A statistically significant association with uterine rupture during a trial of labour after caesarean delivery was found in at least two studies for the following variables: inter-delivery interval (higher risk with short interval), birth weight (higher risk if 4000 g or over), induction of labour (higher risk), oxytocin dose (higher risk with higher doses), and previous vaginal delivery (lower risk). However, no clinically useful risk estimation model that includes clinical variables has been published. A thin lower uterine segment at 35–40 weeks, as measured by ultrasound in women with a caesarean hysterotomy scar, increases the risk of uterine rupture or dehiscence. No cut-off for lower uterine segment thickness, however, can be suggested because of study heterogeneity, and because prospective validation is lacking. Large caesarean hysterotomy scar defects in non-pregnant women seen at ultrasound examination increase the risk of uterine rupture or dehiscence in subsequent pregnancy, but the strength of the association is unknown. To sum up, we currently lack a method that can provide a reliable estimate of the risk of uterine rupture or dehiscence during a trial of labour in women with caesarean hysterotomy scar(s).
Introduction
Uterine rupture is a rare but serious complication of a trial of vaginal birth after caesarean delivery. Therefore, vaginal birth after caesarean delivery should be proposed only to women who are likely to have a low risk of uterine rupture. Is it possible to identify these women? A number of clinical factors might be important as well as the integrity of the hysterotomy scar and the thickness of the lower uterine segment assessed by imaging techniques. A simple and easily available imaging technique to use for this purpose is ultrasound. Possibly, ultrasound assessment of the hysterotomy scar or of the whole lower uterine segment could be used alone or in combination with clinical factors to estimate the likelihood of uterine rupture or dehiscence occurring spontaneously or during a trial of labour. Current evidence on our ability to predict uterine rupture or dehiscence using ultrasound or clinical variables is limited, and is summarised below.
Frequency of uterine rupture after caesarean delivery
We do not know for sure how common uterine rupture is after caesarean delivery. In retrospective studies, the rate of uterine rupture during a trial of labour after caesarean delivery is around 1%. These studies seem to have included only symptomatic uterine rupture. In prospective studies in which women, who had the thickness of their lower uterine segment measured with ultrasound, were followed up with regard to pregnancy outcome, the rate of uterine rupture or dehiscence is on average 6.6% (range 1–46%). The prospective studies include complete uterine rupture, uterine dehiscence, and diagnoses made at elective caesarean, not just rupture occurring during a trial of labour. In an ordinary clinical setting, it is unlikely that all these cases would have been assigned an International Classification of Diseases code indicating uterine dehiscence or rupture. In a prospective observational study by Rozenberg et al., uterine rupture and uterine dehiscence were diagnosed either at caesarean section or by uterine exploration after vaginal delivery; the latter was carried out in all women who gave birth vaginally. The frequency of uterine rupture after caesarean delivery was 2.3% and that of uterine dehiscence 1.6%, for a total frequency of ‘uterine defect’ of 3.9%.
Some might argue that only symptomatic uterine rupture (in all likelihood resulting in an International Classification of Diseases code in the patient’s record) is clinically important, whereas asymptomatic dehiscence or rupture diagnosed at emergency caesarean delivery is clinically unimportant. I disagree. Whether a uterus with a dehisced caesarean hysterotomy scar (or with an extremely thin myometrium in the scar area) will proceed to rupture is likely to depend on the management of labour and on the timing of caesarean delivery.
Uterine rupture before start of labour is extremely rare. Vaknin et al. reported uterine rupture to occur before start of labour in seven of 120,636 (i.e. in one in 17,234) pregnancies 22 gestational weeks or over. In five of the seven cases, the woman had a caesarean hysterotomy scar. Rupture of an unscarred uterus is also rare. Miller et al. reported rupture of an unscarred uterus during labour in 10 out of 168,491 (i.e. in one of 16,849) deliveries.
Frequency of uterine rupture after caesarean delivery
We do not know for sure how common uterine rupture is after caesarean delivery. In retrospective studies, the rate of uterine rupture during a trial of labour after caesarean delivery is around 1%. These studies seem to have included only symptomatic uterine rupture. In prospective studies in which women, who had the thickness of their lower uterine segment measured with ultrasound, were followed up with regard to pregnancy outcome, the rate of uterine rupture or dehiscence is on average 6.6% (range 1–46%). The prospective studies include complete uterine rupture, uterine dehiscence, and diagnoses made at elective caesarean, not just rupture occurring during a trial of labour. In an ordinary clinical setting, it is unlikely that all these cases would have been assigned an International Classification of Diseases code indicating uterine dehiscence or rupture. In a prospective observational study by Rozenberg et al., uterine rupture and uterine dehiscence were diagnosed either at caesarean section or by uterine exploration after vaginal delivery; the latter was carried out in all women who gave birth vaginally. The frequency of uterine rupture after caesarean delivery was 2.3% and that of uterine dehiscence 1.6%, for a total frequency of ‘uterine defect’ of 3.9%.
Some might argue that only symptomatic uterine rupture (in all likelihood resulting in an International Classification of Diseases code in the patient’s record) is clinically important, whereas asymptomatic dehiscence or rupture diagnosed at emergency caesarean delivery is clinically unimportant. I disagree. Whether a uterus with a dehisced caesarean hysterotomy scar (or with an extremely thin myometrium in the scar area) will proceed to rupture is likely to depend on the management of labour and on the timing of caesarean delivery.
Uterine rupture before start of labour is extremely rare. Vaknin et al. reported uterine rupture to occur before start of labour in seven of 120,636 (i.e. in one in 17,234) pregnancies 22 gestational weeks or over. In five of the seven cases, the woman had a caesarean hysterotomy scar. Rupture of an unscarred uterus is also rare. Miller et al. reported rupture of an unscarred uterus during labour in 10 out of 168,491 (i.e. in one of 16,849) deliveries.
Clinical factors associated with uterine rupture after caesarean delivery
Researchers have attempted to determine the clinical factors associated with uterine rupture after caesarean delivery in retrospective cohort studies or case-control studies, including women who underwent a trial of labour after caesarean delivery. These studies seem to have included only symptomatic uterine rupture. The following factors have been examined to assess their ability to predict uterine rupture during a trial of labour after caesarean delivery: gestational age, maternal age, inter-delivery interval inter-pregnancy interval, suture technique for closing the hysterotomy, birth weight, previous vaginal delivery, induction of labour, ethnicity, pre-eclampsia or gestational hypertension, twin pregnancy, labour progress, and number of epidural doses. A statistically significant association with uterine rupture was found in more than one study for the following variables: inter-delivery or inter-pregnancy interval (higher risk with short interval, short inter-delivery interval being defined as 24 months or less or 18 months or less, and short inter-pregnancy interval as less than 6 months ), birth weight (higher risk if 4000 g or more), induction of labour (higher risk), oxytocin dose (higher risk with higher doses), and previous vaginal delivery (lower risk of rupture if the woman ever delivered vaginally). In addition, a systematic review that included 12 studies showed that locked single layer closure of the caesarean hysterotomy increased the risk of uterine rupture compared with double layer closure.
Two research teams tried to create multivariate logistic regression models that include clinical data to estimate the individual risk of uterine rupture (dehiscence not included) during a trial of labour after caesarean delivery. Both studies were retrospective. In a case-control study, Macones et al. created one model that included four variables (maternal age, gestational age, ethnicity, prior vaginal delivery) and another that included six variables (maternal age, gestational age, ethnicity, prior vaginal delivery, cervical dilatation at admission, and labour induction). The variable ‘two or more previous caesarean deliveries’ did not enter any model. No model performed well enough to be useful in clinical practice (area under the receiver operating characteristic curve 0.68 and 0.70, respectively). In a retrospective study including only women with a singleton term pregnancy after one pervious caesarean delivery, Grobman et al. constructed a logistic regression model that included two variables: previous vaginal delivery (lower risk of rupture) and induction of labour (higher risk of rupture). The model, however, had poor diagnostic performance (area under the receiver operating characteristic curve 0.63 and 0.60 on the training and test sets, respectively). Moreover, the predicted rupture risk did not agree well with the observed rate of rupture. Therefore, this model could also not be recommended for clinical use. To sum up current evidence, it seems not to be possible to provide a reliable estimate of the risk of uterine rupture during a trial of labour after caesarean delivery using only clinical information.
Thickness of the lower uterine segment as measured by ultrasound at 35–40 gestational weeks as a predictor of uterine rupture or dehiscence
Jastrow et al., in a systematic review published in 2010, attempted to establish whether the thickness of the lower uterine segment, as measured by ultrasound at 35–40 gestational weeks, predicted uterine rupture or dehiscence. The aim was to estimate the strength of the association between sonographic thickness of the lower uterine segment in women who had undergone caesarean delivery and uterine scar dehiscence or rupture, and to find the best cut-off value for the thickness of the lower uterine segment in predicting uterine dehiscence or rupture.
The review comprised 12 studies published between 1988 and 2009, including in total 1834 women with a previous low transverse caesarean hysterotomy who underwent ultrasound measurement of the thickness of the lower uterine segment at 35–40 gestational weeks. In seven of the 12 studies, the full lower uterine segment thickness was measured, in four studies the myometrial thickness was measured and, in one study, both measurements were taken. Because only three of the 12 studies reported the frequency of uterine rupture separately, the outcome measure was ‘uterine rupture or uterine dehiscence’ (i.e. ‘uterine scar defect’). It is uncertain if the definition of uterine rupture and uterine dehiscence was the same in all 12 studies. A common definition of uterine dehiscence is ‘subperitoneal separation of the uterine scar in the lower uterine segment with the chorioamniotic membrane visible through the peritoneum’. Uterine rupture is usually defined as a complete separation of the uterine scar with communication between the uterine and abdominal cavities. The rate of uterine scar defect was 6.6% (121/1834), the rate ranging from 1–46% in the different studies. Both the full lower uterine segment thickness and the myometrial thickness were associated with uterine scar defect: the area under the summary receiver operating characteristic curve for the full lower uterine segment thickness was 0.83 (SE 0.03) and that of myometrial thickness 0.75 (SE 0.05). Because the 12 studies were heterogenous, it was impossible to suggest an optimal thickness cut-off for predicting uterine defect. The cut-offs suggested for the full lower uterine segment thickness in the different studies varied between 2.0 mm and 3.5 mm, and those for the myometrial thickness between 1.4 mm and 2.0 mm. It is of note that, in nine of the 12 studies included in the review, the ultrasound results were available to the staff managing the pregnancies and deliveries. This is inappropriate when estimating sensitivity and specificity of a diagnostic test. Therefore, the results of these nine studies are likely to be biased. The study most likely to have yielded reliable results is the one by Rozenberg et al. Another study of particular interest is the one by Bujold et al. Both these studies are briefly outlined below.
The aim of the prospective observational study by Rozenberg et al. was to estimate the sensitivity and specificity of the full lower uterine segment thickness as measured by transabdominal ultrasound with regard to uterine rupture or dehiscence. It includes 642 women with a caesarean hysterotomy scar who had their full lower uterine segment thickness measured with transabdominal ultrasound at 36–38 gestational weeks by one single ultrasound examiner. The medical staff was blinded to the ultrasound results. Uterine rupture and uterine dehiscence were diagnosed either at caesarean section or by uterine exploration after vaginal delivery, the latter being carried out in all women who gave birth vaginally.
The full lower uterine segment thickness varied between 1.6 and 12.3 mm. Uterine rupture was diagnosed in 15 (2.3%) women and uterine dehiscence in 10 (1.6%) women (i.e. the rate of uterine defect was 3.9%). The thinner the full lower uterine segment, the higher the risk of uterine rupture or dehiscence. In groups of women with full lower uterine segment thickness of 1.6–2.5 mm, 2.6–3.5 mm, 3.6–4.5 mm, and over 4.5 mm, the frequency of any uterine defect (either dehiscence or rupture) was 16%, 10%, 2% and 0, the frequency of uterine rupture was 10%, 7%, 0.6% and 0, and that of uterine dehiscence was 6%, 4%, 1% and 0. The investigators themselves suggested a cut-off of 3.5 mm to be optimal and suitable for clinical use, values for full lower uterine segment thickness 3.5 mm or less being taken to indicate a high risk of uterine rupture. In their study population, this cut-off had a sensitivity of 88% and a specificity of 73%. This corresponds to a positive likelihood ratio of 3.3 and a negative likelihood ratio of 0.16, which means that this test had poor – or at most moderate – ability to predict uterine dehiscence or rupture.
The prospective observational study by Bujold et al. is the second largest study in the systematic review by Jastrow et al. It includes 236 women who had the full lower uterine segment thickness and the myometrial thickness measured using both transabdominal and transvaginal ultrasound at 35–38 weeks of gestation. The measurement technique was described in detail ( Fig. 1 ). The frequency of uterine rupture was 1.3% (3 out of 236 women) and that of dehiscence 2.5% (6 out of 236 women). The investigators used multivariate logistic regression, and found that a thin lower uterine segment (full thickness less than 2.3 mm), single layer closure of the hysterotomy and short inter-delivery interval (less than 18 months) significantly and independently predicted uterine rupture or dehiscence. Unfortunately, the staff were not blinded to the ultrasound results, and so these results are likely to be biased. Moreover, the study was not large enough to allow assessment of interaction between variables and, with only nine cases of uterine defect, the result of the multivariate analysis are unreliable because of the risk of over-fitting. Multivariate analysis, including both clinical and ultrasound variables, however, could be the way forward and should be used in future much larger studies.
Prospective validation of cut-offs for lower uterine segment thickness
No studies have prospectively validated any of the lower uterine segment thickness cut-offs suggested (i.e. it is not known how well they would perform in other populations than those where they were suggested, or in the hands of new examiners).
Rozenberg et al. introduced measurement of the lower uterine segment with the 3.5 mm cut-off into clinical practice after their observational study had been closed. They compared the results for 1995 to 1996 after the method had been implemented clinically with those in their observational study (1989 to 1994) with regard to the frequency of uterine dehiscence or rupture, the frequency of planned and emergency caesarean delivery, and the use of oxytocin in women with previous caesarean delivery. After measurement of the lower uterine segment had been introduced clinically, the frequency of uterine defect was lower (0.8% v 3.9%), and the use of oxytocin was twice as common in women with previous caesarean delivery. Moreover, in women who had undergone only one previous caesarean delivery, the frequency of elective caesarean delivery was higher (12% v 6%), whereas that of emergency caesarean delivery was lower (20% v 24%) and, in women with two previous caesarean deliveries, the rate of vaginal delivery was higher. Even though these results suggest that ultrasound measurement of the lower uterine thickness might improve the management of delivery in women who have undergone previous caesarean section, they must be interpreted with caution because the study uses historical controls. It is not a randomised-controlled trial.
Measuring thickness of the lower uterine segment during pregnancy by ultrasound
Two ultrasound measurements have been reported to reflect the thickness of the lower uterine segment in the end of pregnancy and to possibly be predictive of uterine rupture or dehiscence: the ‘full lower uterine segment thickness’ and the ‘thickness of the myometrial layer’. These measurements can be taken either transabdominally or transvaginally. No consensus exists on which measurement technique should be used for either transabdominal or transvaginal ultrasound.
In studies reporting on measurements of the thickness of the lower uterine segment, the measurement technique was described with varying detail. Therefore, it is unlikely that exactly the same transabdominal measurement technique or exactly the same transvaginal measurement technique was used in all studies: differences may exist between the degree of bladder filling, the site of measurement, the placement of the callipers, the number of measurements taken and in the measurement selected to be representative if more than one was taken. This means that the measurement results in different studies may not be directly comparable. Clearly, if a cut-off for the sonographic thickness of the lower uterine segment is recommended by a certain research group, the exact same measurement technique must be used as in the study where that particular cut-off was recommended.
The most detailed description of how the measurements of the lower uterine segment were taken is found in a paper by Bujold et al. Their measurement technique (the same for transabdominal and transvaginal measurements) is described in Fig. 1 .
Thickness of ‘normal’ lower uterine segment as measured by ultrasound at 35–40 gestational weeks
No study has reported on the sonographic thickness of the full lower uterine segment at 35–40 gestational weeks in women with an unscarred uterus. One study, however, reported on the thickness of the myometrium at 36–38 gestational weeks in nulliparous women and in parous women who had only delivered vaginally. The mean ± SD (range) myometrial thickness was 2.3 mm ± 1.1 (1.1–5.5) in nulliparous women and 3.4 mm ± 2.2 (1.0–10.3) in parous women. The corresponding thickness for women with caesarean hysterotomy scar(s) was 1.9 mm ± 1.4 (0–9.0).
A summary of published ultrasound measurements of the lower uterine segment thickness at 35–40 weeks in women who gave birth by caesarean is presented in Table 1 . It seems that the sonographic full thickness of the lower uterine segment at 35–40 weeks in women delivered by caesarean is on average 3–4 mm, with a range from 2–19 mm, and that sonographic myometrial thickness ranges from 0–10 mm.
