There were 38 (1.3%) women undergoing a trial of labor in the low transverse group who had a scar disruption detected during the trial of labor, and six (1.6%) women in the low vertical group who had a scar disruption detected (P = .6) (Table 3). Of greater clinical importance, the symptomatic rupture rate also was similar between the two groups, with a rupture rate of 1.0% for those in the low transverse group and 0.8% for those in the low vertical group, (P > .999).
As shown in Table 4, women undergoing a trial of labor with a prior low vertical incision were more likely to have a spontaneous vaginal delivery during the index trial of labor. The percentage of those with Apgar scores less than 7, intrapartum fever, endomyometritis, and transfusion was similar between the groups. The mean change in hematocrit between admission and postpartum day 1 also was similar between the two groups. Overall, there were nine hysterectomies in the low transverse group (0.31%), and one in the low vertical group (0.27%). Three of the hysterectomies in the low transverse group were not associated with uterine rupture.
There were 12 perinatal deaths among those in the low transverse group (0.4%) and four among those in the low vertical group (1.1%). There were 11 antenatal deaths before admission in the low transverse group and one neonatal death of an infant who was born weighing 740 g. Within the low vertical group, there were three antenatal deaths before admission and one neonatal death due to anomalies.
A multiple logistic regression analysis was performed to examine the association of uterine rupture with scar type while controlling for the potential confounding effects of maternal age, parity, length of labor, public aid insurance, gestational age, use of epidural, use of oxytocin, centimeters of cervical dilatation at admission, birth weight, indication for prior cesarean delivery, and year of delivery. In this model, those women who had a prior low vertical uterine incision remained no more likely to have a uterine rupture compared with women who had a prior low transverse uterine incision (odds ratio [OR] = 1.0, 95% confidence interval [CI] 0.29, 3.45). The year of delivery was not a significant predictor of uterine rupture (OR = 1.0, 95% CI 0.9, 1.1).
We then compared the maternal and neonatal outcomes for the symptomatic ruptures in both study groups (Table 5). Outcome in the presence of rupture was similar in the low transverse and low vertical groups. There were no maternal deaths in either group, and the only perinatal death occurred in a patient with a prior low transverse hysterotomy. This particular patient was sent home after a failed induction, and several days later was found to have had a fetal demise. An induction was reinitiated, and during this second induction, she had a uterine rupture. We also calculated the need for blood replacement among the women with uterine ruptures. Within the low transverse group, seven women received packed red blood cells, with a range of between one and nine units of blood and a median value of two units of blood. For the low vertical ruptures, one woman received six units packed red blood cells.
As controversy remains regarding the definition of a low vertical incision, we further characterized the low vertical group. None of the three women having a uterine rupture and 24.7% (87 of 352) of the women without a uterine rupture had their prior low vertical incision performed before 37 weeks' gestation. For low vertical scar disruptions, four of the six prior cesarean deliveries were performed at 38 weeks' gestation or later, and two were performed at 36 and 37 weeks' gestation. The low vertical uterine incision had been closed in two layers for the three women having a uterine rupture in a subsequent trial of labor and also for the three women with detected asymptomatic dehiscences. For the entire low vertical group, closure of the prior low vertical uterine incision was performed in two layers in 77%, in three layers in 3%, and with at least one required additional suture at the upper pole of the incision in 9.4%. The closure method could not be determined from the operative report in 10.5%.
We found no difference in risk of uterine rupture following a trial of labor among women with prior low vertical incisions compared with prior low transverse uterine incisions, even when controlling for multiple confounding factors in a logistic regression analysis. This study has one of the largest number of trials of labor among women with prior low vertical uterine incisions that we were able to identify in the English literature. Our finding of a 0.8% risk of uterine rupture among women with a prior low vertical incision during a trial of labor is consistent with existing literature.4,6 In a recent compilation of case series of trials of labor with a prior low vertical uterine incision, the risk for uterine rupture during a trial of labor was reported to be 1%, based on fewer than 400 trials of labor.6 Of the four prior low vertical–related uterine ruptures in the literature, one was in a parturient with a prior low vertical and then a prior low transverse uterine incision, and one was in a gravida with two prior low vertical incisions.4 The third occurred in a woman with an undocumented uterine scar who was found to have a presumed prior low vertical uterine incision.8 The last was a posterior rupture that occurred in a woman with one prior low vertical incision undergoing a trial of labor.5 Our three ruptures, and all our scar disruptions in the low vertical group, occurred in women who had only one prior low vertical uterine incision, and our rate of uterine rupture was just under 1%. Whereas we also have included those detected asymptomatic dehiscences as a component of all disruptions of the prior uterine scar, this measure is quite unreliable as an indicator of the rate of complication, as it is identified incompletely after successful trials of labor. It is identified easily at cesarean delivery, but it is noted after a vaginal delivery only when a uterine examination is performed. Our definition of a rupture of the prior cesarean scar, a uterine scar disruption that we have defined as symptomatic in some way to parturient or fetus, is the truer measure of morbidity from a trial of labor.
We also compared maternal and neonatal outcome for women with uterine rupture because differences in serious sequelae for mother and infant could result. Among women with uterine ruptures, there was no difference between the two groups in the rate of maternal or perinatal death, and none of the deaths appeared to be related to the trial of labor. Interestingly enough, there were no cases of extrusion of any part of the fetal-placental unit in the low vertical group, although this complication occurred in one-fourth of the uterine ruptures in the low transverse group. Extrusion of the fetus or placenta into the peritoneal cavity is a well-described complication for uterine ruptures with prior vertical uterine incisions extending to the corpus or fundus.9 This complication did not occur in any of the three women with ruptures of a prior low vertical incision in our study. The rates of hysterectomy, oophorectomy, damage to bladder, and transfusion were not statistically different between the low transverse and low vertical groups. However, it is difficult to compare the rate of these rare events because the number of events was small.
The precise definition of a low vertical uterine incision is not clear.10–12 Although any noted uterine incision with extension into the upper contractile portion of the uterus was excluded from this study, a sizable proportion of patients had required closure of the prior uterine incision with more than two layers, or with additional required sutures at the upper pole. We did not exclude these patients from our analysis because they were thought at the time of surgery to have no corpus extension of the uterine incision and therefore met our criteria for a low vertical incision. After the delivery, and with contraction of the uterus, the upper portion of some of these incisions required more than a standard two-layer closure. None of the scar disruptions had prior low vertical incisions that required anything more than the standard two-layer closure. In addition, all of the detected low vertical scar disruptions occurred in women who had their prior cesarean deliveries at gestational ages at which the lower uterine segment should have been well developed. No ruptures occurred in the 87 trials of labor in women whose prior cesarean delivery was performed earlier in gestation. For women with a prior low vertical incision undergoing a trial of labor after cesarean delivery, our data did not suggest an increased risk regardless of the gestational age at which the prior cesarean was performed or the manner in which the incision was closed.
The upper limit of risk considered acceptable for adverse outcome during a trial of labor is a subjective measure unique to each practicing physician and to each gravida planning a delivery after prior cesarean. Although the overall risk of uterine rupture for women with a prior low transverse uterine incision is close to 1%, identified subgroups of these women with higher risks are offered trials of labor routinely. For women with multiple prior cesarean deliveries, or those undergoing oxytocin administration during a trial of labor, the risk for scar disruption during a trial of labor has been reported to range from 1–8%.13–16 In a large study of maternal and fetal outcomes after a prior cesarean delivery involving more than 100 uterine ruptures, the incidence of uterine rupture with two or three prior cesareans is between 2 and 3%.9 For women with prior classical hysterotomy, the risk is even higher, on the order of at least 12%.1 It is generally considered that the risk for women with a prior classical uterine incision is too high to allow a trial of labor, whereas for women with multiple prior cesarean deliveries, the risk is frequently considered acceptable.17 Although we saw no increase in risk of uterine rupture when comparing women with prior low transverse incisions to women with prior low vertical incisions, the potential for a type II statistical error must be considered. We had sufficient power in this study to exclude an increase in uterine rupture rate from 1%, as noted among those with a prior low transverse uterine incision, to 3%, a rate similar to that for a trial of labor among women with multiple prior uterine incisions, or for women receiving oxytocin during a trial of labor.
A limitation of this study bears further discussion. There has been much concern in the literature about the precise definition of a low vertical cesarean incision, specifically in defining the exact boundary of the upper pole of this incision. The subjectivity of this assessment makes conclusions regarding risks related to use of this incision potentially difficult. We sought to exclude any patients with known extension of the uterine incision into the upper contractile portion of the uterus. Undoubtedly, as the data for the study were obtained from the medical record, some patients may have been included if the extension into the upper portion of the uterus was not well documented. Inclusion of these patients in our study could have increased the rupture rate associated with low vertical incisions. The rates, however, of the two groups were the same. In addition, although our numbers are small, we did not see any instances of uterine rupture among women with closure of the prior uterine incision in more than two layers or among women whose prior surgery was done before 37 weeks' gestation when the lower uterine segment may be less well-developed.
This study of labor after cesarean delivery with a low vertical incision suggests that the rate of uterine rupture is comparable to that of women with a prior low transverse incision. The level of uterine rupture (1% and up to 3% given with power calculation) agrees with and is at the lower end of that reported by ACOG (1–7%).18 The clinical recommendation of whether vaginal birth after cesarean is indicated in certain patients is summarized elsewhere.18 Our finding of a similar rate of rupture among women with prior low vertical and low transverse incisions will be useful to women and clinicians making this complex decision.
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© 1999 The American College of Obstetricians and Gynecologists
18. American College of Obstetricians and Gynecologists. Vaginal birth after previous cesarean delivery. ACOG practice bulletin no. 2. Washington, DC: American College of Obstetricians and Gynecologists, 1998.