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Nichapat Pahirah, M.D.*, Wipada Laosooksathit, M.D.*, Kittipong Kongsomboon, M.D., Ph.D.**, Maethaphan
Kitporntheranunt, M.D.*
*Department of Obstetrics and Gynecology, **Department of Preventive and Social Medicine, Faculty of Medicine, Srinakharinwirot University,
Nakhon Nayok, 26120, ailand.
Sonographic Lower Uterine Segment Thickness to
Predict Cesarean Scar Defect in Term Pregnancy
ABSTRACT
Objective: To study the validity of abdominal sonographic lower uterine segment (LUS) thickness in predicting
intraoperative cesarean scar defect (CSD) and thin incision-site uterine wall thickness in term pregnancy.
Methods: is was a cross-sectional study involving 111 full-term pregnant women who were scheduled for repeat
cesarean delivery from April, 2019 to January, 2020. e abdominal sonographic myometrial LUS thickness was
measured prior to surgery. e cesarean scar was assessed using the morphologic classication system as either
grade 1 (a normally formed LUS), grade 2 (a thin LUS, but without visible content), or grade 3 (a thin LUS with
visible content). en, the ophthalmic calipers was used to measure the incision-site uterine wall thickness. e
correlations between the abdominal sonographic measurements and intraoperative ndings were reported. e
sensitivity, specicity, positive predictive value (PPV), and negative predictive value (NPV) were calculated.
Results: ere were two cases (1.8%) of grade 3 CSD. e overall correlation between the abdominal sonographic
and intraoperative incision-site uterine wall thickness showed r=0.559 with p value < 0.001. e sonographic cut-o
value of 1.5 mm could predict CSD and a thin incision-site uterine wall thickness with sensitivity, specicity, PPV,
NPV of 50.0%, 90.8%, 9.1%, 99.0%, and 37.5%, 94.6%, 54.5%, 90.0%, respectively. A receiver operating characteristic
curve was generated to determine the optimum cut-o value at 2.5 mm with a sensitivity of 76.5% and a specicity
of 73.3%. e area under the curve was 0.8 (a 95% condence interval, 0.718-0.885).
Conclusion: Abdominal sonography is a valuable tool for the preoperative predicting of CSD. A myometrial LUS
thickness of more than 1.5 mm is associatedwitha lowerlikelihoodof cesarean scar dehiscence.
Keywords: Sonography; lower uterine segment; cesarean scar; term pregnancy (Siriraj Med J 2021; 73: 330-336)
Corresponding author: Maethaphan Kitporntheranunt
Email: mtp_swu@hotmail.com
Received 1 December 2020 Revised 29 March 2021 Accepted 1 April 2021
ORCID ID: http://orcid.org/0000-0002-5794-9810
http://dx.doi.org/10.33192/Smj.2021.43
INTRODUCTION
Uterine rupture is a devastating complication of
cesarean scar defect (CSD). Several factors inuence
cesarean scar healing, such as the suturing technique, the
suture materials, the anatomical site, and the apposition
of the myometrium.
1,2
e prevalence of a niche in a
cesarean scar at six-weeks postpartum was 64.5% and
continued rising.
3
Previous studies demonstrated CSD
by using various methods, such as vaginal sonography
3
,
3-dimensional (D) abdominal sonography
4
, and pelvic
magnetic resonance imaging.
5
But these methods are
inconvenient, expensive, and require expertise on the
part of the operators. Two-dimensional (2D) abdominal
sonography is a simple, less invasive, more aordable
and readily available method.
ere was a high relationship between the sonographic
LUS thickness and the risk of CSD.
6
Earlier researchers
measured the entire layer of the LUS, including the
Pahirah et al.
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bladder wall, uterine scar brosis, the myometrial layer,
and the chorio-amniotic membranes, without a good
comparator.
7,8
Sen, et al.
9
reported that a cut-o value
of 2.5 mm full LUS thickness is associated with uterine
dehiscence. Recent studies focused only the myometrial
layer of the LUS, with a higher accuracy in detecting
CSD.
10
ere still is no consensus on using antepartum
LUS thickness to evaluate CSD.
11
So, the objective of
this study was to assess the validity of 2D abdominal
sonography in predicting CSD and the thin incision-site
uterine wall thickness in term pregnancy.
MATERIALS AND METHODS
is cross-sectional study was conducted at HRH
Princess Maha Chakri Sirindhorn Medical Center,
Srinakharinwirot University, ailand from April, 2019 to
January, 2020. e study was approved by the Institute’s
Ethical Review Board (SWUEC/F 386/2018) and registered
to ai Clinical Trials Registry number 20190718001.
Participants were the singleton term pregnant women
with at least a prior cesarean section, aged 18 years and
up, and who were scheduled for repeat cesarean delivery
between 38 and 40 weeks of gestation. e exclusion
criteria were women who had labor symptoms, abnormal
placentation, leiomyoma at the LUS, and prior classical
uterine incision. Consent was obtained from all the
participants. eir maternal age, gestational age, body
mass index, parity number, miscarriage number, and
number of previous cesarean sections were recorded.
Preoperative LUS sonography
Two-dimension abdominal sonography was
performed by a well-trained sonographer (NP) within
24 hours before the operation. An Accuvix XG (Samsung
Medison Co Ltd., Seoul, Korea) ultrasound machine was
used in the study. e participants were prepared in a
supine position with a full bladder. e convex probe
(frequency of 1-4 MHz) was placed at the suprapubic area
in the midsagittal plane. A two-layer structure between
the urinary bladder and uterine cavity was identied,
consisting of a hyperechogenic layer (bladder wall)
and a hypoechogenic layer (myometrium).
7
e area
of interest was magnied to occupy up to two-thirds of
the screen. For the myometrial-thickness measurement,
the rst marker was placed at the interface between the
urinary bladder wall and the myometrial layer. e
second marker was placed at the interface between the
myometrial layer and the amniotic membranes (or fetal
scalp) (Fig 1).
10
e same procedures were repeated at
1 cm laterally apart from the rst measurement on both
sides. ree values were calculated into an average value.
e thin sonographic myometrial LUS was dened as
having thickness < 1.5 mm.
12
Intraoperative LUS assessment
In the operative eld, cesarean scar morphology
was classied into three groups by direct visualization
(Fig 2): grade 1= normal-formed LUS; grade 2= thin
LUS without visible content; and grade 3= thin LUS
with visible content or an absence of LUS continuity.
13
e cesarean scar dehiscence was dened as cesarean
scar morphologic grade 3. en, a low-transverse uterine
incision was made until the amniotic membrane was
exposed. e CASTROVIEJO ophthalmic calipers was
applied to the upper uterine ap at the midpoint of the
uterine incision site for measurement, and reapplied at
1 cm apart on both sides (Fig 3). e average value of the
incision-site uterine wall thickness was calculated. A thin
LUS was dened as a uterine wall thickness equal to or
less than 1.0 mm.
10
All obstetricians were trained for the
caliper measurement and blinded from the preoperative
results. If this procedure could not be accomplished,
they were excluded from the study.
Fig 1. e longitudinal abdominal sonogram showing
the myometrial LUS thickness measurement.
Abbreviation: LUS: lower uterine segment
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Pahirah et al.
Fig 2. e Intraoperative cesarean scar morphology:
grade 1 normal LUS (A) and grade 3 thin LUS
with visible content (B)
Abbreviation: LUS: lower uterine segment
2A 2B
Fig 3. e intraoperative incision-site uterine wall thickness measurement
using the CASTROVIEJO ophthalmic calipers.
Statistical analysis
e sample-size calculation was done by using a
prevalence of CSD at 8.5%
6
with an expected sensitivity
of 90%, a condence interval at 95%, and an allowable
number of errors of 15%. e number of participants
needed in this study was 92 pregnant women.
e baseline characteristics were analyzed using
descriptive statistics - namely, mean ± standard deviation
(SD), median with an interquartile range, and percentage,
as appropriate. e calculation for sensitivity, specicity,
positive predictive value (PPV), and negative predictive value
(NPV) were made. e authors used the Chi-square test to
nd the relationship between the two parameters. A p-value
of less than 0.05 was consideredstatisticallysignicant.
RESULTS
120 pregnantwomenwere enrolled in this study. Nine
were excluded from the study (two women had labor pain
and seven couldn’t go through the intraoperative assessment).
Thebaseline characteristicsof the 111participants
arepresentedin Table1.
erewasa statistically signicant correlationbetween
the sonographic LUS thickness and the cesarean scar
morphology ingrade 1, grade2, andall grades (Table 2).
ere were two cases in the grade 3 CSD group that did
not correlate with the incision-site uterine wall thickness.
A cut-off level at 1.5mm for the abdominal
sonographicmyometrialLUS could predict uterine
dehiscence(grade 3 morphology) and a thin incision-site
uterine wall thickness with validity, as shown in Table 3.
Based on our data, areceiveroperatingcharacteristicgraph
wasgenerated (Fig 4). e authors suggested that using
a cut-o point at 2.5mm sonographic LUS thickness
could predict an intraoperative thin-incision site with
a sensitivity of 76.5% and a specicity of 73.3%.
DISCUSSION
e CSDisan abnormalndingmanifestedduring
a repeatcesareansection. e CSD spectrum can present
withscar dehiscence orauterinescarruptureduringlabor.
is devastating complication can be prevented. To do
so, a good screeningtool for the early detection of CSD
is needed.
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TABLE 1. Patient’s characteristics (n=111).
Characteristics Values
Age (years) (mean±SD) 29 ± 6
Gestational age (weeks) (mean±SD) 38.5 ± 0.6
BMI (kg/m
2
) (number (%))
< 18.5 9 (8.1)
18.5 - 22.9 45 (40.5)
23.0 - 24.9 13 (11.7)
25.0 - 29.9 29 (26.1)
> 30 15 (13.5)
Parity (number (%))
1 93 (83.8)
2 16 (14.4)
> 3 2 (1.8)
Miscarriages (number (%))
0 80 (72.1)
1 28 (25.2)
> 2 3 (2.7)
Number of previous cesarean sections (number (%))
1 101 (91.0)
> 1 10 (9.0)
Data are presented as mean ± standard deviation (SD), or number (%)
Abbreviation: BMI, body mass index
TABLE 2. e correlations between the abdominal sonographic LUS thickness and the incision-site uterine wall
thickness in each cesarean-scar morphologic grading.
LUS thickness (mm) measured by
Cesarean scar Ophthalmic Correlation
morphology Number Sonography calipers coefcient P-value
Grade 1 55 3.1 ± 1.0 3.3 ± 0.8 r = 0.559* 0.001
Grade 2 54 2.3 (1.8-2.9) 2.3 (1.7-2.3) r = 0.407** 0.002
Grade 3 2 Case A= 1.4 Case A=1.2 - -
Case B= 3.4 Case B=0.7
Overall 111 2.6 ± 1.0 2.5 ± 1.1 r = 0.559* 0.001
Data are presented as mean±standard deviation or median (interquartile range).
*Pearson correlation, **Spearman correlation
Abbreviation: LUS, lower uterine segment.
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Pahirah et al.
TABLE 3. e validity of the abdominal sonographic LUS thickness to detect cesarean scar dehiscence (grade 3
morphology) and thin incision-site uterine wall thickness.
Sonographic
LUS
thickness(mm) Total Sensitivity Specicity PPV NPV
<1.5 >1.5 number (%) 95%CI (%) 95%CI (%) (%)
Cesarean scar Yes 1 1 2 50.5 40-59 90.8 85-96 9.1 99.0
dehiscence No 10 99 109
Total number 11 100 111
Thinincision-site Yes 3 5 8 37.5 4-71 92.2 87-94 27.3 95.0
uterine wall No 8 95 103
thickness
Total number 11 100 111
Abbreviations: LUS: lower uterine segment; CI: condence interval; PPV: positive predictive value; NPV: negative predictive value
Fig 4. Receiver operating characteristic curve showing the optimal cut-o value for thin LUS (2.5 mm): the area under the curve was 0.8
(95% CI, 0.718-0.885).
Abbreviations: LUS: lower uterine segment; CI: condence interval
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e2Dabdominalsonographyis a simple,noninvasive,
widelyused, andreadily availabledevice inmost hospitals.
Theabdominal sonographicLUSthicknesscan be
usedfor antepartumCSD screening. eoretically, a
thinnerLUS willresult in a more severedegreeofCSD.
14
In
this study, the authorsevaluatedonlythemyometrial
layerofthe LUS,whichdirectly represents the uterine
scar’s integrity. eoverallmean sonographic myometrial
LUSthicknesswas 2.6+1.0 mm, which is comparable to
the Tazion, et al study.
15
Other studies have reported a
thinner sonographic measurement.
10,12
e dierences
involving the sonographic LUS thickness may becaused
bythe variation of participants’ characteristics, gestational
age, uterine-closure techniques, scar-brosis formation,
uterine healing process, and the sonographic protocol
used. However, the number of grade 3 CSD found in
this study was 1.8%, whichis comparabletowhat was
reported in those’ studies.
10,12
ereis a signicantcorrelationbetweenthe overall
sonographic LUSthickness and the incision-site uterine
wall thickness, and that is consistent with a prior study
6
which hada high level ofcorrelation. Surprisingly,
one case in the grade 3 group had a sonographic LUS
thickness of 3.4 mm, while the incision-site uterine
wall thickness was only 0.7 mm. isunexpected result
maybecaused bya poor imaging technique used on
the thick abdominal wall; less urine in the bladder; or
abnormal focal myometrial thickening. e authors
intend to use the specic sonographic protocol and
three-point measurement technique to maximize the
correspondence between the sonographic area of interest
and the cesarean scar site, but mislocation may still occur.
With regard to any prediction of scar dehiscence, the
use of a sonographic myometrial LUS thickness of less
than 1.5 mm hadasensitivityof50.0% and a specicity
of 90.8%, which is quite dierent from what Gizzo,etal.
12
Specically, they reported a high sensitivity of 100% and a
specicity of 85%. A possible reason for the dierences is
the dierent characteristics of the participants, especially
the higher number of previous cesareans. e thicker
sonographic LUS in this study results in a lower number
of positive tests, which can lead to less sensitivity and
more specicity. Ourresults showed a high NPV of99.0%;
thus, when the sonographicLUSthickness is more than
1.5 mm, it is less likely to have CSD.
With regardto the detectionof a thinincision-site
uterine wall thickness, a sonographic myometrialLUS
thickness of less than 1.5mm hadasensitivity of 37.5%.
So, thereis a need toredene the optimum cut-opoint.
Basedon this study,theauthors suggest a cut-o levelat
2.5 mm.
Strengths and limitations
The strengths of this study are that only one
sonographerwas used, so as to minimize interobserver
variations
16
; all assessors were blindedfrom the sonographic
results; and an ophthalmic calipers was employed for
objective measurement.
elimitationof this studywas thesmallnumberof
cesarean scardehiscence cases. Also, there was a possible
errorduringthe incision-site uterine wall thickness
measurement, as the ophthalmic calipers jaws have to grasp
a certain amount of tissuedeep fromtheincisionaledge,
which may result in abnormally thick uterine walls. ere
are dierenttypes of ophthalmic calipers used for LUS
measurement, such as Castroviejo ophthalmic calipers
6
or Vernier calipers
10
, and thismayaect the results.
Further study with more participants and a
longer duration of follow-up should be carried out to
achievethemost accurate method for antepartum CSD
prediction.
CONCLUSION
Preoperative abdominal sonography is a simple
tool for CSD prediction. A myometrial LUS thickness of
more than 1.5 mm is associatedwitha lowerlikelihoodof
cesarean scar dehiscence.
ACKNOWLEDGMENTS
is study got funding from HRH Princess Maha
Chakri Sirindhorn Medical Center, Faculty of Medicine,
Srinakharinwirot University (Contract No.256/2019).
Conict of interest: No potential conict ofinterest
regarding thisarticlewasreported.
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