Commentary
Lessons learned from the history of VBAC
As VBACs became more common, the approach to the procedure became more relaxed. VBACs went from only being performed in tertiary care hospitals...
Failed trials of labor resulting in repeat cesarean deliveries have consistently been associated with higher morbidity than scheduled repeat cesarean deliveries, with the greatest difference in rates for ruptured uterus. In the first MFMU Network study, there were no cases of uterine rupture among a cohort of 15,801 women who underwent elective repeat cesarean delivery, and in the second multicenter study of 25,000 women, this patient group had a rupture rate of 0.004%.
Yet, as ACOG points out, neither elective repeat cesarean deliveries nor TOLAC are without maternal or neonatal risk. Women who have successful VBAC delivery, on the other hand, have significantly lower morbidity and better outcomes than women who do not attempt labor. Women who undergo VBAC also avoid exposure to the significant risks of repeat cesarean deliveries in the long term.
Research unequivocally shows that the risk of placenta accreta, hysterectomy, hemorrhage, and other serious maternal morbidity increases progressively with each repeat cesarean delivery. Rates of placenta accreta have, in fact, been rising in the United States – a trend that should prompt us to think more about TOLAC.
Moreover, TOLAC is being shown to be a cost-effective strategy. In one analysis, TOLAC in a second pregnancy was cost-effective as long as the chance of VBAC exceeded approximately 74% (Obstet Gynecol. 2001 Jun;97[6]:932-41). More recently, TOLAC was found to be cost-effective across a wide variety of circumstances, including when a woman had a probability of VBAC as low as 43%. The model in this analysis, which used probability estimates from the MFMU Cesarean Registry, took a longer-term view by including probabilities of outcomes throughout a woman’s reproductive life that were contingent upon her initial choice regarding TOLAC (Am J Perinatol. 2013 Jan;30[1]:11-20).
Likelihood of success
Evaluating and discussing the likelihood of success with TOLAC is therefore key to the counseling process. The higher the likelihood of achieving VBAC, the more favorable the risk-benefit ratio will be and the more appealing it will be to consider.
According to one analysis, if a woman undergoing a TOLAC has at least a 60%-70% chance of VBAC, her chance of having major or minor morbidity is no greater than a woman undergoing a planned repeat cesarean delivery (Am J Obstet Gynecol 2009;200:56.e1-e6).
There are several prediction tools available that can be used at the first prenatal visit and in early labor to give a reasonably good estimate of success. One of these tools is available at the MFMU Network website (http://mfmu.bsc.gwu.edu). The tools take into account factors such as prior indication for cesarean delivery; history of vaginal delivery; demographic characteristics such as maternal age and body mass index; the occurrence of spontaneous labor; and cervical status at admission.
Prior vaginal delivery is one of the strongest predictors of a successful TOLAC. Research has consistently shown that women with a prior vaginal delivery – including a vaginal delivery predating an unsuccessful TOLAC – have significantly higher TOLAC success rates than women who did not have any prior vaginal delivery.
The indication for a prior cesarean delivery also clearly affects the likelihood of a successful TOLAC. Women whose first cesarean delivery was performed for a nonrecurring indication, such as breech presentation or low intolerance of labor, have TOLAC success rates that are similar to vaginal delivery rates for nulliparous women. Success rates for these women may exceed 85%. On the other hand, women who had a prior cesarean delivery for cephalopelvic disproportion or failure to progress have been shown to have lower TOLAC success rates ranging from 50%-67%.
Labor induction should be approached cautiously, as women who undergo induction of labor in TOLAC have an increased risk of repeat cesarean delivery. Still, success rates with induction are high. Data from the MFMU Cesarean Registry showed that about 66% of women undergoing induction after one prior cesarean delivery achieved VBAC versus 76% of women entering TOLAC spontaneously (Obstet Gynecol. 2007 Feb;109[2 Pt 1]:262-9). Another study of women undergoing induction after one prior cesarean reported an overall success rate of 78% (Obstet Gynecol. 2004 Mar;103[3]:534-8).
Whether induction specifically increases the risk for uterine rupture in TOLAC, compared with expectant management, is unclear. There also are conflicting data as to whether particular induction methods increase this risk.
Based on available data, ACOG considers induction of labor for either maternal or fetal indications to be an option for women undergoing TOLAC. Oxytocin may be used for induction as well as augmentation, but caution should be exercised at higher doses. While there is no clear dosing threshold for increased risk of rupture, research has suggested that higher doses of oxytocin are best avoided.