Six Confusing Facts: What the Large Canadian VBAC Study Really Says
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.
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 deaths 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.
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:
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. 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 so you can increase VBAC access in your community, join the VBAC Facts Membership for Professionals. Click here to learn more.
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As a nationally recognized consumer advocate and Founder of VBAC Facts®, Jen helps perinatal professionals, and cesarean parents, achieve clarity on vaginal birth after cesarean (VBAC) through her educational courses for parents, online membership for professionals, continuing education trainings, 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.