Our study reported that the main factors affecting the success rate of TOLAC included parity, pre-pregnancy BMI, Cervix Bishop score, past vaginal delivery history and neonatal birth weight. These factors were of great value in predicting successful vaginal delivery. According to the logistic regression model, the nomogram model was constructed from these five factors. The AUC in the prediction and validation models was 0.815 (95% CI 0.762–0.854) and 0.730 (95% CI 0.652–0.808), respectively, which demonstrated that the nomogram prediction model had predictive value for successful vaginal delivery and a certain degree of accuracy.
A review reported that TOLAC is safe and feasible for most women with a history of cesarean section. The success rate of vaginal trial delivery is more than 75%, and the serious complications are less than 1% [13]. In a study conducted in Japan, there were 1532 women who tried TOLAC successfully gave birth with success rate of 88.6%, and 8 cases (0.46%) of uterine rupture [14]. Moreover, a very recent prospective study showed that a big proportion of patients (64.1%) chose TOLAC in Japan, and the final success rate was 91.3% with remarkably low uterine rupture rate (0.6%) [15]. The TOLAC success rate reported in our study was 76.48%, which was just moderate. When compared with the TOLAC failure group, the patients who successfully performed TOLAC had less blood loss and blood transfusion rate, and higher 1-min APGAR score. In general, TOLAC is potential strategy for decreasing the cesarean section rate and successful trials would reduce some important adverse outcomes.
Accordingly, one of the major concerns when conducting TOLAC is uterine rupture. A study has reported that the incidence of uterine rupture in patients undergoing TOLAC with a transverse incision in the lower uterus ranged from 0.5 to 0.9% [16]. It was generally suggested that the risk of uterine rupture is related to the thickness of the scar in the lower uterus. The continuity of the healing of the lower uterine myometrium has been proposed to more accurately predict whether the lower uterus will rupture. Since the current measurement of scar thickness and continuity standards are not uniform, the accuracy of B-ultrasound in evaluating the thickness and continuity of uterine scar myometrium is still controversial. In this study, the thickness of uterine scar was not significantly related to the success of TOLAC. The possible reason would be that if the thickness of the uterine scar of pregnant women was too low and not suitable for TOLAC, it would not be included in the study. Therefore, the thickness of the uterine scar was unlikely to be a factor affecting the outcome of TOLAC in our study. Other factors, such as advanced cervical opening, effacement, gravidity, parity, and prior vaginal delivery were also associated with successful vaginal birth [17]. In our study, the main factors related to the success of TOLAC were parity, pre-pregnancy BMI, cervical score, past vaginal delivery history, and neonatal birth weight.
The success rate of TOLAC is suggested closely related to the neonatal birth weight. Studies have confirmed that fetal weight is of high value in predicting TOLAC’s success [18, 19]. The larger the weight of the fetus, the lower the success rate of vaginal trial delivery. The reason may be that when the fetus is too heavy and the fetal head would be blocked, it may cause excessive traction of the lower uterine fibers, resulting in incomplete or complete rupture of the muscle layer of the lower uterus. This would then lead to failure of the vaginal trial. A previous study pointed out that compared to a neonate with a normal weight, the cesarean section rate of pregnant women with a baby weighing of more than 3450 g increased by 3 times, and the probability of VBAC was reduced by 50% for those with a neonatal weight of more than 3700 g [17]. Similarly, according to the results in our study, for pregnant women who are planning to undergo TOLAC, weight management can be carried out during pregnancy to keep the fetal weight within 3300 g.
Here, we found that the pre-pregnancy BMI of pregnant women in the TOLAC successful group was obviously smaller than that in the TOLAC failure group. After logistic regression analysis excluding the influence of confounding factors, pre-pregnancy BMI was still reported to had a significant impact on the success of TOLAC (OR = 2.40, 95% CI 1.40–4.14). Interestingly, there was no significant difference in weight gain during pregnancy between the two groups. Whether weight gain during pregnancy affects TOLAC is currently unclear. Previous study has provided evidence that weight gain during pregnancy and maternal BMI both associate with successful VBAC [20, 21]. Our results were consistent with a previous retrospective cohort study which emphasized that excessive weight gain during pregnancy was not a risk factor for failed TOLAC, even in obese patients [22]. Therefore, more studies are needed to confirm whether weight gain during pregnancy will affect the success rate of TOLAC.
A satisfactory prediction model could be clinically important to identify women with greater opportunities of a successful TOLAC. In a previous study, a nomogram model established had good performance at the high estimated probability of successful TOLAC for about 93% of women with an estimated ≥ 90% having a vaginal birth [21]. Although the probability in our study was relatively low, the TOLAC prediction model constructed here was useful to terminate the pregnancy in a timely manner, and to monitor closely during the labor process, which would increase the success rate of TOLAC and reduce the maternal and fetal complications related to cesarean section.
Some of the study limitations were that it was only a single-center study, and the scope of the survey was relatively narrow, which affected the extrapolation of the prediction model and may not reflect the actual situation of the Chinese population. Second, this study was a retrospective study, which will inevitably lead to the lack of some data analyses. In addition, the limited sample size may affect the extrapolation of the results of this study to a certain extent. Despite these limitations, there were a number of strengths associated with the study. First, this study conducted single-factor and multi-factor regression analysis on various factors that may affect TOLAC results, and identified several factors that had an impact on TOLAC results. By intervening these factors, the success rate of TOLAC can be improved clinically. In addition, a visual model has been constructed in our study, which can be tested in clinical practice. Also, this will provide a basis for future studies to explore the usefulness of the visual model in clinical practice.