What We Have Learned About Cervical
Ultrasound
John Owen* and Jay D. Iams† for the National Institute of Child Health
and Human Development Maternal-Fetal Medicine Units Network
The National Institute of Child Health and Human Development Maternal-Fetal Medicine Units
Network has completed 2 prospective, blinded observational studies of cervical ultrasound for the
prediction of preterm birth and the identification of high-risk women who might benefit from
interventions aimed at preventing spontaneous preterm birth. These reports and other clinical data
support the concept that cervical competence is not a dichotomous variable, but more likely functions
along a continuum that is reflected by the relationship between cervical length and reproductive
outcomes. Because of its safety, availability, patient acceptance and reproducibility, the sonographic
evaluation of the uterine cervix has become an important investigational tool that has advanced our
understanding of the preterm birth syndrome and should allow a more focused approach to the most
persistent and challenging problem in modern obstetrics.
© 2003 Elsevier Inc. All rights reserved.
he cervix has a dominant role in maintaining the intrauterine gestation until the
spontaneous onset of term labor. For most of the
pregnancy the cervical sphincter remains firmly
closed, providing both mechanical support for
the products of conception and protection
against ascending pathogens. However, pathologic cervical ripening may occur at any time in
a preterm gestation: the underlying causes are
generally unknown.
In the United States in 1998, the incidence of
preterm birth was 11.8%.1 The prevention of
preterm birth, and its associated short- and longterm neonatal morbidities, is the most challenging problem in modern obstetrics. Prematurity is
usually the result of spontaneous preterm birth:
only a minority of patients are delivered for
maternal or fetal indications.2 Chief compo-
T
From the Departments of Obstetrics and Gynecology, Maternal-Fetal
Medicine Divisions, *the University of Alabama at Birmingham,
AL; and †Ohio State University, Columbus, OH.
Supported by grants from the National Institute of Child Health and
Human Development (U10-HD 21410, U10-HD 27917, U10-HD
27915, U10-HD -27869, U10-HD 27905, U10-HD 27861,
U10-HD 27860, U10-HD 27889, U10-HD 27883, U10-HD
21414, U10-HD 19897, U10-HD 36801, U10-HD 34116,
U10-HD 34201, U10 HD 34208, U10-HD 34136).
Address reprint requests to John Owen, MD, Department of Obstetrics and Gynecology, 619 19th St South, OHB 458, Birmingham,
AL 35249-7333; e-mail: johnowen@uab.edu
© 2003 Elsevier Inc. All rights reserved.
0146-0005/03/2703-0000$30.00/0
doi:10.1016/S0146-0005(03)00021-1
194
nents of contemporary models of the preterm
birth syndrome include decidual activation and
premature membrane rupture, preterm labor,
and diminished cervical competence (or alternatively, premature cervical ripening).3 The concept that the cervix is either competent or not
has been challenged by both clinical data4-7 and
interpretive reviews.8-10 It has become increasingly apparent that cervical competence probably functions along a biologic continuum that is
reflected in reproductive outcomes. Thus, research into the etiology and prevention of spontaneous preterm birth has focused on the cervix,
and its underlying role in pathways to prematurity.11
Traditionally, clinicians have evaluated the
cervix by visual examination and digital palpation, the latter often summarized into a Bishop
score.12 Although women with early cervical
changes, as detected by palpation, are more
likely to experience a preterm birth,13 low predictive value and considerable inter-observer
variability have limited its clinical utility. Moreover, the examining hand can neither assess the
entire cervical length nor detect early changes in
the lower uterine segment and internal cervical
(os). Recent investigations have shown that cervical effacement begins at the internal os.14 Funneling at the internal os may be effacement in
progress. Recently, technical advances in vaginal
ultrasonography and wide patient acceptance of
the technique have circumvented many of the
Seminars in Perinatology, Vol 27, No 3 (June), 2003: pp 194-203
Cervical Ultrasound Can Predict Preterm Birth
limitations of both digital examinations and earlier transabdominal methods of cervical sonography.15
Evaluation of the Cervix With Vaginal
Sonography
For the examination, the patient should have an
empty bladder and be placed in dorsal lithotomy
position. A high-resolution (ⱖ5 MHz) endovaginal probe, protected by a lubricated sterile transducer cover, is gently inserted along the vaginal
canal. Essentially all endovaginal probes currently in use are based on a convex, switched
array of active elements, which create a fan- or
sector-shaped image. The sector should encompass at least a 120° field of view to create an
image that facilitates simultaneous visualization
of the required landmarks without excessive
probe manipulation. The image “trapezoid”
should stay in a vertical orientation to maintain
a sagittal view of the cervix and lower segment.
Three landmarks are crucial for the measurement of cervical length: 1) External os, 2) Endocervical canal, and 3) Internal os. By sweeping
the probe slightly from left to right, one or more
of these landmarks can be visualized. Although
there is significant biologic variation among patients (and also over time of gestation), the internal os can usually be recognized as a small
notch or triangle at the interface between the
amniotic cavity and the endocervical canal. With
experience, various patterns of internal os anatomy can be appreciated. In some cases, a normal
appearing internal os can never be visualized,
because either funneling, a poorly developed
lower uterine segment, or placenta previa
should be suspected. These situations will be
covered in more detail below.
Once the internal os has been recognized, its
image should be maintained while the transducer is manipulated slightly in order to visualize the entire endocervical canal, ending at the
external os. The external os is generally more
difficult to visualize than the internal os because
the vaginal mucosa is well applied to the ectocervix and has a similar echogenicity. The external os is often characterized by an echogenic
area several millimeters inside of cervico-vaginal
interface. With high quality sonographic equipment, more details of the external os can be
recognized, and it commonly has the appear-
195
ance of the small triangle or notch. The intervening cervical canal may have a varied appearance, depending on the echogenicity of the
glands, mucous, and the canal itself. The canal is
commonly more echolucent than the surrounding cervical and glandular tissue, but it may also
appear as a faint echo-dense line.
Once these 3 landmarks are simultaneously
visualized, the probe must be manipulated one
last time before acquiring the cervical length.
The probe should be withdrawn slowly (while
maintaining a view of these landmarks) until the
image blurs. At this point, the sonologist should
increase the insertion pressure slightly, just until
a satisfactory image returns. This technique minimizes pressure on the cervix and prevents mechanical distortion of the surrounding anatomy.
The image should be frozen and electronic calipers should be used to measure the distance
from the internal os to the external os. At times,
the canal will have a curved or bent contour, and
a line connecting the 2 os will not follow the
endocervical canal. This biologic artifact can be
avoided by measuring the endocervical canal in
2 linear segments, which connect at the point of
maximum deflection along the canal. If the
maximal deflection is ⱖ5 mm, we use the sum of
the 2 linear segments to define the cervical
length. It is advisable to repeat the measurement
several times to find the shortest cervical length
associated with the best quality image. Experience has shown that the first measurement is
typically several millimeters longer than subsequent assessments.
If a normal appearing internal os cannot be
visualized, the cervix should be assessed further
to determine whether funneling is present. The
appearance of funnels has been well characterized in the literature.16 We generally diagnose a
funnel if the “notch,” which often defines the
internal os, is greater than 5 mm from the end of
the endocervical canal to its “shoulder” (seen
more cephalad; Fig 1). Funnels may be subcategorized as primarily V-shaped or U-shaped, depending on their dominant appearance. When
funneling is seen, the presumption is that the
endocervical canal at the internal os has begun
to dilate, but one can only estimate its (former)
location. In this setting, we measure cervical
length from the external os to the functional
internal os, where the apex of the funnel meets
the closed endocervical canal. In rare cases, a
196
Owen and Iams
Figure 1. Endovaginal sonograph (sagittal view) of a
cervix with funneling at the internal os at 18 weeks of
gestation. Cervical length is measured from the internal os to functional internal os (approximately 23
mm). Note asymmetric anterior and posterior “shoulders” cephalad to the internal os and lack of a welldefined “shoulder” landmark for caliper placement.
Tick marks visible along the top of the image represent a 1-cm scale.
funnel can replace essentially the entire endocervical canal (Fig 2) and the cervical length is
zero. (This phenomenon can also be thought of
as complete canal dilation.) Funnel measurements are inexact since the landmarks are not
always distinct and may change during the examination. The point along the “shoulder”
Figure 2. Endovaginal sonograph (sagittal view) of a
cervix with complete canal dilation of approximately
8 mm at 20 weeks’ gestation in a twin pregnancy.
Because the entire canal is dilated, the cervical length
is defined as zero. Note the prolapsing chorioamnion
at the external os.
where the caliper should be placed is operator
dependent, and at times, asymmetric “shoulders” are recognized (Fig 1). This diminishes the
reproducibility of the measurement.
Another confounder of cervical length measurement is a poorly developed lower uterine
segment (Fig 3). This phenomenon is probably
due to either a delay in the normal maturing
process of the lower segment, or, in some cases,
the result of spontaneous muscular contractions.
It can be visualized in approximately 15% of
midtrimester gestations. Although it is a subjective diagnosis, we have published diagnostic criteria for this phenomenon (Table 1).17 In these
cases, the internal os cannot be visualized, and
thus a cervical length cannot be accurately measured. At times, a poorly developed lower uterine segment may spontaneously resolve during
the scan, after which the cervical length can
then be measured.
When performing vaginal sonography, we often use a provocative measure to assess for any
dynamic changes. Moderate fundal pressure (insufficient to cause any patient discomfort) is
applied while maintaining the image of the internal os to determine if a funnel develops or
the cervical length shortens. If so, the cervical
length should be remeasured. Cervical length
shortening may also occur spontaneously, and
Figure 3. Endovaginal sonograph (sagittal view) of a
poorly developed lower uterine segment at 18 weeks
of gestation. Note appreciable cervical length (⬎50
mm), areas of differing echogenicity, considerable
distance between the bladder reflection and the amniotic cavity, and a presumptive internal os located
notably cephalad to the inferior edge of the bladder
reflection.
Cervical Ultrasound Can Predict Preterm Birth
Table 1. Criteria for a Poorly Developed Lower
Uterine Segment
Unusually long-appearing cervix (eg, length greater
than 50-55 mm)
S-shaped endocervical canal
Two distinctly different areas of echogenicity within
the “cervix“
Apparent internal os located appreciably cephalad
to the inferior edge of the bladder reflection
Notably increased distance between the bladder
reflection and the amniotic fluid
Asymmetric anterior and posterior lower uterine
segment widths, cephalad to apparent internal os
this phenomenon and the associated shortened
cervical length measurement should also be recorded. We have determined that utilizing the
cervical length associated with serial mid-trimester evaluations, and including either fundal
pressure-induced or spontaneously occurring
dynamic shortening, increases the predictive
value of cervical length for spontaneous preterm
birth.17 To observe these dynamic changes, we
recommend that cervical scans last a minimum
of 5 minutes.
Sonographer Certification and Proficiency
During 2 National Institute of Child Health and
Human Development (NICHD) Network observational studies of cervical ultrasound,5,17 the
sonographers and sonologists at the participating centers were given a video tape and study
guide that showed the visualization of critical
landmarks and the measurement of cervical
length (and other ancillary characteristics). Subjective findings were also shown and explained.
To become certified for these studies, and prior
to patient enrollment, each sonographer submitted a video tape of 10 vaginal scans and their
accompanying data sheets, which demonstrated
proper technique, visualization, measurements,
and subjective assessments. All of these tapes
were reviewed by the respective primary investigator, who was blinded to obstetric outcomes.
Based on the performance, additional taped examinations might be requested to confirm adherence to the sonographic protocol. Once enrollment began, the data coordinating centers
randomly chose videotaped patient examinations from the participating centers to ensure
protocol compliance and provide continuing
197
feedback to the sonographers. These were periodically reviewed by the primary investigator,
who was unaware of the obstetric outcomes and
also by a member of the data coordinating center.
Additionally, we limited the number of certified sonographers. Because the volume of enrolled patients per center was not large, no more
than 2 sonographers could be certified at a single site; if one of the certified sonographers left
during the study, his/her replacement was certified in a similar fashion. Based on our collective experience with this process of sonographer
certification and continuing quality assurance,
we believe that formal supervised education and
feedback is crucial to maintaining high standards of quality and reproducibility. These principles have been confirmed in other studies as
well.18 Thus, centers that plan to use cervical
ultrasound should develop their own protocols,
because many sonographers in current practice
did not receive formal educational experience
in vaginal ultrasound and cervical assessment.
Although some sonographers may have learned
to evaluate the cervix by the transabdominal
method, this technique has significant technical
limitations, and the results are not reproducible.
During these quality assurance reviews, the
most common pitfalls included the use of excessive probe pressure (which can be easily assessed
by confirming approximately equal cervical
widths on the anterior and posterior sides of the
endocervical canal), inconsistent funnel measurements (understandable, given the biologic
variation), and failure to recognize dynamic cervical shortening and a poorly developed lower
uterine segment. However, from a practical
standpoint, if a poorly developed lower uterine
segment is not recognized and the cervical
length is “measured,” a long value is consistently
observed. This is unlikely to adversely influence
patient management. Conversely, excess pressure or failure to recognize dynamic shortening
could yield a falsely reassuring cervical length
measurement.
Because these were blinded studies, the
sonographers could only notify the managing
physicians in cases of fetal death or placenta
previa. Because the placenta often appears to be
low-lying in the midtrimester, it might appear to
encroach on the internal os, and thus we defined previa as placental implantation at least 1
198
Owen and Iams
cm on both sides of the internal os. Most cases of
suspected placenta previa in the midtrimester
resolve with advancing gestation.19 In selected
cases, follow-up evaluations are necessary to rule
out a previa, if the sonographic suspicion is high.
Cervical Ultrasound and the Prediction of
Spontaneous Preterm Birth
If cervical competence functions along a biologic continuum of reproductive performance,
then assessment of cervical characteristics
should provide useful information about at least
one component of the preterm birth syndrome,
and possibly clues about interactions with the
other commonly recognized components in the
model. Of the features of the cervix and lower
uterine segment anatomy that can be visualized
with vaginal sonography, the assessment of cervical length has been the most widely investigated. The sonographic measurement of cervical length has been well standardized20 and is
reproducible.18 Moreover, it represents an objective linear measurement that lends itself well
to assessment of its utility as a screening test and
predictor of various pregnancy outcomes.
In a novel retrospective study of 461 women,
past obstetric performance was correlated with cervical length assessment in the next pregnancy.4
The prior reproductive outcomes comprised a
wide spectrum from clinically defined cervical incompetence to normal term births. The underlying hypothesis was that if cervical competence
functioned as a dichotomous, “all or none,” biologic parameter, and if cervical length were a reasonable surrogate for competence, then women
with cervical incompetence should have significantly shortened cervical length. Women, who
were delivered prematurely for other causes
should have uniformly longer cervical lengths, similar to those women who had experienced normal
pregnancy outcomes. Instead, the investigators observed a continuum of cervical lengths (measured
at several points in midgestation) directly proportional to the gestational age of the prior spontaneous preterm birth. This report provided fundamental insight into the relationship between
cervical length and cervical competence as a continuous, rather than a dichotomous function of
reproductive performance and confirmed the utility of cervical ultrasound as an important investigational tool.
Cervical ultrasound images have also been
employed to predict obstetric performance, by
assessing cervical length and other cervical and
lower uterine segment characteristics prospectively at various points in gestation. If these characteristics could identify women at significantly
increased risk for preterm birth (presumably
from a clinically significant component of diminished cervical competence), then focused
interventions could be evaluated in clinical trials.
In 1996, the NICHD Maternal-Fetal Medicine
Units Network reported the results of a prospective, blinded study of nearly 3,000 unselected
women with singleton gestations who were receiving their prenatal care at the 10 participating university centers.5 Vaginal ultrasonographic
evaluations to assess cervical length and funneling at the internal os and concurrent digital
examinations were scheduled at 24 and 28
weeks’ gestation. The investigators observed a
strong inverse relationship between cervical
length and the likelihood of spontaneous preterm birth ⬍35 weeks’ gestation (Fig 4). Using a
cutoff at the 10th population percentile of cervical length (⬍26 mm) at the 24-week scan, there
was an associated positive predictive value of
only 18%, due, in large part, to the low frequency of preterm birth in the study population,
(spontaneous preterm birth ⬍35 weeks’ ⫽
Figure 4. Estimated probability of spontaneous preterm delivery before 35 weeks of gestation from the
logistic regression analysis (dashed line) and observed
frequency of spontaneous preterm delivery (solid
line) according to cervical length measured by endovaginal ultrasonography at 24 weeks in an unselected
population. (Reprinted with permission.5 Copyright
娀 2003 Massachusetts Medical Society.)
Cervical Ultrasound Can Predict Preterm Birth
4.3%). The findings from the 28-week evaluation were even less predictive. Moreover, visualization of funneling did not improve the screening compared to the sole assessment of cervical
length. Cervical length was more reliably evaluated than funneling, which was not consistently
observed across the 10 centers. Although the
information from the digital examinations
(Bishop score) had some predictive value for
eventual spontaneous preterm birth, its performance was inferior to cervical length.
These findings were significant because they
supported the concept that cervical competence, and its surrogate, cervical length, function along a biologic continuum. They also confirmed the previously recognized comparative
disutility of digital examinations and cast doubt
on whether the recognition of funneling added
appreciable information to the measurement of
cervical length. This report also showed that, in
a low-risk population, cervical ultrasound was
not an efficient screening tool for spontaneous
preterm birth.
As a result of the first investigation,5 we hypothesized that evaluations in high-risk women
obtained earlier in pregnancy would improve
the predictive value of cervical ultrasound, and
that serial mid-trimester evaluation of length
and other ultrasonographic characteristics
would further improve the clinical utility of cervical ultrasound to identify women at high risk
for spontaneous preterm birth. In 2001, the Network reported the results of a second investigation, designed to test these hypotheses and others not previously evaluated. We enrolled only
women with a prior early (⬍32 weeks’) spontaneous preterm birth, because the gestational age
of the prior birth has been consistently and inversely related to the risk of recurrence.21 Since
it was not known when in gestation or how rapidly nonreassuring cervical findings could develop, we began the sonographic evaluations at
16 to 18 weeks’ gestation. Follow-up examinations were scheduled every 2 weeks until 23 to 24
weeks’ gestation. Women who had received a
prophylactic cerclage in the current pregnancy
because of a prior clinical history of cervical
incompetence were excluded.
A total of 590 vaginal sonograms were performed by certified sonographers and sonologists on 183 high-risk women. At each evaluation, we recorded cervical length and internal os
199
funneling; fundal pressure was then applied to
determine if provoked cervical shortening occurred. However, if a poorly developed lower
uterine segment was recognized, the cervical
length was not measured. Any spontaneous dynamic cervical length shortening that occurred
during the examination was noted, including
resolution of a poorly developed lower uterine
segment. Other cervical and lower uterine segment features identified or measured included:
1) posterior cervical width, 2) lower uterine segment thickness, 3) cervical orientation (vertical/
horizontal), 4) minor degrees of canal dilation
(⬍5 mm) with or without, 5) membrane prolapse, and 6) the angle formed by the intersection of the endocervical canal and the posterior
lower uterine segment. The primary study endpoint was spontaneous preterm birth ⬍35
weeks’ gestation, and the observed population
incidence was 26%.
When the shortest ever observed cervical
length, both from the serial evaluations and the
result of dynamic shortening, was evaluated for
the prediction of preterm birth, women whose
cervical lengths were ⬍25 mm had a relative risk
of spontaneous preterm birth ⬍35 weeks’ of 4.5
(95% confidence interval 2.7-7.6). From a logistic regression model, we determined that for
each 5-mm increase in cervical length, the odds
of spontaneous preterm birth fell by 43%. The
⬍25-mm threshold was associated with a sensitivity of 69%, a specificity of 80% and a positive
predictive value of 55%.
We also studied funneling and dynamic
changes as potentially independent risk factors
for spontaneous preterm birth. Clearly, women
with either or both of these findings were at higher
risk for spontaneous preterm birth, but neither
was consistently associated with shorter cervical
length. Reviews of the videotaped examinations
indicated that, although funnel recognition was
reliably recorded, funnel depth measurement was
unacceptably operator dependent, owing to the
lack of reliable landmarks and to significant biologic variation among funnels. After controlling
for cervical length, neither funneling nor dynamic
shortening was a statistically significant predictor.
We concluded from these observations that the
process by which the cervix shortens may not be as
important as the fact that it does shorten.
Because serial sonographic evaluations were
performed (median 3 evaluations per patient),
200
Owen and Iams
we could determine the rate of cervical shortening and its relation to spontaneous preterm
birth. Independent of the baseline cervical
length, higher rates of cervical shortening (mm/
week) were significantly associated with higher
odds of spontaneous preterm birth. Inclusion of
cervical length shortening from dynamic
changes and data from serial evaluations significantly improved the utility of cervical ultrasound as a screening test for spontaneous preterm birth in a receiver operator characteristic
curve analysis (Fig 5). From this curve, a cervical
length cutoff of ⬍25 mm was the best compromise between sensitivity and specificity (1-falsepositive rate), and maintained a clinically significant positive predictive value of 55%.
Cervical and lower uterine segment characteristics other than cervical length, funneling
and dynamic changes were also reported from
this study.21 We chose to study these characteristics both because they were novel and because
they were plausibly related to the syndrome of
spontaneous preterm birth. For example, posterior cervical width was considered a surrogate
for overall cervical size. Whereas smaller cervixes
Figure 5. Receiver operating characteristic curves of
cervical length cutoffs for the prediction of spontaneous preterm birth less than 35 weeks’ gestation. Solid
line depicts the performance at the initial 16- to 18week evaluation before any dynamic shortening.
Dashed line depicts the shortest ever observed cervical length from 16 weeks’ to 23 weeks’ 6 days’ gestation and also considers any recognized dynamic shortening. For comparison, the solid dots on the curves
represent a cervical length cutoff of less than 25 mm.
(Reprinted with permission.17 Copyrighted 2003,
American Medical Association.)
may have been a risk factor for cervical hypoplasia, we observed similar mean dimensions in
women who were delivered prior to, as opposed
to, beyond 35 weeks of gestation. Similarly, we
measured the lower uterine thickness adjacent
to the bladder reflection as a marker of lower
segment thinning. This measurement did not
distinguish women who would deliver before 35
weeks’ gestation. The cervical canal contour
(straight v curved), dominant cervical position
(horizontal/vertical), and prominent vascularity
of the lower uterine segment also did not identify women who delivered before 35 weeks’.
However, chorioamnionic membrane visualization at the internal os, implying mechanical or
inflammatory disruption of the local membranedecidual interface, was a significant risk factor
for spontaneous preterm birth. Also significant
were minimal degrees (1-4 mm) of endocervical
canal dilation. Both chorioamnion visualization
and canal dilation remained significant predictors after controlling for initial cervical length at
the 16- to 18-week scan.
In a secondary analysis of the Network study
of high-risk women, we reasoned that, if cervical
length was a reasonable surrogate for the competency status of the cervix, then spontaneous
preterm birth associated with shortened cervical
length should preferentially occur earlier as opposed to later in gestation.22 In unselected populations, the frequency of spontaneous preterm
birth increases throughout gestation and is highest in the period just prior to term (37 weeks’).23
Instead, we observed an inverse relationship, in
that a shortened cervical length (defined as ⬍25
mm and 25-29 mm) was preferentially associated
with birth in the midtrimester as opposed to
later preterm birth. Only in women with longer
cervical lengths (ⱖ30 mm) did we observe the
typical delivery pattern associated with the natural history of spontaneous preterm birth. We
further hypothesized that, if cervical length were
a biologically credible measure of reproductive
performance, it should also predict the interval
from the measurement of the shortest observed
cervical length to delivery. Here, we confirmed a
very strong positive correlation (P ⬍ .0001, r2 ⫽
.42) between cervical length and the scan-todelivery interval. These analyses suggest that
shortened mid-trimester cervical length implies
a clinically significant component of diminished
Cervical Ultrasound Can Predict Preterm Birth
cervical competence which may be amenable to
therapeutic interventions.
Cervical Ultrasound and the Incompetent
Cervix
Recently, clinicians have sought to diagnose cervical incompetence and recommend cerclage
based on nonreassuring cervical ultrasonographic findings in the midtrimester. As the
above-mentioned studies confirm, we recognize
a compelling theoretical basis for the presumption that shortened midtrimester cervical length
may indicate clinically significant decreased cervical competence. Nevertheless, the diagnosis of
cervical incompetence with ultrasonography has
been extremely problematic. A review of published criteria for the sonographic diagnosis of
cervical incompetence indicates marked disparity among investigators (Table 2).7,24-39 Confounding the interpretation of many of these
reports has been the use of non-blinded or retrospective study designs, differences among
study populations and uncontrolled interven-
201
tions. Moreover, in contemporary models of cervical competence as a continuum, the associated
adverse obstetric outcome could result from premature cervical ripening related to exogenous
processes such as infection, inflammation, hormonal effects, genetic (multifactorial) predisposition or even occult uterine activity. Therefore,
the underlying cause of diminished cervical
competence might not be uniformly related to
inherent mechanical weakness. Thus, the response to mechanical support (ie, cerclage) is
impossible to predict accurately.
As cervical length and the risk of spontaneous
preterm birth are inversely related along a continuum (Fig 4), regardless of the population
studied, selecting an appropriate action point
for intervention has been difficult. When vaginal
sonography is used to measure cervical length as
a screening test in low-risk or unselected populations, the positive predictive values at nominal
cutoffs (eg, ⬍25 mm) are generally too low to be
clinically relevant.40 Even though the negative
predictive values tend to be quite high, the overall clinical utility of this observation is suspect,
Table 2. Published Reports of the Sonographic Diagnosis of Cervical Incompetence
Author, Year
Population
GA
Brook,24 ’81
Vaalamo,25 ’83
Elective cerclage
“At risk”
MT
MT
Varma,26 ’86
“At risk”
10-32 weeks
Michaels,27 ’86
Ayers,7 ’88
“At risk”
Prior MT loss
N/S
N/S
Podobnik,28 ’88
Michaels,29 ’89
“At risk”
DES exposed
MT
N/S
Michaels,30 ’91
Twin gestations
MT
Joffe,31 ’92
History of CI
10-28 weeks
Guzman,32 ’94
“At risk” or history
of classic CI
8-25 weeks’
Fox,33 ’96
Guzman,34 ’97
Guzman,35 ’97
Wong,36 ’97
Guzman,37 ’98
Kurup,38 ’99
MacDonald,39 ’01
“At risk”
“At risk”
“At risk”
“High-risk”
“At risk”
All “urgent” cerclages
“High-risk”
14-28 weeks’
N/S
MT
17-33 weeks
MT
N/S
⬍24 weeks’
Criteria for Cervical Incompetence
Width of internal os ⬎1.9 cm
Detached membranes at internal os bulging
into a dilated cervical canal
Not explicitly stated. Cervical canal width ⬎8
mm implied
Membrane prolapse ⬎6 mm and “short cervix“
CL ⬍40 mm (⫺2 standard deviations from the
mean)
Not explicitly stated
Combination of membrane protrusion,
progressive herniation, cervical dilation and
shortening
Combination of cervical shortening, canal
dilation and membrane protrusion
Progressive shortening and beaking of the
chorioamnion
Dilation of the internal os with the membranes
protruding into canal or funneling & cervical
shortening
⬎1 cm decrease in CL and/or funneling
CL ⬍10 mm or cervix dilated on physical exam
Progressive cervical changes to a CL ⬍26 mm
33% decrease in CL
CL ⬍20 mm with or without fundal pressure
Beaking of amniotic fluid at internal os
CL ⬍10 mm with or without fundal pressure
Abbreviations: GA, gestational age; MT, midtrimester; CL, cervical length; CI, cervical incompetence; N/S, not stated.
202
Owen and Iams
particularly in the absence of effective intervention strategies. However, when used in populations with increased risk for spontaneous preterm birth, the summary predictive values
improve,40 because of the increased population
incidence of preterm birth and the greater likelihood that women at high-risk for recurrent
spontaneous preterm birth have some degree of
diminished cervical competence. We believe
that women with a history of early spontaneous
preterm birth and a shortened midtrimester cervical length represent an ideal population for a
randomized trial of cerclage. Based on our previous findings (Fig 5), a cervical length cutoff of
⬍25 mm would appear to be the optimal action
point for such a trial, because it represents the
best compromise between sensitivity, specificity,
and positive predictive value.17 A randomized
trial of cerclage in a population of women with
appreciable risk of recurrent spontaneous preterm birth has the potential to determine
whether temporary mechanical support of the
cervix can treat a syndrome that has many causes
and interactive pathways. Recently, the NICHD
approved funding for such a trial, which will be
undertaken by a consortium of centers in the
United States.40
Summary
The cervix has an increasingly recognized role
in the etiology of spontaneous preterm birth
and has become an appropriate focus of research initiatives. Across numerous investigations and study populations, the relationship between shortened midtrimester cervical length
and spontaneous preterm birth is consistent and
compelling. This relationship, however, is also
influenced by interactions with other components of the preterm birth syndrome. Thus, the
real clinical utility of these observations will be
appreciated only through further investigation
into the underlying pathophysiologic events,
which culminate in spontaneous preterm birth
and the results of well-designed clinical intervention trials in appropriate populations of women
at risk for spontaneous preterm birth.
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