Understanding VBAC Success Factors: What the Evidence Tells Us

If you're pregnant after a previous cesarean birth, you're likely weighing your options carefully. Should you plan for a repeat cesarean, or is a vaginal birth after cesarean (VBAC) right for you? One of the most common questions we hear is: "What are my chances of having a successful VBAC?"

The good news is that researchers have studied this question extensively, and we now have a solid body of evidence to help guide your decision-making process. In this article, we'll walk through the factors that influence VBAC success rates and help you understand what the research actually says—so you can have informed conversations with your care provider and make the choice that's right for you.

What Is VBAC, and What Is TOLAC?

Before we dive into the evidence, let's clarify some terminology. VBAC stands for vaginal birth after cesarean—this refers to a successful vaginal delivery following a previous cesarean birth. TOLAC stands for trial of labour after cesarean, which means attempting a vaginal birth rather than scheduling a repeat cesarean. Not every TOLAC results in a VBAC; some people who attempt vaginal birth will ultimately need a cesarean during labour.

Understanding this distinction matters because when we talk about "success rates," we're really asking: of the people who attempt a TOLAC, how many achieve a VBAC?

Overall VBAC Success Rates

Research consistently shows that VBAC success rates are higher than many people expect. According to large studies and systematic reviews, approximately 60 to 80 per cent of people who attempt a TOLAC will have a successful vaginal birth. The American College of Obstetricians and Gynaecologists (ACOG) cites success rates of 60 to 80 per cent for appropriate candidates, with some studies showing rates as high as 85 percent in certain populations.

These numbers are encouraging, but they represent averages across diverse groups of people. Your individual likelihood of success depends on several factors, which we'll explore below.

Factors That Increase the Likelihood of VBAC Success

Previous Vaginal Birth

One of the strongest predictors of VBAC success is whether you have ever had a vaginal birth—either before or after your cesarean. Research shows that people with a history of vaginal delivery have significantly higher success rates, often exceeding 85 to 90 per cent. A vaginal birth demonstrates that your pelvis has accommodated a baby before, and this prior experience is associated with more favourable outcomes.

If your previous vaginal birth occurred after your cesarean (meaning you've already had a successful VBAC), your chances of another successful VBAC are even higher. Studies suggest success rates above 90 per cent in this group.

Reason for Your Previous Caesarean

Why you had your first cesarean matters, research indicates that people whose previous cesarean was performed for a non-recurring reason—such as breech presentation, placenta previa, fetal distress, or cord prolapse—tend to have higher VBAC success rates than those whose cesarean was for "failure to progress" or "cephalopelvic disproportion."

This makes intuitive sense. If your previous cesarean happened because the baby was in an unfavourable position rather than because of labour complications, there's no reason to assume the same situation will occur again. However, it's worth noting that even people with a prior cesarean for failure to progress still have reasonable success rates, often around 50 to 60 per cent or higher, depending on other factors.

Spontaneous Labour Onset

How your labour begins plays a significant role in VBAC outcomes. Evidence consistently shows that spontaneous labour is associated with higher success rates compared to induced labour. When your body initiates labour on its own, it suggests that your cervix is ready and that the hormonal cascade supporting labour is already underway.

Studies have found that VBAC success rates with spontaneous labour can be 10 to 20 percentage points higher than with induction. This doesn't mean induction should be avoided in all cases—sometimes induction is medically necessary—but it's an important factor to discuss with your provider when considering timing and labour management.

Cervical Status at Admission

If you present to the hospital already in active labour with a dilated and effaced cervix, your chances of VBAC success are higher than if you arrive in early labour. Research shows that cervical dilation of 4 centimetres or more at admission is associated with improved outcomes. This finding aligns with what we know about labour progress generally—being further along when you arrive suggests your body is responding well to labour.

Body Mass Index

Studies have found an association between body mass index (BMI) and VBAC success rates, with higher BMI linked to somewhat lower success rates. However, this relationship is more nuanced than it might appear. Many people with higher BMIs have successful VBACs, and BMI alone should not be used to discourage someone from attempting TOLAC.

It's also important to acknowledge that weight bias in healthcare can affect how providers counsel patients and how they manage labour. The goal should be individualised care that considers the whole person, not decisions based solely on a number on the scale.

Maternal Age

Some research suggests that younger maternal age is associated with slightly higher VBAC success rates. However, the differences are modest, and many people over 35—and even over 40—have successful VBACs. Age alone is not a contraindication to TOLAC, and decisions should be based on a comprehensive assessment of individual factors.

Interpregnancy Interval

The time between your cesarean birth and your next pregnancy has been studied as a potential factor in both VBAC success and uterine rupture risk. While some research suggests that a very short interval (less than 18 to 24 months between delivery and next conception) may be associated with slightly lower success rates and marginally higher rupture risk, the absolute differences are small. Most professional organisations do not set a strict minimum interval for TOLAC candidacy.

Factors That May Decrease the Likelihood of VBAC Success

Induction of Labour

As mentioned above, induction is associated with lower VBAC success rates compared to spontaneous labour. The method of induction also matters. Mechanical methods, such as Foley balloon catheters, are generally considered safer for VBAC candidates than prostaglandin medications like misoprostol, which have been associated with increased uterine rupture risk and are typically avoided in people with a uterine scar.

If induction is recommended, having a thorough conversation with your provider about the reasons for induction, the methods that will be used, and how this might affect your VBAC goals is essential.

Labour Augmentation with Oxytocin

The use of synthetic oxytocin (Pitocin) to strengthen or speed up labour has been studied in the context of VBAC. High doses of oxytocin have been associated with increased uterine rupture risk, likely because stronger, more frequent contractions place more stress on the uterine scar. When oxytocin is used during TOLAC, careful dosing and monitoring are important considerations.

No Prior Vaginal Delivery

While people without a prior vaginal birth can absolutely have successful VBACs, the absence of this history is associated with somewhat lower success rates—typically in the 60 to 70 per cent range rather than the higher rates seen in those with prior vaginal births.

Previous Cesarean for Failure to Progress

As noted earlier, having a prior cesarean due to labour dystocia (failure to progress) is associated with lower success rates compared to non-recurring reasons for cesarean. The concern is that whatever factors contributed to the previous labour stalling—whether related to the pelvis, the baby's position, or uterine contractility—might recur. However, many variables differ between pregnancies, including baby size, baby position, and the person's overall health, so a prior cesarean for this reason does not preclude a successful VBAC.

Increased Maternal Weight Gain

Excessive weight gain during the current pregnancy has been associated in some studies with decreased VBAC success. This may be related to estimated fetal size or other metabolic factors. As with BMI, this should be one piece of information among many rather than a deciding factor.

What About VBAC Prediction Calculators?

You may have heard of VBAC prediction calculators or scoring systems designed to estimate your individual probability of success. The most well-known is the Maternal-Fetal Medicine Units (MFMU) Network calculator, developed from a large cohort of people attempting TOLAC.

These tools take into account factors such as maternal age, BMI, prior vaginal delivery, prior VBAC, and reason for previous cesarean to generate a percentage likelihood of success. While they can be useful conversation starters, they have significant limitations.

First, these calculators were developed using specific populations and may not accurately predict outcomes for everyone. Second, they cannot account for many important variables, such as the quality of labour support, the patience of your care provider, or hospital policies that might influence how your labour is managed. Third, some versions of these calculators have included race as a variable, which reflects systemic inequities in care rather than biological differences—a practice that has been rightfully criticised.

If your provider uses a VBAC calculator, it's reasonable to ask how the results should be interpreted and what factors the tool does and does not consider. A lower predicted success rate does not mean VBAC is impossible, and a higher rate does not guarantee success.

The Importance of Provider and Birth Setting

One factor that significantly influences VBAC outcomes—but is often underemphasized—is the support you receive from your care provider and birth setting. Research has shown wide variation in VBAC rates between hospitals and between individual providers, even when accounting for patient characteristics. This suggests that institutional culture, provider philosophy, and labour management practices play a meaningful role.

Choosing a provider who is genuinely supportive of VBAC and has experience attending vaginal births after cesarean can make a substantial difference. Questions to ask include: What is your VBAC success rate? How do you typically manage labour for TOLAC patients? Under what circumstances would you recommend moving to a cesarean? Are there hospital policies that might affect my labour?

Continuous labour support from a doula has also been shown in research to improve birth outcomes generally and may be particularly valuable for people attempting VBAC, both for physical support during labour and for advocacy in the birth space.

Making an Informed Decision

Understanding your individual likelihood of VBAC success is just one component of decision-making. Equally important is understanding the risks and benefits of both TOLAC and planned repeat cesarean—including short-term outcomes, long-term implications for future pregnancies, and what matters most to you personally.

For many people, attempting a vaginal birth is important for their physical recovery, emotional well-being, or future fertility plans. For others, the predictability of a scheduled cesarean or concerns about uterine rupture may lead them to choose a repeat surgical birth. Neither choice is inherently right or wrong; what matters is that you have accurate information and feel empowered in your decision.

The Bottom Line

VBAC success rates are generally favourable, with the majority of people who attempt TOLAC achieving a vaginal birth. Factors associated with higher success include prior vaginal birth, spontaneous labour onset, a non-recurring reason for the previous cesarean, and a favourable cervical status at admission. Factors associated with lower success include induction, no prior vaginal delivery, and a previous cesarean for failure to progress.

Prediction calculators can provide a rough estimate of individual success probability but have significant limitations and should not be the sole basis for decision-making. Perhaps most importantly, the support you receive from your care provider, your birth team, and your birth setting can meaningfully influence your experience and outcomes.

If you're considering VBAC, we encourage you to seek out evidence-based information, ask questions, and find a provider who respects your autonomy and supports your goals. You deserve to make this decision from a place of knowledge and empowerment.