop Score as a dichotomous or as a continuous variable,

to obtain summary OR comparing women with higher

versus lower Bishop Score, summary OR per unit increase

in the Bishop Score and summary HR per unit increase in

the Bishop Score. Since only two studies presented RR (but

also OR), we did not compute the summary RRs.

First, analyses were performed based on crude ORs

using dichotomized Bishop Score. Because few studies

presented adjusted ORs using dichotomized Bishop Score,

these results were considered as crude ORs. Then, we

conducted analyses that allowed obtaining summary OR

estimates based on adjusted ORs and using the Bishop

Score as continuous variable. Because only one study

presented unadjusted OR using Bishop Score as continuous

variable, the results of this study were considered as

adjusted OR. Finally, we performed analyses to compute

summary HRs, based on adjusted HRs and using Bishop

Score as a continuous variable.

Stratified analyses allowed obtaining summary ORs

according to the definition of labour induction success

considered as: the achievement of active phase of labour

with no time limit for this to occur; the achievement of

active phase of labour occurring within a certain time

interval; vaginal delivery achieved within a certain time

interval; vaginal delivery occurring with no time limit.

Studies evaluating more than one of these outcomes were

included repeatedly according to the definition of labour

induction success. When a study presented more than one

option for the time interval to the outcome, we considered

the lowest. Regarding studies presenting HR as a measure

of association, stratified analyses were also conducted to

obtain summary HR for the time interval until achievement

of the active phase of labour and for the time interval until

achievement of vaginal delivery.

Specifically for the studies presenting the Bishop Score

as dichotomous variable, we considered all cutoff points

that were possible to obtain from each study to compute

summary ORs stratified by the Bishop Score cutoffs within

each group according to the definition of labour induction

success. When the same study presented more than one

cutoff point for the Bishop Score, the study was repeatedly

introduced according to the cutoff point.

Summary OR and HR and respective 95 % confidence

interval (95 % CI) were obtained, using random effects

method (DerSimonian–Laird method). Heterogeneity was

quantified using the I2 statistics [33]. All analyses were

conducted with STATA�, version 9.0 (StataCorp LP,

College Station, TX).

Results

As described in detail in the flowchart (Fig. 1), the search

strategy yielded 59 studies meeting the established criteria

[9, 14–31, 34–73]. The main characteristics of the studies,

including the outcomes evaluated, are described in Table 1.

Five studies included only nulliparous women [14, 16, 18,

51, 65], two included only multiparous women [29, 30],

seven presented analyses according to parity [17, 36, 39,

44, 52, 60, 67], a study presented a separate analysis

including only nulliparous women [70] and another study

did the same for multiparous women [73]. Some studies

were restricted to women with preeclampsia [47], PROM

[56] or post-term pregnancy [24, 38, 49, 52, 53], those

performing labour induction for nonmedical reasons [39,

57] and those with initial Bishop Score lower than five [9,

43, 45, 49, 64], six [31, 55, 58, 59], seven [60, 62] or eight

[21, 40], and those with initial Bishop Score higher than

two [38] or higher than five or six according to parity [57].

Regarding the methods to induce labour, cervical rip-

ening was reported in several studies by using misoprostol

(PGE1) [9, 14, 18, 21, 23, 24, 28, 47, 48, 51, 56, 58–60, 62,

70], dinoprostone (PGE2) [9, 14–17, 20, 22, 24–27, 30, 31,

37, 39, 40, 42–47, 49–55, 58–61, 63–67, 70, 72, 73], a

Foley balloon [28, 34, 51, 64] or laminaria [47]. Sixteen of

those studies report that all participants received prosta-

glandins [9, 16, 21, 25, 42, 43, 45, 48, 49, 56, 58–60, 62,

66], although doses could depend on Bishop Score values.

The use of direct oxytocin and/or amniotomy, for all par-

ticipants, with no previous ripening was reported by seven

studies [19, 29, 35, 36, 38, 57, 68] and one study did not

describe the method used to induce labour [71]. For the

remaining studies, there were different proportions of

women receiving prostaglandins, depending on the

Arch Gynecol Obstet (2012) 286:739–753 741

123

presence of unfavourable cervix. However, the Bishop

Score values to define unfavourable cervix were different

across studies, ranging from a value lesser than three to one

lesser than nine.

The outcomes evaluated by the studies as successful

induction were the achievement of vaginal delivery with no

limit of time for this to occur [9, 14–16, 19, 22–24, 36–40,

42–45, 47, 49–52, 54, 57, 58, 60–64, 67–73], within 12 [41,

48, 56] or 24 h [17, 18, 21, 25, 26, 28, 35, 36, 46, 48, 54,

56, 60], or within 12 or 60 h with direct oxytocin and with

topical prostaglandins, respectively [27]; the achievement

of active phase of labour with no time limit [29, 31, 44],

within 8 [41] or 48 h [20, 42], within 14 h for multiparous

and 20 h for nulliparous women [15], or within 12 h after

initiating oxytocin and within 21 h after the insertion of a

dinoprostone vaginal insert [30, 65]. Two studies [20, 55]

evaluated the induction to active phase time interval and

four studies evaluated the induction to vaginal delivery

time interval [27, 53, 59, 66].

Two studies [41, 70] reported that the Bishop Score was

determined using a 10-point scale and although several

studies did not report the scale used [26, 31, 42, 47, 49, 50,

55–57, 60, 64, 69, 71], the majority of the studies reported

a 13-point scale for the Bishop Score.

In the studies presenting adjusted measures of associa-

tion (OR and HR), the variables considered as potential

confounders were parity [20, 22, 26, 27, 47, 53, 54], ges-

tational age at delivery [20, 22, 30, 47, 54, 65], maternal

age [24, 26, 54, 65], maternal body mass index [19, 22, 54],

maternal height [22], birth weight [19, 65], cervical length

[19, 20, 27, 30, 53, 55, 59, 65], cervical position [47],

dilatation [55] and use of cervical ripening agents [47].

Fig. 1 Selection of studies

included in the meta-analysis


123

Ta

ble

1P

rin

cip

alch

arac

teri

stic

so

fth

est

ud

ies

incl

ud

edin

the

met

a-an

aly

sis

Stu

dy

(countr

y)

Sam

ple

n

(null

ipar

ous

%)

Mat

ernal

age

(yea

rs)

Ges

tati

onal

age

(wee

ks)

Ther

apeu

tic

indic

atio

ns

for

induct

ion

Ele

ctiv

e

induct

ion

(%)

%W

om

enw

ith

cerv

ical

ripen

ing

(type)

Outc

om

esan

alyse

d(%

succ

ess)

Mea

n±

SD

mea

n[r

ange]

med

ian

(ran

ge)

Mea

n±

SD

mea

n[r

ange]

med

ian

(ran

ge)

Post

-dat

e

(%)

Hyper

tensi

ve

Dis

ord

ers

(%)

PR

OM

(%)

Oth

ers

(%)

Fri

edm

an[2

9]

(US

A)

408

(0.0

)N

AN

A1.5

2.2

6.6

5.6

84.1

0.0

Act

ive

phas

eof

labour

(93.4

)

Orh

ue

[34

]

(Nig

eria

)

931

(24.3

)24.7

[14–42]

[36

51.5

*31.7

*0.0

17.9

*0.0

28.9

(Fole

ybal

lon)

Vag

inal

del

iver

y(9

0.4

)

Bak

os

[35

]

(Sw

eden

)

223

(42.6

)27

[21–43]

NA

38.6

24.2

0.0

35.9

8.5

0.0

Vag

inal

del

iver

yw

ithin

24

h(8

0.3

)

Dhal

l[3

6]

(India

)200

(68.5

)N

A[3

6–43]

25.5

38.5

15.0

21.0

0.0

0.0

Vag

inal

del

iver

yw

ithin

24

h(7

1.5

)

Vag

inal

del

iver

y(8

1.0

)

Pat

erso

n-B

row

n

[37

](U

K)

50

(52.0

)[2

1–42]

[37–42]

60.0

12.0

0.0

20.0

8.0

NA

(PG

E2)

Vag

inal

del

iver

y(8

5.3

)

Sch

reyer

[38

]

(Isr

ael)

61

(29.0

)27.2

±2.4

(FB

M?

)

41.7

±0.3

(FB

M?

)

NA

0.0

NA

NA

NA

0.0

Vag

inal

del

iver

y(9

3.4

)

26.6

±2.6

�41.4

±0.3

�

Mac

er[3

9]

(US

A)

253

(30.4

)29.9

±4.9

39.4

±1.0

0.0

0.0

0.0

0.0

100.0

7.5

(PG

E2)

Vag

inal

del

iver

y(8

5.4

)

Sch

nei

der

[40

]

(Ger

man

y)

467

(51.0

)N

AN

A37.0

11.0

24.0

28.0

0.0

100.0

(PG

E2)

Vag

inal

del

iver

y(8

5.9

)

Bla

nch

[41

](U

K)

103

(46.6

)27

[15–39]

[37

39.8

15.5

0.0

42.1

12.6

NA

(PG

E2)

Act

ive

phas

eof

labour

wit

hin

8h

(NA

)

Vag

inal

del

iver

yw

ithin

12

h(N

A)

Alb

eric

o[4

2]

(Ita

ly)

133

(67.7

)N

A[3

5–43]

37.6

21.8

8.3

NA

NA

100.0

(PG

E2)

Act

ive

phas

e�w

ithin

48

h

(85.7

)

Vag

inal

del

iver

y(8

0.5

)

Pan

el[4

3]

(Fra

nce

)

186

(56.5

)28.2

[15–43]

39.2

[33–43]

17.2

24.7

0.0

58.1

0.0

100.0

(PG

E2)

Vag

inal

del

iver

y(7

8.0

)

Xen

akis

[44]

(US

A)

597

(42.2

)25.3

±6.6

(BS

\4)

38.6

±3.6

(BS

\4)

22.0

32.0

0.0

46.0

0.0

21.6

(PG

E2)

Act

ive

phas

e§(9

6.3

)

Vag

inal

del

iver

y(8

0.2

)

25.1

±6.1

(BS

[6)

40.6

±2.3

(BS

[6)

26.1

±6.8

�39.7

±2.4

�

Car

uso

[45

](I

taly

)290

(79.0

)32.1

±5.5

(par

ous)

39.8

±1.6

(par

ous)

36.9

18.6

6.2

38.3

0.0

100.0

(PG

E2)

Vag

inal

del

iver

y(7

2.8

)

29.6

±4.2

�40.3

±1.6

�

Gonen

[46

]

(Isr

ael)

86

(47.7

)28.4

[19–42]

40.2

[34–42]

55.8

8.1

0.0

36.1

0.0

37.2

(PG

E2)

Vag

inal

del

iver

yw

ithin

24

h(6

2.8

)


123

Ta

ble

1co

nti

nu

ed

Stu

dy

(countr

y)

Sam

ple

n

(null

ipar

ous

%)

Mat

ernal

age

(yea

rs)

Ges

tati

onal

age

(wee

ks)

Ther

apeu

tic

indic

atio

ns

for

induct

ion

No

med

ical

induct

ion

(%)

%W

om

enw

ith

cerv

ical

ripen

ing

(type)

Outc

om

esan

alyse

d(%

succ

ess)

Mea

n±

SD

mea

n[r

ange]

med

ian

(ran

ge)

Mea

n±

SD

mea

n[r

ange]

med

ian

(ran

ge)

Post

-dat

e

(%)

Hyper

tensi

ve

Dis

ord

ers

(%)

PR

OM

(%)

Oth

ers

(%)

Hen

dri

x[1

5]

(US

A)

365

(43.0

)24.5

±6.5

38.1

±2.5

24.9

28.8

15.3

31.0

0.0

NA

(PG

E2)

Act

ive

phas

e§w

ithin

14

or

20

h(8

9.6

)

Vag

inal

del

iver

y(6

9.3

)

Nas

sar

[47

]

(US

A)

145

(NA

)26.4

±7.5

(succ

ess)

31.2

±2.0

(succ

ess)

0.0

100.0

0.0

0.0

0.0

42.8

(PG

E2

or

PG

E2

lam

inar

ia)

Vag

inal

del

iver

y(4

8.3

)

28.8

±8.0

�30.8

±2.3

�

Liu

[48

]

(Tai

wan

)

89

(NA

)30.4

±1.4

(BS

\5)

C37

32.6

7.9

34.8

13.5

11.2

100.0

(PG

E1)

Vag

inal

del

iver

yw

ithin

12

h(6

8.5

)

29.9

±1.4

�V

agin

aldel

iver

yw

ithin

24

h(7

8.7

)

Sco

llo

[49

]

(Ita

ly)

152

(80.3

)[1

6–44]

C42

100.0

0.0

0.0

0.0

0.0

100.0

(PG

E2)

Vag

inal

del

iver

y(8

4.7

)

Chan

dra

[24

]

(Can

ada)

120

(64.0

)27.9

±5.2

41.2

±0.3

100.0

0.0

0.0

0.0

0.0

70.5

(PG

E2

or

PG

E1)

Vag

inal

del

iver

y(8

0.0

)

Gab

riel

[50

]

(Fra

nce

)

179

(48.0

)N

A[

37

53.6

20.7

0.0

25.7

0.0

74.3

(PG

E2)

Vag

inal

del

iver

y(7

0.4

)

Win

g[2

1]

(US

A)

1,3

73

(49.3

)27

±7

(succ

ess)

39.6

±2.1

(succ

ess)

11.4

18.4

0.0

70.2

0.0

100.0

(PG

E1)

Vag

inal

del

iver

yw

ithin

24

h(4

7.9

)

26

±7

�39.3

±2.9

�

Bah

ar[2

5]

(Sau

di

Ara

bi)

241

(NA

)33.5

±6.4

(PG

E2

=1.5

mg)

40.1

±1.5

(PG

E2

=1.5

mg)

51.0

14.5

0.0

34.5

0.0

100.0

(PG

E2)

Vag

inal

del

iver

yw

ithin

24

h(8

4.2

)

28.1

±5.5

(PG

E2

=3

mg)

40.2

±1.6

(PG

E2

=3

mg)

Johnso

n[5

1]

(US

A)

2,2

22

(100.0

)26.8

±5.8

39.8

±1.4

NA

NA

0.0

NA

NA

NA

(PG

E2

or

PG

E1

or

Fole

ybal

lon)

Vag

inal

del

iver

y(7

6.8

)

Ran

e[5

2]

(UK

)

382

(50.3

)29

(16–45)

41.4

(41.4

–42.1

)100.0

0.0

0.0

0.0

0.0

NA

(PG

E2)

Vag

inal

del

iver

y(8

1.4

)

Ran

e[5

3]

(UK

)

382

(50.3

)29

(16–45)

41.4

(41.4

–42.1

)100.0

0.0

0.0

0.0

0.0

NA

(PG

E2)

Induct

ion-v

agin

aldel

iver

yin

terv

al

Tim

eli

mit

=24

h

Ran

e[5

4]

(UK

)

604

(43.5

)30

(16–47)

40.1

(35.0

–42.7

)36.6

14.1

9.4

32.6

7.3

NA

(PG

E2)

Vag

inal

del

iver

yw

ithin

24

h(6

4.2

)

Vag

inal

del

iver

y(8

0.1

)

Yan

g[2

0]

(South

Kore

a)

105

(74.3

)27.3

±3.2

40.5

±1.2

56.2

4.8

0.0

38.0

1.0

67.6

(PG

E2)

Act

ive

phas

e#w

ithin

48

h(8

8.6

)

Induct

ion–ac

tive

phas

ein

terv

al

Rom

an[3

1]

(Fra

nce

)

106

(NA

)31.2

±5.3

39.8

±1.2

57.5

1.9

0.0

40.6

0.0

64.2

(PG

E2)

Act

ive

phas

e§(8

4.9

)

[38–41]

Rom

an[5

5]

(Fra

nce

)

106

(NA

)31.2

±5.3

39.8

±1.2

[38–41]

57.5

1.9

0.0

40.6

0.0

64.2

(PG

E2)

Induct

ion–A

ctiv

ephas

e§in

terv

al

Ara

[56

]

(Pak

ista

n)

104

(26.9

)N

A[

37

0.0

0.0

100.0

0.0

0.0

100.0

(PG

E1)

Vag

inal

del

iver

yw

ithin

12

h(5

6.7

)

Vag

inal

del

iver

yw

ithin

24

h(9

4.2

)


123

Ta

ble

1co

nti

nu

ed

Stu

dy

(countr

y)

Sam

ple

n

(null

ipar

ous

%)

Mat

ernal

age

(yea

rs)

Ges

tati

onal

age

(wee

ks)

Ther

apeu

tic

indic

atio

ns

for

induct

ion

No

med

ical

induct

ion

(%)

%W

om

enw

ith

cerv

ical

ripen

ing

(type)

Outc

om

esan

alyse

d(%

succ

ess)

Mea

n±

SD

mea

n[r

ange]

med

ian

(ran

ge)

Mea

n±

SD

mea

n[r

ange]

med

ian

(ran

ge)

Post

-dat

e

(%)

Hyper

tensi

ve

Dis

ord

ers

(%)

PR

OM

(%)

Oth

ers

(%)

Nie

lsen

[57

]

(US

A)

93

(NA

)N

AC

39

00

00

100.0

0.0

Vag

inal

del

iver

y(9

2.5

)

Buen

o[2

6]

(Spai

n)

196

(75.0

)29.4

±5.5

[17–41]

40.3

±1.4

[37–42]

54.6

1.5

5.6

NA

NA

NA

(PG

E2)

Vag

inal

del

iver

yw

ithin

24

h(5

9.5

)

Roze

nber

g[5

8]

(Fra

nce

)

266

(71.4

)M

edia

n=

29.1

Med

ian

=39.7

28.2

18.4

12.8

40.6

0.0

100.0

(PG

E2

or

PG

E1)

Vag

inal

del

iver

y(8

2.0

)

1st q

uar

tile

=26.5

1st q

uar

tile

=38.4

3rd q

uar

tile

=32.8

3rd q

uar

tile

=41.3

Roze

nber

g[5

9]

(Fra

nce

)

266

(71.4

)M

edia

n=

29.1

Med

ian

=39.7

28.2

18.4

12.8

40.6

0.0

100.0

(PG

E2

or

PG

E1)

Induct

ion–vag

inal

del

iver

yin

terv

al

1st q

uar

tile

=26.5

1st q

uar

tile

=38.4

3rd q

uar

tile

=32.8

3rd q

uar

tile

=41.3

Das

kal

akis

[16

]

(Gre

ece)

137

(100.0

)24.3

[19–37]

40.0

[37–42]

71.5

10.9

0.0

17.6

0.0

100.0

(PG

E2)

Vag

inal

del

iver

y(6

7.2

)

Dodd

[60

]

(Aust

rali

a)

620

(58.9

)27.6

±5.4

(morn

ing)

[36

50.2

21.9

0.0

27.9

0.0

100.0

(PG

E2

or

PG

E1)

Vag

inal

del

iver

y(7

5.6

)

28.4

±5.6

�V

agin

aldel

iver

yw

ithin

24

h(3

1.8

)

Elg

horo

ri[6

1]

(UK

)

104

(51.9

)26.5

[14

–37

]41

[37

–42

]82.7

4.8

0.0

11.5

1.0

80.8

(PG

E2)

Vag

inal

del

iver

y(8

3.7

)

Per

egri

ne

[22

]

(UK

)

267

(58.8

)31.6

±6.0

40.1

±1.4

58.8

7.5

5.7

23.1

4.9

NA

(PG

E2)

Vag

inal

del

iver

y(7

0.0

)

Szc

zesn

y[6

2]

(Norw

ay)

99

(47.4

)28.7

±5.1

(succ

ess)

40.0

±2.1

(succ

ess)

17.2

25.3

26.3

31.2

0.0

100.0

(PG

E1)

Vag

inal

del

iver

y(8

3.8

)

32.0

±4.5

�39.6

±2.0

�

Tan

[63

]

(Mal

aysi

a)

152

(42.8

)N

A[3

7–

42]

42.8

*14.5

*0.0

NA

NA

78.9

(PG

E2)

Vag

inal

del

iver

y(7

7.0

)

Cro

mi

[64

]155

(76.1

)31.5

±5.1

39.7

(34–42)

27.7

24.5

0.0

47.8

0.0

100.0

(PG

E2

and

Fole

yca

thet

er)

Vag

inal

del

iver

y(6

8.4

)

Go

mez

Lae

nci

na

[27

]

(Spai

n)

177

(70.1

)31.2

[18–46]

39.6

[36–42]

28.8

4.0

24.9

41.2

1.1

64.4

(PG

E2)

Vag

inal

del

iver

yw

ithin

12

haf

ter

oxyto

cin

(65.0

)

Vag

inal

del

iver

yw

ithin

60

haf

ter

PG

E2

(64.0

)

Kee

pan

asse

ril

[14

](I

ndia

)

137

(100.0

)25

(20–35)

38

(34–41)

26.1

42.8

1.4

29.7

0.0

56.2

(PG

E2

or

PG

E1)

Vag

inal

del

iver

y(7

6.6

)

Oboro

[28]

(Nig

eria

)

197

(67.0

)28.1

±4.9

[16–43]

39.5

±1.6

[28–43]

44.7

12.7

10.2

32.4

0.0

NA

(PG

E1

or

Fole

y

cath

eter

)

Vag

inal

del

iver

yw

ithin

24

h(6

2.4

)


123

Ta

ble

1co

nti

nu

ed

Stu

dy

(countr

y)

Sam

ple

n

(null

ipar

ous

%)

Mat

ernal

age

(yea

rs)

Ges

tati

onal

age

(wee

ks)

Ther

apeu

tic

indic

atio

ns

for

induct

ion

No

med

ical

induct

ion

(%)

%W

om

enw

ith

cerv

ical

ripen

ing

(type)

Outc

om

esan

alyse

d(%

succ

ess)

Mea

n±

SD

mea

n[r

ange]

med

ian

(ran

ge)

Mea

n±

SD

mea

n[r

ange]

med

ian

(ran

ge)

Post

-dat

e

(%)

Hyper

tensi

ve

Dis

ord

ers

(%)

PR

OM

(%)

Oth

ers

(%)

Par

k[6

5]

(South

Kore

a)161

(100.0

)30.6

±3.6

(succ

ess)

39.8

±1.4

(succ

ess)

26.7

1.2

0.0

68.4

3.7

91.3

(PG

E2)

Act

ive

phas

e#w

ithin

12

haf

ter

oxyto

cin

or

21

haf

ter

PG

E2

(65.8

)

29.9

±3.5

�38.8

±1.5

�

Riz

zo[6

6]

(Ita

ly)

49

(64.0

)32

(20–43)

41

(40.2

–40.1

)N

AN

A0.0

NA

NA

100.0

(PG

E2)

Vag

inal

del

iver

y(6

1.2

)

Tan

[67

](M

alay

sia)

249

(43.4

)30.1

±4.8

(succ

ess)

39.9

±1.3

(succ

ess)

34.9

*15.3

*0.0

NA

NA

79.2

(PG

E2)

Vag

inal

del

iver

y(7

7.9

)

30.8

±4.5

�39.8

±1.5

�

Yan

ik[6

8]

(Turk

ey)

73

(58.9

)28.1

±5.3

(succ

ess)

41.0

±0.7

(succ

ess)

83.6

2.7

0.0

11.0

2.7

0.0

Vag

inal

del

iver

y(3

9.7

)

25.4

±4.7

�41.3

±0.7

�

Eggeb

o[2

3]

(Norw

ay)

275

(47.0

)30

(18–45)

40

(37–43)

25.8

7.6

19.6

35.6

12.0

58.9

(PG

E1)

Vag

inal

del

iver

y(8

7.0

)

Tan

ir[1

8]

(Turk

ey)

43

(100.0

)26.7

±1.3

(succ

ess)

38.2

±2.1

(succ

ess)

27.9

23.3

0.0

48.8

0.0

NA

(PG

E1)

Vag

inal

del

iver

yw

ithin

24

h(5

1.1

)

25.5

±2.7

�38.6

±2.4

�

Jokhi

[69

](U

K)

205

(46.0

)29.0

±6.5

40

[37–42]

33.7

13.2

0.0

NA

NA

50.2

(NA

)V

agin

aldel

iver

y(8

3.9

)

Mei

jer-

Hoogev

een

[70

]

(Net

her

lands)

102

(67.0

)31

(21–41)

41

(37–43)

48.0

19.6

0.0

27.5

4.9

34.3

(PG

E2)

Vag

inal

del

iver

y(8

5.3

)

Pev

zner

[9]

(US

A)

1,2

74

(61.9

)26.3

±5.9

40.0

±1.4

31.6

18.6

4.5

28.8

16.5

100.0

(PG

E2

or

PG

E1)

Vag

inal

del

iver

y(7

1.8

)

Tan

[17

](M

alay

sia)

231

(42.9

)N

A[3

7–42]

35.1

*15.6

*0.0

NA

NA

76.2

(PG

E2)

Vag

inal

del

iver

yw

ithin

24

h(6

8.0

)

Par

k[3

0]

(Kore

a)110

(0.0

)32.5

±3.1

(succ

ess)

39.4

±1.4

(succ

ess)

27.2

5.5

0.0

57.3

10.0

39.1

(PG

E2)

Act

ive

phas

e#w

ithin

12

haf

ter

oxyto

cin

or

21

haf

ter

PG

E2

(86.4

)

33.4

±3.3

�38.9

±1.2

�

Ray

amaj

hi

[71

](N

epal

)156

(62.2

)N

AN

A51.3

8.3

17.3

23.1

0.0

NA

Vag

inal

del

iver

y(6

5.4

)

Ver

hoev

en[7

2]

(Net

her

lands)

240

(57.9

)31.1

±4.7

41

(36–42)

34.6

14.6

14.6

27.4

8.8

45.0

(PG

E2)

Vag

inal

del

iver

y(8

8.8

)

Uyar

[19

](T

urk

ey)

189

(71.4

)24.2

±4.1

40.2

±1.4

62.4

0.0

16.4

21.2

0.0

0.0

Vag

inal

del

iver

y(8

3.1

)

Cheu

ng

[73

](C

hin

a)460

(61.1

)29.4

±5.0

[37–41]

79.3

3.0

0.0

17.7

0.0

28.6

(PG

E2)

Vag

inal

del

iver

y(7

3.9

)

NA

not

avai

lable

,P

RO

Mpre

mat

ure

ruptu

reof

mem

bra

nes

,P

GE

2din

opro

stone,

PG

E1

mis

opro

stol,

FB

M?

pre

sence

of

feta

lbre

athin

gm

ovem

ents

*S

om

epar

tici

pan

tshad

more

than

one

reas

on,

�ce

rvic

aldil

atat

ion

C3

cm,

�oth

erw

ise,

§ce

rvic

aldil

atat

ion

C5

cm,

#ce

rvic

aldil

atat

ion

C4

cm


123

Fig. 2 Forest plot of OR for

achieving active phase with no

time limit (a) and within a

certain time interval (b),

according to the Bishop Score

cutoff. Percentage of women

with cervical ripening is

presented when available


123

Summary ORs based on crude ORs according to the

Bishop Score cutoff are presented in Figs. 2, 3 and 4. In all

circumstances, women with lower Bishop Score were

considered as reference group.

There was a positive association between the Bishop

Score and the achievement of the active phase of labour

with no limit of time for this to occur (Fig. 2a); within

groups defined according to the Bishop Score cutoff, the

Fig. 3 Forest plot of OR for vaginal delivery with no time limit, according to the Bishop Score cutoff. Percentage of women with cervical

ripening is presented when available


123

summary ORs ranged from 4.34 (95 % CI: 1.31–14.43;

I2 = 0.0 %) to 6.79 (95 % CI: 2.79–16.55; I2 = 0.0 %).

Conversely, there was no association when successful

induction was defined as the achievement of the active

phase of labour within a certain time interval (Fig. 2b); we

obtained summary OR only taking as cutoff point for the

Bishop Score a value of 4 (ORsummary = 2.55; 95 % CI:

0.91–7.17; I2 = 74.9 %).

Women with higher versus lower Bishop Score values

were more likely to achieve vaginal delivery with no limit of

time for this to occur (Fig. 3); the lowest and the highest

magnitudes for this association were observed taking as cut-

off point for the Bishop Score values of 4 (ORsummary = 1.98;

95 % CI: 1.58–2.48; I2 = 36.6 %) and 8 (ORsummary = 5.48;

95 % CI: 1.67–17.96; I2 = 0.0 %), respectively. There was

also a positive association between the Bishop Score and the

achievement of vaginal delivery occurring within a certain

time interval (Fig. 4); according to the Bishop Score cutoff,

the summary ORs ranged from 2.15 (95 % CI: 1.36–3.40;

I2 = 0.0 %) to 4.22 (95 % CI: 2.48–7.17; I2 = 68.2 %).

Figure 5 presents the summary ORs (A) and HRs

(B) per unit increase in the Bishop Score, computed from

adjusted ORs and adjusted HRs, respectively. There was a

positive association between the Bishop Score values and

the chance of vaginal delivery, either with no time limit for

this to occur (ORsummary = 1.33; 95 % CI: 1.13–1.56

I2 = 66.1 %) or occurring within a certain time interval

(ORsummary = 1.52; 95 % CI: 1.37–1.70; I2 = 42.4 %,

respectively). Similarly, there was a positive association

between the Bishop Score and the achievement of the

active phase of labour within a certain time interval

(ORsummary = 1.23; 95 % CI: 1.02–1.49; I2 = 0.0 %). The

pooled HRs and respective 95 % CI showed a significant

increase in the chance of vaginal delivery (HRsum-

mary = 1.28; 95 % CI: 1.21–1.36; I2 = 0.0 %) per unit

increase in Bishop Score, but not significant for the chance

of achieving active phase of labour (HRsummary = 1.21;

95 % CI: 0.88–1.68; I2 = 70.7 %).

Comment

This meta-analysis summarizes the available evidence

describing the association between the Bishop Score and

the success of labour induction. The article search com-

plemented by backward citation tracking yielded 59

Fig. 4 Forest plot of OR for

vaginal delivery within a time

interval according to the Bishop

Score cutoff. Percentage of

women with cervical ripening is

presented when available


123

studies. The main strength was the attempt to create con-

sistent groups regarding the definitions of labour induction

success and the Bishop Score cutoff points which were

possible to extract from the studies.

As a result of this meta-analysis, it is important to

highlight the association between the Bishop Score and the

achievement of vaginal delivery. Higher Bishop Score

values increased the likelihood of vaginal delivery, even if

the factor time was included in the outcome, considering as

successful labour induction the achievement of vaginal

delivery within a certain time interval, as recommended by

the Royal College of Obstetrics and Gynecology (RCOG)

[2]. This association remains after adjustment for other

factors associated with the success of induced labour, such

as parity, gestational age and maternal age or maternal

body mass index. Furthermore, we observed in this meta-

analysis a significant association between the Bishop Score

and the induction to vaginal delivery time interval.

It is arguable whether failed labour induction should be

defined as the inability to reach active phase of labour

(cervical dilatation 4–5 cm) after a minimum of 12–18 h

since the start of labour induction [74]. Some studies report a

strong association between the Bishop Score and the length

of latent phase of labour [75–77]. Despite this, our meta-

analysis showed conflicting results when successful labour

induction was defined as the achievement of active phase of

labour within a certain time interval. Though pooled OR

computed from adjusted ORs revealed a positive association

between the Bishop Score and the chance of this outcome, no

such association was observed when we considered the

summary OR based on crude ORs and the summary HR.

However, as few studies included in this meta-analysis used

such definition of successful labour induction [15, 20, 30, 41,

42, 65], it prevented us from reaching definitive conclusions.

We must be aware that any judgment about pooled

results considered in this meta-analysis is weakened

because of the large variability across studies. There was a

wide range regarding the sample size, proportion of nul-

liparous women, proportion of indications for induction

and methods used to induce labour. Across studies using

the same definition of successful induction, we observed an

ample range regarding the proportion of women achieving

that. The vaginal delivery rates varied between 39.7 and

93.4 % if no time limit was imposed and between 31.8 and

94.2 % otherwise. There are many maternal and foetal

factors associated with the likelihood of caesarean section

that can explain this ample range, such as maternal age [1,

9, 51], BMI [6, 9, 22], birth weight [1, 6, 9, 51] and parity

[1, 9, 23, 78]. It is likely that the indication for the

induction itself accounts for the increase in the caesarean

section rate after labour induction. There is evidence that

hypertension [79, 80], post-date pregnancy [79–81] and

diabetes [82] are independent risk factors for caesarean

section. Thus, it is expectable that differences in labour

induction success defined as vaginal delivery would depend

on the proportion of these indications. Furthermore, the

differences in obstetric practice facing prolonged latent

phase [74, 83] and variation according to the medical

experience for diagnosis of failure to progress or non-

reassuring foetal status [84] could influence the caesarean

section rates. These factors are difficult to assess, but they

are probably important factors that explain the variability

of vaginal delivery rates observed in this meta-analysis.

Even within groups of studies using the same definition

of labour induction success and the same Bishop Score

cutoff, heterogeneity remains. It is known that cervical

Fig. 5 Forest plot of adjusted OR (a), and HR (b), based on Bishop

Score as a continuous variable and according to the outcome.

Percentage of women with cervical ripening is presented when

available


123

dilatation [75, 85, 86] and station [75, 86] are the compo-

nents of the Bishop Score which are more discriminative for

successful labour induction. The same Bishop Score value

could have different magnitudes of association with labour

induction success, depending on the components that con-

tribute to that Bishop Score value. Furthermore, Bishop

Score is inherently subjective, not reliable with high intra-

and inter-observer variation [22, 87, 88] and this issue could

contribute to the heterogeneity observed across studies

included in this meta-analysis.

Another noteworthy issue is the selection of the method

for labour induction. Low Bishop Score values reveal an

unfavourable cervix requiring prostaglandins to modify

cervical favourability [10, 13, 89]. Differences have been

reported in the induction to delivery time interval [90–93]

and in caesarean delivery rates [90, 94, 95] according to the

methods for labour induction, and according to the different

routes for administration of induction agents. Some authors

state that to be successful, oxytocin seems be more depen-

dent on the cervical condition than other methods of labour

induction [34, 35]. Since the Bishop Score is used to select

women receiving different methods for labour induction, or

different doses of cervical ripening agents, there is a

potential bias due to lack of standardization regarding the

method of induction. It may not be possible to know the

independent influence of the Bishop Score on successful

labour induction and to draw definitive conclusions from it.

Cervical assessment before labour induction is essential

to select candidates for elective induction. The Bishop

Score by digital cervical examination up to now has been

the most popular method for cervical assessment before

induction of labour [1, 11, 78]. It is simple to use, easy to

determine and does not require any special equipment

[22, 35]. Moreover, it provides information on cervical

consistency and station which are associated with out-

comes of labour induction [86], but are not accurately

determined by transvaginal ultrasound [61]. Transvaginal

ultrasound and biochemical findings, such as foetal fibro-

nectin, allow a more objective cervical assessment [1, 11].

However, neither of these newer methods has been shown

to be superior to the Bishop Score regarding the prediction

of successful labour induction [1], and cervical length

evaluated by transvaginal ultrasound seems to be a poor

test in predicting labour induction success [96].

The Bishop Score still seems to be the most cost-

effective and accurate method for predicting the likelihood

of vaginal delivery after induced labour.

Conflict of interest The authors declare that they have no conflict

of interest to disclose.

Ethical approval No ethics approval was warranted for the work

reported.

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