Is the VBAC Success Calculator Accurate?

Updated: August 2024

If you are planning your VBAC, it’s likely you’ve heard of a VBAC calculator. Especially in the last few years, the idea of a VBAC calculator is a hot topic in the birth world and for good reason. . There is outdated evidence, updated evidence and conflicting evidence surrounding its accuracy and use for those attempting a trial of labor after Cesarean (TOLAC). 

While there may be various kinds of VBAC calculators in circulation, the most widely used is this one created (and recently updated) by The Maternal Fetal Medicine Units Network (MFMU). This VBAC success calculator uses statistics from more than 11,000,000 VBAC attempts in the United States between 1999 and 2002 to determine how likely YOU are to have a VBAC.

If you have listened to our VBAC podcast at all, you know we have a strong opinion on the VBAC score provided by these calculators.  We hope this article provides an overview of what you should know about the VBAC success rate and, most importantly, what else to consider when calculating your personal decision to VBAC. 

Factors included in your VBAC score 

One of the biggest reasons we struggle to support use of the VBAC calculator is that it is against women of size and reduces your score based on medical factors that might not even be true. Providers who use the VBAC calculator to make a decision about your ability to VBAC and / or their willingness to support you are making that decision based on a small piece of the puzzle. That being said, one very positive thing to note is that the VBAC calculator was revised in 2021 to remove race and ethnicity. Previously, the calculator showed lower probabilities for those identifying as Black or Hispanic.

However, according to the updated calculator, your score of having a successful VBAC is impacted by the following

• Having a BMI over 30
• Are over 35 years old
• Have not had any prior vaginal deliveries
• Have a Cesarean that was labeled failure to progress or failure to descend

It’s important to note that the VBAC calculator data only accounts for those with one Cesarean and a low transverse incision. Right off the bat, those considering VBA2C, VBA3C or those with special scars are excluded. 

Even with the updated criteria, research shows that the calculator predicts your ability to VBAC comparably to the original model. Essentially, you should know that this calculator has limitations and should not be the sole decision-maker regarding a woman’s decision to attempt a VBAC. 

A more important thing to do is to have an educated conversation with your provider about your medical history, past birth experiences, your incision type and plans for your family size. 

One of my favorite VBAC stories is of my first doula client, who was given a 4% (yes FOUR) chance of success by her provider using the VBAC calculator. She ended up pushing her baby out in twenty minutes.

Curious to see what your success rate is? Play around with the VBAC calculator and adjust different variables to see how it impacts your “chances” of VBAC. I also posted a video, the exact one we use in our VBAC course for parents, playing around with the calculator so you can see its inherent flaws. Watch it here on YouTube.

ACOG and the VBAC Calculator

ACOG Practice Bulletin 205  from 2010 discusses using the VBAC calculator for providers when counseling patients who want a VBAC:

A VBAC calculator may be used to provide more specific information about the chance of VBAC, which can be used to further the process of shared decision making.” (emphasis added)

The ACOG bulletin also focuses on how population-based statistics cannot accurately predict an individual’s VBAC success odds. It goes on to say:

It is ill-advised to use statistics as a primary indicator when making VBAC decisions.

and

A predicted success rate of less than 70% is not a contraindication to TOLAC. The decision to attempt TOLAC is a preference-sensitive decision, and eliciting patient values and preferences is a key element of counseling.

In a more updated article on ACOG’s website from September 2023, they refer to VBAC calculators as “not perfect” and offering only an estimate of your chances of success. Furthermore, “Remember, if you and your ob-gyn use a VBAC calculator, the results should be discussed along with your goals and the benefits and risks of a VBAC for you.”

Is my VBAC score accurate? 

In 2019, a cohort study was performed to see if predicted rates for VBAC were accurate. Are prediction models for vaginal birth after cesarean accurate?. 

While I know the calculators were more likely to predict those who WOULD achieve a VBAC vs. those who would not, I was surprised to see the data. Let’s break it down. Your VBAC score can either be a number (0-25) or, a percentage (out of 100%).

I absolutely love these charts from the AJOG journal, which break down the numbers from the study referenced above using both the score-based and percentage-based calculators. As you look at the numbers, pay close attention to the “Calculated VBAC Score” and compare it to the “Actual VBAC Success Rate.” 

While the number of participants is small, and more research still needs to be done (especially research using the updated version of the calculator), you can see how inherently flawed these numbers can be. It saddens me to think that many parents have not been given a chance for TOLAC because of a lower VBAC score.

Putting VBAC score in perspective 

Of particular interest, we think, is a literature review published in 2023 that included provider perceptions on the use of the VBAC calculator. This review found that many providers find the calculator to be inaccurate and underestimating VBAC success. 

One critical conclusion is that there is no single right answer on estimating the probability of any individual attempting VBAC. Even people with similar predicted success rates will make different decisions, have different providers, and face different birth circumstances. 

The most critical factors to consider when choosing between VBAC or repeat Cesarean are

• A VBAC friendly doctor or midwife who truly supports you
• A supportive birth location

Some providers promise to support your VBAC, and as the due date approaches, they change their tune, restrict your options, or come up with reasons to schedule a repeat Cesarean. They may suggest reasons such as your baby being too big, testing positive for gestational diabetes, not going into labor by 40-weeks or any other non-evidence-based reason. 

Having a supportive provider and birthing at a VBAC friendly location makes a more significant impact on your VBAC success chances than any other factor. 

VBAC Calculator FAQ’s 

The VBAC calculator can be difficult to wrap your head around if you have a lower than expected score. For my first VBAC, I had a 42% VBAC score. I had a VBAC, then two more after that. Even now, my predicted success rate is just 72% because I am older and slightly fluffier around the edges. 

How accurate is the VBAC calculator?

While VBAC calculators attempt to predict your odds of having a vaginal birth, they have many flaws. They should not be considered the ultimate decision-maker when deciding how to birth. 

What is a good VBAC score?

Some believe that a VBAC score of 60%-70% or higher is a good score to allow a trial of labor. However, up to 80% of parents who choose to attempt a VBAC will succeed, regardless of their score. 

Is VBAC considered high risk?

VBAC is generally considered safer than a repeat Cesarean and is not regarded as high risk. 

Who is a good candidate for VBAC?

Most women are great candidates for VBAC. Finding a provider that is VBAC supportive will ensure you are set up for success. 

If you want to learn everything you can about VBAC, so you can make informed birth decisions every step of the way, check out our VBAC Course for Parents.