Research title: Transvaginal
ultrasound measurement of cervical volume prior to induction of labour and its
relationship with outcome of induction of labour
Background and justification
Induction of labour (IOL) is defined as planned
initiation of labour prior to spontaneous onset.1When compared to expectant
management it has shown to reduce peri-natal mortality and morbidity in potentially
compromised foetuses and in certain maternal diseases. In current obstetric
practice approximately 15-20% of women undergo induction of labour due to
various indications.2 Approximately 70% of induced labours
deliver by vaginal delivery without further intervention,
while about 15% need assisted vaginal delivery and 22% need emergency caesarean
Sri Lanka has the highest prevalence of IOL in the region (35%) according to
In addition to failed induction, IOL may also be
associated with adverse outcomes such as uterine hypertonus, uterine rupture,
cord prolapse and fetal distress. Inappropriate patient selection stands as a
leading cause of adverse outcomes and failed induction. Assessment of suitable
candidates based only on digital cervical assessment is one contributing factor
for this. Though the physiological trigger for labour remains uncertain in
human, cervical changes in association with regular uterine contractions is the
‘sine qua non’ (element factor) of labour.4
Digital cervical assessment remains the best
available method of assessing uterine cervix to date. Bishop et al.5 introduced a pelvic
scoring system for elective induction that can be used as a tool to predict the
outcome. However Bishop introduced this scoring system to predict the post
induction outcome in multigravidae, which was later modified with addition of
cervical length instead of effacement. Digital cervical length assessment is a
subjective assessment, which is observer dependent, and shows wide variations
hence difficult to standardize.6
Several recent studies have proposed that
transvaginal sonographic assessment of cervix uteri is more sensitive in
prediction of obstetric outcome in induction of labour7-9.
Almost all these studies assessed the sonographic cervical length, and none of
them measured the cervical volume. Cervical volume calculation includes both
the length and the diameter of the cervix which covers two aspects of cervical
scoring systems. The objective of this study is to comparatively assess the
predictive value of cervical volume in IOL, and to obtain an index cervical
volume, to predict on induction of labour outcome.
1.2 Literature review
in any obstetric intervention it is the paramount importance to ensure that the
indication for induction of labour still exists. Assessment of the states of
uterine cervix remains the most reliable indicator of success at induction of
labour. There are several previous studies which have shown association between
sonographic cervical length and induction delivery interval. Most of these
studies compared Bishop score system with sonographic assessment and some of
the results are conflicting.
et al.10 examined 86 women
prior to induction and found significant associations between both the Bishop
score and sonographically measured cervical length with successful induction of
labour and the induction to delivery interval.
et al.11 examined 50 study
participants prior to induction and reported that, Bishop score associate
significantly with successful vaginal delivery but the score does not
satisfactorily predict the outcome of induction of labour. In addition they
reported cervical volume measured sonographically, was not significantly associated
with either the Bishop score or induction to delivery interval. However the
sample size of this study was small since it was done as a pilot study.
et al.12 examined 53 women
prior to induction and found that, although sonographically measured cervical
length correlated with the duration of the latent phase of labour , there was
no significant association with the induction to delivery interval. Since the
study sample was small the predictive value of the cervical length measurement
in success of IOL could not be elicited in this study.
et al.8 carried out
pre-induction sonographic measurement of cervical length in the prediction of
successful induction of labour, in a relatively larger sample (n=240) and
concluded that both sonographic cervical length assessment and the Bishop score
successfully predicted vaginal delivery within 24h from start of induction.
They reported that the cervical length appears to be a better predictor than
the Bishop score, with a sensitivity of 0.87 and a specificity of 0.71 compared
to 0.58 and 0.77 for digital assessment. Although the study compared the
score with ultrasonic cervical length assessment, the dilatation of the
endocervical canal and the width of the cervix were not measured. Since Bishop
score incorporates the degree of effacement of the cervix, above two parameters
remain important to compare the two systems.
is hypothesized that these shortcomings can be prevented by the assessment of
cervical volume, instead of cervical length alone, since the former measures
both the cervical dilatation and the width.
02. Objectives of the study
2.1 General Objective:
a cutoff for a favorable cervical volume for induction of labour and to assess
the predictive value of cervical volume measurement in successful induction of
2.2 Specific Objectives:
To describe the mean
volume of cervix at term and its variability with parity
To obtain a cut off
cervical volume as an index to predict success at induction of labour.
To compare cervical
volume with Modified Bishop Score for assessing favorability of cervix in IOL.
To compare the
predictive values of sonographic cervical length and cervical volume for
success at IOL
3.1 Study design
This would be a cross sectional study on a sample of
women undergoing induction of labour after completing 37 weeks of POA.
study will be carried out
Study population would be selected from
women who are admitted for induction of labour to the study unit. Pre induction
cervical assessment will be done by both the modified Bishop score and
transvaginal ultrasound. Suitable women who fulfil following criteria will be
included in the study after informed written consent.
· Singleton pregnancy
· Term pregnancy at 37-42 gestation
· Previous uterine scar
· Pre labour rupture of membranes
· Evidence of fetal distress
· Previous cervical surgeries- LLETZ, cone
biopsy or cervical circlage.
· Any condition that contraindicates the unit
protocol of induction of labour
3.4.1 Ultrasonic cervical assessment
induction ultrasonographic cervical volume assessment will be carried out
within 30 minutes of decision for IOL.
Cervical volume assessment will be done by a single investigator with
expertise and accreditation on ultrasound scanning.
subjects will be made comfortable in dorsal recumbent position. They will be
advised to empty the bladder immediately prior to the assessment and the
transvaginal probe will be placed in the vagina one to two centimetres away
from the cervix with minimal pressure to avoid angulations and distortion of
its cylindrical geometry.
ultrasound will be performed with a sector phased array 7 MHz probe (Sonoscape,China),
offset at 30 degrees to give a field of view of 90 degrees11.
Adequate amount of sterile contact jelly will be applied both inside and
outside of the disposable sterile condom. Sagittal view of the cervix will be
obtained and internal and external os would be identified with the echogenic
endocervical mucosa adjoining them. Cervical length will be measured by
“tracing” the distance between internal and external os, the furthest points at
which cervical walls were juxtaposed.7,13,14.
The ‘trace’ option of the ultrasound machine will be used for this measurement.
Shortest cervical length of three consecutive measurements would be documented
in order to minimize the effect of funnelling13.
Measurement of antero-posterior diameter
of the cervix will be obtained at the midpoint of the cervix, right angled to
the endocervical canal
Cervical volume would be assessed
assuming the cervix is a cylinder in geometric view.
When cervix is dilated, volume will be
calculated by deducting the volume of the dilated tunnel from the total volume.
Digital cervical assessment
Pre induction Modified Bishop Score will
be assessed by an experienced member of the obstetric team who is unaware of
the sonographic findings.
Induction of labour
Induction of labour will be performed
according to the unit policy based on WHO recommendations for IOL issued in
20113. Those with an unfavourable cervix