The principal finding of this study is that women with a PVD were 5 times less likely to experience uterine rupture during TOLAC, as compared with women who had not experienced a PVD. Moreover, the presence of a vaginal delivery in the obstetrical history of a women attempting TOLAC was the single and most important independent protective factor from uterine rupture. These parturients were also significantly more likely to experience a successful VBAC as compared to their counterparts with no such history. Importantly, overall rates of uterine rupture during TOLAC are low (0.5%), and yet, safety issues which cause patients as well as medical professionals to refrain from considering TOLAC, arise every day in the clinical milieu and led to a decline in TOLAC rates in the U.S to a nadir of 16% in 2010 [1,16,17]. For the patients, this kind of “anti-TOLAC” policy exposes them to the risks associated with repeated CDs in the short and long term, and overlooks the benefits VBAC offers, for both mother and offspring [14,18].
A recent meta-ethnographic review of women’s birth choices after CD by Black and colleagues [17] categorized women to 3 decision groups; predetermined for elective re-cesarean section (ERCS), predetermined for VBAC, and those with an “open minded” approach. As factors influencing decision making are various and derive from cultural, social and environmental influences in all three groups, women in the open-minded approach group sought and relied on facts and professional advice communicated by their healthcare professionals. In these cases, the health care professionals personal view and ability to assess the individual risk, were crucial in the decision-making process.
Several previous studies addressed the issue of the magnitude of PVD as a protective factor from uterine rupture, revealing mixed results. In concurrence with our results, Zelop et al [12] reported a compelling risk reduction to one fifth the risk of uterine rupture in these women, as compared to women with no prior VD. However their cohort included a relatively small number of parturients (1000), considering the rarity of uterine rupture.
Similar findings were reported by Shimonovitz et al [13] who demonstrated a statistically significant risk reduction for uterine rupture during TOLAC after a previous VD based on approximately 5000 women attempting TOLAC, between the years 1980-1997. The authors reported an incidence of uterine rupture of 0.59% for all women attempting TOLAC. The vast majority (81%) of the 26 women who experienced uterine rupture, were attempting their first TOLAC, and the risk decreased dramatically for women with a prior successful VBAC. Although these findings are derived from a larger cohort, the study was based on data from over 30 years ago.
A different conclusion was published by Grobman et al. [14] who evaluated the success rates of labor induction in women with and without PVD. The authors concluded that women without PVD were at a greater risk for uterine rupture only if their labor involved induction. Women in the comparison group (no PVD) delivering spontaneously, did not differ in the risk for uterine rupture.
Two prospective studies by Landon et al [19,20] showed association between TOLAC and increased risk for uterine rupture, although the absolute risk was low in total. It was noted that higher risk for uterine rupture was associated with labor augmentation (0.9%, OR 2.42) and the highest risk was associated with labor induction (1.0%, OR 2.86). No sub analysis was made in these studies to assess uterine rupture risk in the context of PVD history.
Hendler et al [21] showed that although PVD taking place before and after the previous CD was associated with higher rates of TOLAC success, it was also a significant risk factor for uterine scar dehiscence. The authors hypothesized that VD causes uterine scar stretching that inclines the scar to dehisce during TOLAC in a subsequent pregnancy. However, the authors claimed that the higher rate of dehiscence does not necessarily translate into a higher rate of uterine rupture.
There are scarce data regarding the role of previous vaginal delivery timing in relation to the prior CD. A recent study by Atiya et. al [22] addressed an important question regarding the risk of uterine rupture with regards to the timing of the previous vaginal delivery ; before the CD or after (i.e. a history of VBAC). They showed that prior VBAC was associated with higher rates of TOLAC success and a reduced risk of uterine rupture. Interestingly, they also showed that women with a history of a vaginal delivery prior to their CD (and no VBAC) had similar uterine rupture rates compared to women without any vaginal delivery (before or after the CD ), and were 5 times more likely for uterine rupture as compared to the prior VBAC group. This surprising result supports the notion that a proven scar is a protecting factor from uterine rupture, but questions the protective effect of a history of a vaginal birth before the CD on uterine rupture. It is important to note that in their study, women with prior vaginal delivery were significantly more likely to be diabetic, deliver macrosomic babies, have low bishop scores upon admission and were more likely to be augmented by Oxytocin - all of which are possible confounders that were not controlled for. Also, their prior CD was significantly more likely to be due to arrest of descent.
Our findings strongly support PVDs' role as a predominant protective factor from uterine rupture during TOLAC. We show that although women in the no-PVD group were younger, less likely to have gestational diabetes and to undergo labor induction, and with smaller newborns, all of which are considered favorable and protective factors, [20,21,23,24], they were still prone to fail TOLAC and had 5 times higher chances for uterine rupture.
A possible explanation for our findings may be that women with no PVD, much like nulliparous women, experience longer labors and use epidural analgesia more often. The prolonged duration of contractions and the relatively slow progress in these women, puts them in greater risk for uterine rupture. This concept was described by Omole-Ohonsi and colleagues [25] in the context of a successful VBAC. The authors showed that the cervical dilatation rate was in accordance with successful VBAC – as progress rate was higher, so were the chances for a successful VBAC. Others have also concluded that labor dystocia during TOLAC, and specifically during the later stages of labor, may be a sign of impending uterine rupture, and warrant intensive monitoring and frequent examinations [26].
The strengths of this study includes its large cohort, being one of the largest to date addressing this issue, and the high-quality data, collected from two tertiary centers with uniformity of labor and TOLAC management protocols and documentation.
Our study has a few limitations. First, the retrospective nature of the data with its inherent faults. Another limitation is that the data was collected from two medical centers characterized by a population motivated toward having large families and avoiding CD, and so, our findings may not be generalizable. Lastly, we had no access to data regarding the timing of the PVD and its relation to the previous CD (i.e. before or after the previous CD). These data could have enhanced the precision of the counseling provided. Certainly, future studies should address these issues.
To summarize, our findings highlight the overwhelming importance of PVD in predicting the course of an attempted TOLAC, in a combined effort to responsibly and safely raise TOLAC rates and reduce re-CD rates. Our data contributes to both patients and healthcare providers in the challenging decision-making process and strongly supports the feasibility and safety of TOLAC in general, and in women with a history of prior vaginal delivery in particular.