Six Confusing Facts: What the Large Canadian VBAC Study Really Says

by | May 7, 2018 | Cesarean section, Hospital birth, Inductions, Infant Outcomes, Maternal Outcomes, Using statistics, Uterine Rupture, VBAC | 11 comments

A VBAC study out of Canada was published today which found, “Absolute rates of severe maternal morbidity and mortality were low but significantly higher after attempted vaginal birth after cesarean delivery compared with elective repeat cesarean delivery.”

After reading the abstract, and full text, I could quickly see how this study will be misinterpreted by many, so let me walk you through it.

As I really dislike the terms successful vs. failed VBAC, I’m going to use my own terminology here and I hope you adopt it:

Rather than “Attempted VBAC,” I’m going to use “Labor After Cesarean” (LAC).
Rather than “Failed VBAC,” I’m going to use “Cesarean Birth After Cesarean” (CBAC).
Rather than “Successful VBAC,” I’m going to use just VBAC.

Whenever I read medical studies, there are many questions that I am looking to answer. Four of which are:

What did they measure?
What definitions did they use?
How did they group events?
What was the absolute versus relative risk?

So let’s begin…

1. The study population

This is a very large study including 197,540 women with one prior cesarean and no prior vaginal deliveries, who were pregnant with one baby and delivered at term. Data was pulled from all hospital deliveries in Canada (excluding Quebec) between April 2003 and March 2015.

2. Scar type

The source of this study’s data is the Discharge Abstract Database of the Canadian Institute for Health Information. Surprisingly for a VBAC study, there is no mention of scar type. So it’s unclear if they just included low transverse scars or they included anyone with a prior cesarean.

From how the study reads, I suspect it’s the latter. That could impact outcomes as we know scars that go into the upper part of the uterus have higher uterine rupture rates. So, that is an unknown that we need to pack away in our minds.

3. Induction rate

As we know that induction increases the risk of uterine rupture, this is a good factor to parse out. Young reports that 3.7% of their study population was induced. Unfortunately, they don’t break out any outcomes by induction.

Was there a higher rate of postpartum hemorrhage, hysterectomy, neonatal complications or death among those induced births? We don’t know. Outcomes are grouped by mode of delivery.

4. Defining uterine rupture

I was shocked as I read their findings on uterine rupture. The overall uterine rupture rate for those who attempted VBAC was 0.99%. This is relatively high for a population with one prior cesarean given that only 3.7% of the total population were induced.

Note that the rate of rupture among elective repeat cesareans (ERC) is actually higher than the rate among VBACs. It’s important to remember that while elective repeat cesareans decrease the incidence of uterine rupture, they do not eliminate it.

Uterine rupture

ERC: 1.75 per 1000 (0.17%)
LAC: 9.93 per 1000 (0.99%)
VBAC: 1.30 per 1000 (0.13%)
CBAC: 18.5 per 1000 (1.85%)

It was only when I saw the next section labeled, “Rupture not including dehiscence” did it become clear.

They included full thickness uterine ruptures and typically asymptomatic uterine dehiscences under the label “uterine rupture.” This is how they arrived at such a high overall “uterine rupture” rate.

It is these two events combined that they are comparing throughout the entire study anytime they use the term “uterine rupture.” This is why asking the questions “what did they measure,” “what definitions did they use,” and “how did they group events” is so important.

So when we look at just full thickness uterine ruptures, we can see that the rate among successful VBACs is just one one-hundredth higher than ruptures among ERC:

Rupture not including dehiscence

ERC: 0.50 per 1000 (0.05%)
LAC: 3.3 per 1000 (0.33%)
VBAC: 0.51 per 1000 (0.051%)
CBAC: 6.08 per 1000 (.608%)

Additionally, Young found that 372 repeat cesareans would need to be performed to prevent one uterine rupture. This is an important number to keep in mind as we consider all the risks and benefits of VBAC and repeat cesarean to the birthing parent and infant as well as the birthing parent’s future fertility, pregnancies, deliveries, and children.

5. What did they compare?

Here again we ask the question, “How did they group events?” This study used ERC and LAC as its two comparison groups.

There’s no right or wrong here, but just different ways to slice and dice the data. And in order to interpret this study, you have to be aware of how the authors chose to slice it.

Remember that attempted VBACs includes anyone who planned a VBAC. So those who had a VBAC, as well as those who needed a cesarean during their planned VBAC, are included under the umbrella of attempted VBAC.

Using attempted VBACs as a comparison group is going to result in a greater difference in outcomes than if successful VBACs were used.

This is why: as with any birth, vaginal deliveries are associated with lower overall complication rates than cesareans that occur during labor. This is the same whether someone has a prior cesarean or not.

So it’s not a surprise when you read in the abstract, “Absolute rates of severe maternal morbidity and mortality were low but significantly higher after attempted vaginal birth after cesarean delivery compared with elective repeat cesarean delivery.”

This matters because most people will just read the abstract. And they may or may not catch the word “attempted.” They will just note the phrase “significantly higher” and go from there.

If they were to compare ERC to VBAC, their abstract would tell a different story. Again, no right or wrong or good or bad, just different ways to slice the data that results in a different overall story.

6. Maternal mortality & morbidity

Given how the abstract references “absolute rates of severe maternal morbidity and mortality,” you would think that the rate of maternal mortality would be reported as a separate figure than morbidity. But it’s not.

Which makes it impossible to determine the actual number of maternal mortalities associated with LAC vs. ERC. Past studies have reported the risk of LAC maternal mortality as 1 in 26,000 with a 3.5 increased risk of maternal mortality associated with ERC. (Guise, 2010) Given this study’s ample sample size, it would have been great to add their data to the larger story.

Again, under the category of “what are we measuring,” let’s look at what this study included under maternal morbidity and mortality, “death, acute myocardial infarction, heart failure, pulmonary edema, disseminated intravascular coagulation, cardiac arrest, assisted ventilation, cardiac complications from anesthesia, cardiopulmonary resuscitation, adult respiratory distress syndrome, acute/unspecified renal failure, blood transfusion given PPH, shock procedures to control bleeding given PPH, cesarean hysterectomy given PPH, total hysterectomy open approach given PPH, subtotal hysterectomy open approach given PPH and repair of injury to bladder and urethra.”

Maternal morbidity and mortality

ERC: 5.65 per 1000 (0.56%)
LAC: 10.7 per 1000 (1.07%)
VBAC: 7.14 per 1000 (0.71%)
CBAC: 14.3 per 1000 (1.43%)

But once we remove transfusions due to postpartum hemorrhages, we get what they call “Restricted severe morbidity:”

Restricted severe morbidity

ERC: 4.65 per 1000 (0.46%)
LAC: 7.19 per 1000 (0.71%)
VBAC: 2.49 per 1000 (0.24%)
CBAC: 11.9 per 1000 (1.19%)

The pattern we have seen here has been replicated many times. We see that VBAC has the lowest complication rate followed by ERC, LAC, and CBAC. This study affirms what others have found.

7. Neonatal death and morbidity

Young concluded, “Failed vaginal birth after cesarean delivery was associated with a threefold higher rate of neonatal death.”

That is a statement that gets your attention quick.

So this is when we ask the question of relative versus absolute risk. This sentence gives us the relative risk: how the risks of CBAC compares to ERC. But let’s actually look at the absolute risk: what the numbers are.

Neonatal death

ERC: 0.08 per 1000 (0.008%)
LAC: 0.22 per 1000 (0.022%)
VBAC: <0.18 per 1000 (0.018%)
CBAC: 0.29 per 1000 (0.029%)

Overall, the risk is very low across the board. Yes, it’s lowest with an ERC. But a 0.022% risk of neonatal death with a LAC is still very low.

This is how focusing exclusively on the relative risk can be used by providers to coerce a pregnant woman or unintentionally misinform the public.

And while accurate, it doesn’t give the full story. We always must consider the absolute and relative risk when discussing medical options. Here is another way to communicate absolute risk of neonatal death:

Neonatal death

ERC: 1 in 12,500
LAC: 1 in 4,545
VBAC: Less than 1 in 5,555
CBAC: 1 in 3,448

While these small numbers reflect real babies and real parents experiencing real loss, it is still a pretty rare occurrence. This is what parents pregnant after a cesarean need to know. Yes, the risk is there. But it is low.

In fact, Young found that 9,056 repeat cesareans need to be performed to prevent one neonatal death.

This is another area where there are no right or wrong answers. It’s because the absolute risk is low that many medical organizations including the American College of OB/GYNs and the National Institutes of Health encourage women to have access to VBAC.

The Bottom Line

So, what does this all mean? This study found what many others have before. Yes, there is a higher relative risk with VBAC, but the absolute risks are low.

This is why medical organizations around the world have supported increasing access to VBAC. The challenge is that hospitals and providers can be very slow to follow the evidence and pervasive fear surrounding VBAC persists. This is why it is so important for all of us to know the facts so we can confidently debunk myths one by one.

I hope this quick overview gave you a better understanding of what this study measures and what it all means. As you can see, reading and interpreting medical studies is a skill set of its own. Plus it’s mighty time consuming. And if you get it wrong, it’s not only embarrassing and potentially misleading but it’s your reputation at stake.

This is why I cull through the research and present a monthly literature review as well as online continuing education trainings. If you want to stay on top of the research relative to VBAC, repeat cesarean, hospital policy, and informed consent so you can increase VBAC access in your community, this is the way to do it! Click here to learn more and join our community of VBAC professionals!


Resources Cited

Young, C. B., Liu, S., Muraca, G., Sabr, Y., Pressey, T., Liston, R., & Joseph, K. (2018). Mode of delivery after a previous cesarean birth, and associated maternal and neonatal morbidity. CMAJ, 190, E556-64. Retrieved from

What do you think?
Leave a comment.

What do you think? Leave a comment.


  1. Thank you for your simplified classification:LAC, VBAC, CBAC, ERC, but don’t you think we should just be looking at the morbidity and mortality of ERC AND LAC, VBAC is successful LAC, and CBAC is failed LAC.

  2. As I understand medical studies of interventions, the gold standard is to use ‘Intention to Treat’ – meaning all patients that were originally in the study and dropped out for whatever reason. Thus, the inclusion of all women that attempted a VBAC is methodologically appropriate.

  3. I won’t comment at length the study recently published on VBAC, because I just read the abstract, a few minutes ago. But it reminds me of the study done by Lydon-Rochelle published in 2001,, on uterine rupture rates according to different situations related to VBAC. The title about that article in the newspaper I read was something along the lines that ‘VBAC is dangerous’ , which the journal editorial emphasized also. When one read the article, though, one realized that what was dangerous was the induction of VBAC and that should have been the focus of the article, its’ title, etc.. Given the alarming induction rates in labour and births in our country, I have trouble believing that only 3.7 % of VBAC women were induced. This is happening close to 20 years after Lydon-Rochelle study. And how about the approach used in these VBAC labors ? medicalized or centered on physiology and natural rythms ? Etc. Let’s read the article before commenting more that recent study.

  4. Thanks for taking the time to break down the stats! I agree that we need to provide good information in a context that patients can understand. Yes, the relative risk is higher, but it is still exceedingly small. That helps patients decide what risk they are comfortable accepting.

  5. I’m interested in how the LAC and CBAC numbers contribute to an overall understanding of risk. The LAC numbers could fall under either VBAC or CBAC as they used an intention to treat analysis. The CBAC numbers all fall under the planned vaginal birth group.

    I’m not sure that comparing VBAC to ERC is the way to go as those higher risk categories are enveloped in the CBAC group and one only knows in retrospect which category one will fall into. This is not an argument to your analysis. Just a thought about discussing risk.

    I think that the comparator group in the this study creates bias in the analysis. Any possible morbidity and mortality related to vaginal birth will be captured in the planned vaginal birth cohort and missed in the ERC cohort. Thus skewing the results. An option would be to compare neonatal outcomes with ERC and planned vaginal birth compared to outcomes with first vaginal births. At least in the discussion it would be nice to also acknowledge that long term outcomes for the caesarean born babies might not be as good.

    Finally, composite outcomes are really misleading. Especially when they are not broken down by risk factor as you mentioned in your piece. Some of the morbidities crossed one. Not statistically significant. And the most common morbidities did not include patient relevant outcomes. For example, if one receives a blood transfusion and leaves hospital feeling fine and not recovering from surgery than this “morbidity” becomes less compelling.

    Anyway, great to have to jump on the analysis of this!

    • I wonder how these maternal and infant deaths compare to those who had normal vaginal deliveries?

  6. Thank you for reviewing this and giving us all “another way” to look at the data. I will be sure to share your review with our medical staff.

  7. Also – the devil is in the details. More needs to be known about the hospitals where LAC was taking place. How attentive and skilled was the nursing care? How available was the obstetrician and the OR? I work in a hospital where everyone is on it and our results are shining.

  8. Thank you for breaking down the statistics for us to see! It is so important to be informed so we can make these tough decisions!

  9. …Also, how many of the labors were augmented (or are you including that under “induced”)?

    • Great question Jill. No mention of augmentation. Just induction. I’m not sure if that isn’t indicated on their source data or if they just didn’t include it. I suspect the former.



  1. Wednesday Wrap Up May 2018 – Andrea Lythgoe Doula Salt Lake City Utah - […] Kamel of VBAC Facts looks at the recent Canadian study on VBAC. I appreciate her outlining her approach to…

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Jen Kamel

As a nationally recognized consumer advocate and Founder of VBAC Facts®, Jen Kamel helps birth professionals, and cesarean parents, achieve clarity on vaginal birth after cesarean (VBAC) through her educational courses, training programs, and consulting services. She speaks at conferences across the country, presents Grand Rounds at hospitals, advises advocates seeking legislative change in their state, and serves as a expert witness in legal proceedings. She envisions a time when every pregnant person seeking VBAC has access to unbiased information, respectful providers, and community support, so they can plan the birth of their choosing in the setting they desire.

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