How to manage delivery in the woman who previously gave birth via cesarean? That is the subject of much controversy in our
specialty, even as cesarean rates rise despite the federal government's attempts to lower them.1 Much effort has gone into trying to identify risk factors for uterine rupture so that we can predict which patients are
likely to have a successful vaginal birth after cesarean (VBAC).2 With our country's increasing cesarean rates, too, have come increased risks of placenta previa and accreta for women in
subsequent pregnancies.3-5 Ultrasound has proved useful in helping to settle other obstetric controversies. Might it also have a role in assessing
uterine scars before pregnancy and predicting risk of uterine rupture? Let's examine the evidence.
Are scars significant?
In early studies of uterine scars, transabdominal U/S was used to distinguish between classical and transverse uterine incisions.
More recent efforts with transvaginal U/S have shown that it is more effective for such screening.6
In 2001, Monteagudo and colleagues used saline infusion sonohysterography (SIS) to detect and characterize prior uterine incisions.
With this technique, saline is infused into the uterus during sonographic evaluation to distend the uterus and delineate the
endometrial cavity's contours. The authors found they could detect a "niche" ranging from 2.5 to 11.5 mm in the anterior uterine
wall of all of the women who had previously delivered by cesarean.7 In addition, in 33% of the patients, the cesarean scar could be seen before the saline infusion as a fine, hyperechoic region
extending anteriorly from the "niche."7 Monteagudo and colleagues speculated that risk of uterine dehiscence and rupture might be related to the depth of the niche
or the thickness of the overlying myometrium.
In another study using SIS, Regnard and colleagues investigated the frequency of sonographic dehiscence in women with a history
of cesarean delivery. They saw a "niche" in 57.5% of patients (Figure 1), and in 6% of patients, a "deep niche" extending
through 80% of the myometrium was visible.8
There have been reports describing repair of uterine defects detected by U/S. One case report describes the repair of a uterine
dehiscence in pregnancy detected at 28 weeks' gestation with subsequent conservative management and elective delivery at 35
weeks.9 Another recent series describes the laparoscopic and vaginal repair of uterine defects detected by U/S in five nongravid women.
One woman in this series went on to have an uncomplicated, term repeat C/S.10
Whether these findings—and the depth of the niche—-are clinically significant remains unknown. What we do know, however, is
that conventional U/S and SIS can be used to identify uterine scars in women who have delivered by cesarean, before they become
pregnant again. Several sonographic findings, including "deep niches," are suggestive of a uterine wall defect, but it's unclear
whether they have clinical significance or are related to subsequent uterine rupture.
Intrapartum uterine rupture during a trial of labor after prior cesarean is rare, occurring in about 0.5% of women, when cervical
ripening with oxytocin or prostaglandin is not required.11 Uterine niches, in contrast, are seen in 6% to 42% of women who have delivered by cesarean, making them too common to show
a clear relationship between a niche and uterine rupture. Their clinical significance and relationship to adverse pregnancy
outcomes also have yet to be determined. At this time, repair of uterine niches is considered experimental.
What about scars at term?
U/S also has been used by clinicians to diagnose uterine rupture before the onset of labor, and recently, researchers have
tried to predict which women may be at increased risk of uterine rupture.
Figure 2. Myometrial thickness of term uterus with history of cesarean delivery.
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During pregnancy, the uterine wall gets progressively thinner. An unscarred uterus decreases from a mean thickness of 6.7
mm in the second trimester to a mean of 3.0 mm by 39 weeks, but it will always measure more than 2.0 mm.12,13 The uterus in a woman with a history of cesarean delivery has a similar thickness early in pregnancy, but is significantly
thinner at term.12 Figure 2 shows a typical sonogram of myometrial thickness in a woman with a history of redundant cesarean delivery, which
demonstrates a normal lower uterine segment measured at the bladder reflection in the second trimester. Whether there is a
correlation between a thin lower uterine segment on U/S and risk of uterine dehiscence and rupture is less clear.
Rozenberg and colleagues used transabdominal U/S to evaluate uterine thickness in 642 women at 36 to 38 weeks. The patients
were divided into one of four groups, according to myometrial thickness. The investigators correlated the measurements with
obstetric outcome and found that the thinner the myometrium, the greater the risk of uterine dehiscence and rupture (Table
1).14 At a cutoff of 3.5 mm, U/S had 88% sensitivity for detection of uterine defects, 73.2% specificity, positive predictive
value of 11.8%, and negative predictive value of 99.3%.14 Similarly, Gotoh and colleagues found evidence of uterine wall separation and only visceral peritoneum covering the uterine
contents at time of elective repeat cesarean in 91% of women whose myometrial thickness was less than 2 mm in the second trimester.12
Other investigators have used U/S to prospectively assess risk of uterine dehiscence and rupture. Asakura and colleagues measured
myometrial thickness at 37 to 40 weeks and evaluated the uterus at the time of repeat cesarean or with internal examination
after successful VBAC.15 They defined dehiscence as separation of the muscular layer with intact serosa or palpation of the serosa by vaginal examination
without an intervening muscular layer (confirmed by U/S). This study found no uterine ruptures. But with a cutoff of 1.6 mm,
antepartum U/S had sensitivity of 77.8% for uterine dehiscence, specificity of 88.6%, positive predictive value of 25.9%,
and negative predictive value of 98.7%.16
Table 1. Correlation between myometrial thickness and uterine dehiscence
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How can we apply Asakura and colleagues' results to clinical practice? Their data on U/S assessment of the uterine scar in
the third trimester may help in the decision-making process when weighing a trial of labor versus a repeat cesarean. The a
priori risk for intrapartum uterine rupture after one prior low-transverse cesarean delivery is 0.5% to 1.0%, which we generally
accept as a reasonable risk when attempting a vaginal delivery.11 However, the risk rises to 25.9% when the thinnest part of the lower-segment myometrium measured in a sagittal plane is
less than 1.6 mm at 37 to 40 weeks. That is higher than the 5% to 9% risk of uterine rupture for labor with a prior classic
cesarean delivery, which is generally considered too high to consider a trial of labor.16 The bottom line: We need studies to determine if risk of intrapartum uterine dehiscence or rupture is significantly reduced
when the myometrium measures more than 1.6 mm.
Both scarred and unscarred uteri progressively thin through the second and third trimester, but scarred uteri seem to become
significantly thinner. Furthermore, the degree of thinning appears to be related to the risk for uterine dehiscence. In the
future, evaluating the thickness of the uterine wall at term may be one way to assess a woman's risk of intrapartum uterine
dehiscence. And stratifying the risk may help clinicians counsel patients about the decision to attempt VBAC. It should be
noted, however, that as many as 25% of women at elective repeat C/S may have an abnormally thinned lower uterine segment or
occult dehiscence.12,17
Uterine scars and abnormal placentation
Age, parity, and number of prior cesarean deliveries are all independent risk factors for placenta accreta.3-5,18 Recent data from the Maternal-Fetal Medicine Unit Network show that the risk of placenta accreta increases from 0.2% with
one prior C/S to 2.1% with four prior C/S and 6.7% with more than five prior C/S.18 When placenta previa is also present, however, the risk for accreta rises to 3.3%, 11%, 40%, and 61% with one, two, three,
and four prior C/S deliveries, respectively.19 Therefore, evaluating abnormal placentation in women with a prior C/S delivery is an important component of prenatal diagnosis.
Figure 3. Representative ultrasound images of placenta accreta
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Both U/S and magnetic resonance imaging (MRI) have been used for antepartum diagnosis of placenta accreta and increta.20,21 They perform equally well in identifying abnormal placentation, but MRI is better at diagnosing posterior placenta accreta.21,22 By comparison, U/S has been shown to be 82.4% sensitive and 96.8% specific for diagnosis of placenta accreta, with positive
predictive value of 87.5% and negative predictive value of 95.3%.23 U/S with power Doppler can help delineate neovascularization, potentially making possible diagnosis of placenta accreta
as early as the first trimester.24,25 Some researchers have not been able to demonstrate increased vascularity, but have instead relied on the gestational sac's
position in the endometrial cavity to indicate likelihood of placenta accreta later in pregnancy.26 
Table 2. Sonographic findings in placenta accreta
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In the second and third trimester, sonography has become the diagnostic standard for differentiating simple placenta previa
from placenta previa complicated by accreta. Comstock and colleagues reviewed all suspected and confirmed cases of placenta
accreta at their institution between 1990 and 2002 to determine which sonographic findings best correlated with placenta accreta.
They found that 54.5% of placenta accreta diagnoses were false positive (sensitivity 85.7%, specificity 98.9%, positive predictive
value 36.4%, negative predictive value 99.9%), most of which were because the echolucent area between the placenta and myometrium
could not be seen.27 The most specific diagnostic criterion for placenta accreta was visualization of irregular vascular spaces within the placenta
(placental lacunae as originally described by Guy and colleagues28 ), with sensitivity and specificity of 93%.27 Figure 3 shows representative images of placenta accreta and Table 2 lists common sonographic findings.
It appears, then, that while both U/S and MRI can be used to diagnose placenta accreta, U/S is more attractive for screening
because it is cheaper and more readily available. Imaging is reasonably sensitive for diagnosis of placenta accreta, but it
reportedly produces high false-positive rates. Antepartum diagnosis of likely placenta accreta should heighten your suspicion
for accreta, although you still need to make the final diagnosis in the operating room.
Conclusion
Pregnancy after C/S continues to be an obstetric challenge. Assessing the uterine scar in women who have had a C/S, before
they become pregnant again, holds promise for determining risk of uterine dehiscence or rupture in the next gestation. More
research is needed, however, into clinical implications of sonographic findings before screening can be integrated into risk
assessment for uterine dehiscence and rupture.
U/S is also evolving as a tool for use in late pregnancy to assess risk of intrapartum uterine dehiscence and rupture. Finally,
U/S can also be used in assessment of abnormal placentation, which is an important component of antepartum evaluation of a
scarred uterus. Much remains to be learned about these new roles for U/S, but they have potential to help patients and clinicians
make more informed decisions about pregnancy after a C/S.
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