Predicting Incomplete Uterine Rupture With
Vaginal Sonography During the Late Second
Trimester in Women With Prior Cesarean
HIDEO GOTOH, MD, HIDEAKI MASUZAKI, MD, ATSUSHI YOSHIDA, MD,
SHUICHIRO YOSHIMURA, MD, TSUNETAKE MIYAMURA, MD, AND
TADAYUKI ISHIMARU, MD, PhD
Objective: To evaluate the usefulness of serial transvaginal
ultrasonographic measurement of the thickness of the lower
uterine segment in the late second trimester for predicting
the risk of intrapartum incomplete uterine rupture in
women with previous cesarean delivery.
Methods: Serial transvaginal ultrasonography with full
bladder was performed in 374 women without previous cesarean delivery (control group) and 348 women with previous
cesarean delivery (cesarean group) from 19 to 39 weeks’ gestation. The thickness of the lower uterine segment was measured
in the longitudinal plane of the cervical canal.
Results: The thickness of the lower uterine segment decreased from 6.7 ⴞ 2.4 mm (mean ⴞ standard deviation [SD]) at
19 weeks’ gestation to 3.0 ⴞ 0.7 mm at 39 weeks’ gestation in
the control group, but the thickness was more than 2.0 mm
throughout this period in each control subject. In the cesarean
group, the thickness decreased from 6.8 ⴞ 2.3 mm at 19 weeks’
to 2.1 ⴞ 0.7 mm at 39 weeks’ gestation and was significantly
thinner than that of the control group after 27 weeks’ gestation
(P < .05). Eleven of 12 women (91%) with lower uterine
segment less than the mean control ⴚ 1 SD in the late second
trimester had a very thin lower uterine segment at cesarean
delivery with fetal hair being visible through the amniotic
membrane, ie, incomplete uterine rupture. In 17 of 23 women
(74%) with lower uterine segment less than 2.0 mm in thickness
within 1 week (4 ⴞ 3 days) before repeat cesarean delivery,
intrapartum incomplete uterine rupture developed.
Conclusion: Transvaginal ultrasonography is useful for measurement of the uterine wall after previous cesarean delivery.
(Obstet Gynecol 2000;95:596 – 600. © 2000 by The American
College of Obstetricians and Gynecologists.)
3869.1– 4 The most common cause of uterine rupture is
scar separation of a previous cesarean delivery. The risk
of uterine rupture during a trial of labor in women with
previous cesarean delivery is 0.3– 4.0%.5–10 The different
results reported by these studies may be due partly to
various definitions used for uterine rupture. Uterine
scar separation includes a spectrum of abnormalities
varying from asymptomatic scar dehiscence to overt
uterine rupture with complete fetal extrusion from the
uterus into the maternal abdomen.11
A number of methods have been used to evaluate the
lower uterine segment after cesarean delivery. Hysterography of uterine scar,12 pelvic examination,13 amniography,14 and x-ray pelvimetry15 have not proved useful for
estimation of the risk of uterine rupture; however, several
reports have suggested that transabdominal ultrasonography may detect a defective uterus after previous cesarean
delivery.16 –18 At present, however, there is no consensus
that prenatal ultrasonography can always detect uterine
rupture.
In the present study, we performed serial transvaginal ultrasonography between 19 and 39 weeks’ gestation to measure the thickness of control and surgically
repaired lower uterine segment by previous cesarean
delivery. We compared these values and the macroscopic findings at cesarean delivery. These data were
used later to predict the development of intrapartum
uterine rupture by using the thickness of the lower
uterine segment measured during pregnancy.
The prevalence of spontaneous uterine rupture has
been estimated at between one in 149 and one in
Materials and Methods
From the Department of Obstetrics and Gynecology, Nagasaki University School of Medicine, Nagasaki, Japan.
The study population consisted of 374 pregnant women
(374 measurements) without a history of cesarean delivery (control group) and 348 pregnant women (348
596 0029-7844/00/$20.00
PII S0029-7844(99)00620-1
Obstetrics & Gynecology
Figure 1. Transvaginal ultrasonography showing the lower uterine
segment and bladder full. Open arrow indicates uterine wall; solid arrow
indicates bladder wall.
measurements) with previous cesarean delivery (cesarean group). Only a single measurement was made for
each fetus. Between January 1995 and December 1998,
722 women were examined prospectively by transvaginal ultrasonography between 19 and 39 weeks’ gestation to measure the thickness of the lower uterine
segment and to detect the presence of any uterine defect
with a full bladder. All mothers were healthy, with
uncomplicated singleton pregnancies. Patients represented all women with uncomplicated pregnancies who
attended the antenatal clinic at our department within
the above time periods. None of the patients reported in
this study attempted vaginal birth after cesarean
(VBAC).
The thickness of the lower uterine segment was
measured after the bladder was identified in the longitudinal plane of the cervical canal at transvaginal ultrasonography (Figure 1). Ultrasonography was performed by using Mochida equipment with a 7.5-MHz
transvaginal transducer (Sonovista Ex model meu-1581,
Mochida, Japan). This system provides clear freezeframes and the use of on-screen calipers for direct
measurements. All subjects were delivered after 37
weeks’ gestation. Measurement of the lower uterine
segment was repeated at least three times in each
examination, and the minimal value was reported (total: 722 measurements; control: 374; cesarean delivery:
348). Intraobserver and interobserver variabilities in the
calculation of the thickness of the lower uterine segment obtained for the first 50 women in the control
group were 4.7% and 5.5%, respectively. The sonographers were masked to the type of patient (control
compared with cesarean) to eliminate any possible bias.
VOL. 95, NO. 4, APRIL 2000
Sonographic results at antepartum were compared
with direct intraoperative observation at cesarean delivery. Incomplete uterine rupture represented separation of the uterine wall and visceral peritoneum covering the uterus.11 We examined differences in the
thickness of the lower uterine segment between control
pregnant women and those with previous cesarean
delivery at each gestational week, as well as the pattern
of change in the lower uterine segment during pregnancy in each group. We also used the antenatal and
intraoperative data to examine whether incomplete
uterine rupture during delivery could be predicted
from lower uterine segment measurement during pregnancy. The surgeons were masked to the sonographic
findings at the time of cesarean delivery.
The study protocol was approved by the Ethics
Review Committee of our institution, and a signed
consent form was obtained from each subject. Data
were expressed as mean ⫾ standard deviation (SD), or
median and range. Student t test and Mann-Whitney U
test were used for comparison of continuous variables.
Differences in uterine thickness between control group
and cesarean group were evaluated by unpaired t test,
and those between the second and third trimesters were
evaluated by paired t test. Fisher exact test was used to
examine the association between ultrasonographically
determined thickness of the lower uterine segment at
different stages of pregnancy and incomplete uterine
rupture at the time of cesarean. Significance was considered P ⬍ .05. The sample size was sufficient to detect
differences at 5% level of significance with 80% power.
Results
Patients in the cesarean group did not differ significantly from those in the control group with respect to
maternal age, parity, gestational age at delivery, infant
birth weight, and 1-minute and 5-minute Apgar scores
(Table 1). The thickness of the lower uterine segment in
the control group decreased steadily from 6.7 ⫾ 2.4 mm
at 19 weeks’ gestation to 3.0 ⫾ 0.7 mm at 39 weeks’
gestation, but the thickness was greater than 2.0 mm in
all cases (Figure 2). In the cesarean group, the thickness
of the lower uterine segment decreased from 6.8 ⫾
2.3 mm at 19 weeks’ gestation to 2.1 ⫾ 0.7 mm at 39
weeks’ gestation (Figure 2). Whereas the thickness of
the lower uterine segment was not different between
the two groups at 19 –26 weeks’ and 28 weeks’ gestation, it was significantly thinner in the cesarean than in
the control group at 27 weeks’ and every week after 29
weeks’ gestation (P ⬍ .05). Serial measurements of the
lower uterine segment during pregnancy showed that
in a subgroup of women with previous cesarean delivery, the uterine wall was persistently thin from the
Gotoh et al
Measurement of Uterine Thickness 597
Table 1. Characteristics of Controls and Women With
Previous Cesarean Delivery
n
Maternal age (y)
Parity
Gestational age at
delivery (wk)
Birth weight (g)
Apgar score 1-min
Apgar score 5-min
Control
Cesarean
P
374
28.9 ⫾ 5.3
1.9 ⫾ 0.9
38.9 ⫾ 1.5
348
31.9 ⫾ 4.6
1.4 ⫾ 0.8
38.2 ⫾ 1.2
—
NS*
NS*
NS*
3121 ⫾ 458
9 (8 –10)
9 (8 –10)
2953 ⫾ 483
8 (7–10)
9 (8 –10)
NS*
NS†
NS†
NS ⫽ not significant.
Data are presented as mean ⫾ standard deviation, or median
(range).
* Student t test.
†
Mann-Whitney U test.
second trimester until the termination of pregnancy
(thin group: lower uterine segment was less than
mean ⫾ 1 SD, n ⫽ 12, Figure 3). In another group of
women with previous cesarean delivery, the uterine
wall decreased in thickness progressively, in a manner
similar to that of the control group (nonthin group:
lower uterine segment was mean ⫾ 1 SD or greater, n ⫽
27, Figure 3). Eleven of the 12 women (91%) with a
lower uterine segment less than the mean control ⫺ 1
SD in the late second trimester had a surprisingly very
thin uterine wall at cesarean delivery; the fetal hair was
visible through the amniotic membrane. In all 27
women with lower uterine segment greater than the
mean control ⫺ 1 SD, an intrapartum incomplete uter-
Figure 3. Correlation between thickness of the lower uterine segment
at late second trimester and third trimester in the thin group and the
nonthin group. Note that in subjects in whom the lower uterine
segment was thin from the late second trimester, intrapartum incomplete uterine rupture was more likely to develop. NS ⫽ not significant.
ine rupture at cesarean delivery did not develop (nonthin group, P ⬍ .001, Table 2).
Further analysis of transvaginal ultrasonographic
findings at 7 days before cesarean delivery (4 ⫾ 3 days)
and those at cesarean delivery showed that 17 of 23
women (74%) with lower uterine segment less than
2.0 mm before repeat cesarean delivery had an incomplete uterine rupture at cesarean delivery. On the other
hand, none of the 45 women with lower uterine segment of 2.0 mm or greater thickness demonstrated an
incomplete uterine rupture at cesarean delivery (P ⬍
.001, Table 2).
Table 2. Association Between Ultrasonographically
Determined Thickness of the Lower Uterine
Segment at Different Stages of Pregnancy and
Intrapartum Incomplete Uterine Rupture at
Cesarean
Thickness of lower uterine
segment
(mm)
Figure 2. The mean thickness of the lower uterine segment in controls
and women who have had a previous cesarean delivery. The thickness
of the lower uterine segment was not different between the two groups
at 19 –26 weeks’ and 28 weeks’ gestation (*). The lower uterine segment
was significantly thinner in the cesarean group than in the control
group at 27 weeks’ and 29 weeks’ gestation (P ⬍ .05, unpaired t test)
(**). The lower uterine segment was significantly thinner in the
cesarean group than in the control group at every week after 30 weeks’
gestation (P ⬍ .01, unpaired t test) (***). SD ⫽ standard deviation.
598 Gotoh et al
Measurement of Uterine Thickness
At second trimester
⬍Mean control ⫺ 1 SD
ⱖMean control ⫺ 1 SD
Within 7 d (4 ⫾ 3 d) of
cesarean
⬍2 mm
ⱖ2 mm
Intrapartum incomplete
uterine rupture
Yes
(%)
No (%)
11 (91)
0 (0)
1 (9)
27 (100)
17 (74)
0 (0)
6 (26)
45 (100)
SD ⫽ standard deviation.
(At second trimester and within 7 d of cesarean: P ⬍ .001).
Obstetrics & Gynecology
Discussion
In 1988, ACOG recommended that carefully selected
patients be encouraged to have a trial of labor after a
single previous cesarean delivery.19 Several investigators have emphasized the efficacy and safety of vaginal
birth after cesarean delivery in a large series of studies.20,21 The most common complication of vaginal
delivery after a previous cesarean delivery is uterine
rupture, and the frequency varies between 0.5 to 0.8%
with previous lower uterine segment incision, as reported in a recent study.22 Although maternal mortality
is rare, rupture of the uterus may be associated with
significant morbidity for the mother and fetus in
women with a history of low transverse cesarean delivery. Jones et al23 and Scott24 reported four perinatal
deaths among 20 cases of uterine rupture with a previous low-segment cesarean scar. Two of these infants
had long-term neurologic impairment, and three
women required hysterectomy.
Several investigators have examined the accuracy of
various diagnostic procedures in the detection of uterine rupture in women with previous cesarean delivery.
Hysterography,12 pelvic examination,13 and amniography14 have been used, but their usefulness has not been
confirmed. Hebisch et al25 compared the results of
ultrasonography with those of magnetic resonance imaging (MRI). They demonstrated that vaginal ultrasonography provided more accurate information about
the condition of the scarred myometrium of the isthmus
than MRI. Other studies have shown that ultrasonography may predict uterine rupture in women with previous cesarean delivery. Rozenberg et al17 indicated that
the risk of uterine rupture in the presence of a defective
scar was related directly to the degree of thinning of the
lower uterine segment as measured by transabdominal
ultrasonography at or near 37 weeks’ gestation. In
particular, they demonstrated that this risk increased
significantly when the thickness was 3.5 mm or less.
The use of this cutoff value showed an excellent sensitivity (88.0%) for ultrasonography, with a negative
predictive value of 99.3%. On the other hand, Fukuda et
al18 examined 41 low-segment transverse cesarean delivery scars by transperineal and transvaginal longitudinal scans. A wall thickness of 2 mm or less was
considered a sign of poor healing. At operation, they
measured the thickness of the lower uterine segment by
ophthalmic calipers just after incising the uterus and
before rupture of the membranes, and reported no
false-positive or false-negative results when using ultrasonography.
Our results showed that when the thickness measured by transvaginal ultrasonography is less than
2 mm within 1 week of delivery, the lower uterine
VOL. 95, NO. 4, APRIL 2000
segment may show an incomplete uterine rupture. The
positive and negative predictive values were 73.9% and
100%, respectively. Furthermore, a thickness of less
than the control mean-SD at the second trimester is
highly predictive of the development of incomplete
uterine rupture at delivery. The positive and negative
predictive values were 91.7% and 100%, respectively. A
careful examination of our results showed that the
predictive value at full term was lower than that at the
second trimester. This finding is probably due to further
thinning of the lower uterine segment at term as the
wall is stretched by the presenting part. Although the
defects of the lower uterine segment can be detected in
the second trimester, our results showed that the most
clinically useful information regarding the status of the
lower uterine segment is obtained when ultrasonography is performed before 36 weeks’ gestation. Measurements obtained at that time avoid problems associated
with engagement of the presenting part in the pelvis or
the physiologic reduction in the volume of amniotic
fluid occurring at later stages of pregnancy. Previous
studies that examined the importance of detecting uterine rupture by ultrasonography emphasized the value
of such evaluation at term,17,18 but our study stresses
the importance of performing such measurement during the late second trimester. We showed that serial
measurements of the lower uterine segment by transvaginal ultrasonography during the second trimester
might predict incomplete uterine rupture at term.
Although none of our cases with incomplete uterine
rupture progressed to a complete uterine rupture during labor, incomplete uterine rupture represents a high
risk for complete uterine rupture because in some cases
with incomplete uterine rupture, uterine contents are
separated from the peritoneal cavity only by the visceral peritoneum covering the uterus.
In women with previous cesarean delivery, uterine
rupture may occur after labor even in those with
lower-uterine segmental incision. Thus, early prediction
of uterine rupture before the onset of labor should also
reduce the frequency of uterine rupture after delivery.
Transvaginal ultrasonography could therefore be considered the standard diagnostic procedure that can
predict accurately intrapartum uterine rupture in
women with previous cesarean delivery. Our results
emphasize the importance of measuring the uterine scar
in the late second trimester in women with previous
cesarean delivery.
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Address reprint requests to:
Hideo Gotoh, MD
Department of Obstetrics and Gynecology
Nagasaki University School of Medicine
1-7-1 Sakamotomachi
Nagasaki, 852-8501
Japan
Received June 21, 1999.
Received in revised form October 5, 1999.
Accepted October 15, 1999.
Copyright © 2000 by The American College of Obstetricians and
Gynecologists. Published by Elsevier Science Inc.
Obstetrics & Gynecology