Digesting VBAC Stats: Know What You’re Eating

Chocolate by André Karwath aka Aka – Own work. Licensed under CC BY-SA 2.5 via Wikimedia Commons – httpcommons.wikimedia.orgwikiFileChocolate.jpg#mediaviewerFileChocolate.jpg

If you’re a South Australian mother researching VBAC, chances are that you’ve stumbled across the Pregnancy Outcome Unit’s ‘Pregnancy Outcome in South Australia’ reports. (If you haven’t already, you just did.) Seriously, if you love stats which are actually *relevant to a local context*, these publications are to the World of VBAC Stats what Pana chocolate is to the world of cocoa-based confectionery. (And, if you haven’t yet stumbled across Pana chocolate, make sure you do – it will help you stay up late reading stats.)

Whet your appetite!

Now, these documents are really a bit of a beast. There is so much information in them that they are best consumed piece by piece (a bit like Pana chocolate). It pays to pace yourself – take notes, highlight things, digest them. Come back in the morning with fresh eyes and re-read them again. Take more notes and eat more chocolate.

One of the tastiest pieces of information for a VBACer is found under ‘Clinical and Maternity Performance Indicator: Vaginal Birth following Caesarean Section’ (shortened to ‘VBAC’ when ‘VBFC’ would actually be correct in this case… I guess it makes it sound like VBACers have their own footy club, though). This is where you will find the magical number which answers the question: ‘How many women VBAC in SA?’

Let’s hold that thought – how far through that block of Pana are you right now? Is it still tasting sweet? Is it melting in your mouth and slipping down your throat like a silken river caressing your insides? Well, enjoy that, because what I’m about to say will leave a bitter taste in your mouth.

For years, the VBAC rate in South Australia has consistently been at about 17%. (And, if you’re playing from interstate, the national rate is usually around 16% – you can track this in the Australian Mothers and Babies reports.)

I can hear you suddenly choking on your chocolate. What?! Did I read that correctly?! Only 17% of VBACers actually VBAC? And, because you’ve read them: Don’t the South Australian Perinatal Practice Guidelines cite a VBAC success rate of 72-76%??


In the name of (stat) love, I want to make a really important point whilst you catch your breath…

The reported rate of VBAC in these cases IS NOT A VBAC SUCCESS RATE. This is simply the rate at which women in a defined cohort VBAC. Assuming these two things are synonymous is like thinking you can subsitute Cadbury for Pana (which, I assure you, you cannot).

A hypothetical

Take a group of 100 women who are potential chocolate connoisseurs. Sarah Wilson has successfully reached 66 of these women, so they have no interest in eating chocolate. 34 of them are after a sugar-fix and indicate they’d like to eat Pana, but 17 of them get Cadbury and the other 17 get Pana. The overall rate of Pana chocolate distribution is 17%, but the rate of successful distribution to the women who wanted it is 50%. So, whilst Pana sales aren’t as great as they could be, it’s maybe not so bad because half of the women who wanted Pana got their fix. (And, from Sarah’s perspective, these stats are looking pretty damn fine.)

Alternatively, here’s the chocolate-/sugar-/dairy-/gluten-/caffeine-/ramble-free breakdown:

Take a group of 100 women, 34 of them plan a VBAC and 17 of them go on to have a VBAC, this means:
– The VBAC rate for this group of 100 women is 17%
– The VBAC *success rate* for this group of women is 50%.

To make my point – taking the VBAC rate of 17% and describing it as a *success rate* is categorically incorrect. VBAC rates and VBAC success rates are *two different sets of stats*.

Furthermore, the group of women in this cohort are “women giving birth vaginally following a previous primary (first) caesarean section and having NO intervening pregnancies greater than 20 weeks gestation.” Do you fit the bill? If you’re having a VBA2C, this stat might be of little relevance to you. If you’re planning a second or third VBAC, you might also feel the need to find this information via other avenues.

So, how do we find out more about VBAC success rates? Well, until our health bureaucrats start recording how many women *attempt VBAC*, we will not know what the success rate is. The best thing you can do here is to ask your care provider AND hospital two questions:

– What is your VBAC rate?
– What is your VBAC success rate? (Otherwise known as ‘How many women in your care attempt VBACs? How many of these go on to VBAC?)

Further Food For Thought

Once you have a VBAC success rate, is it going to satisfy your craving for reassurance? Possibly, but maybe not.

Think about this: if an obstetrician has an 80% success rate, he/she sounds like a keeper, yeah? What about if this obstetrician only oversaw 5 attempted VBACs in one year, 4 of which were successful (hence the 80% success rate)? Does the stat indicate this care provider is skilled and experienced at attending VBAC, or does it simply suggest that this care provider lucked out? And, just whose ‘success’ is this stat, anyway?

The message you can take home in a foil swan

Stats are great to help us get our heads around the unknown, to get a feel for birth culture and to play to our best odds. But, it is really important to read the fine print – adding the word ‘success’ to what is simply a VBAC rate creates a BIG difference. Having the confidence to question stats, care providers and ‘birth experts’ takes intestinal fortitude, but seeking accurate information from a variety of sources makes for a balanced diet… Even if it is predominantly made up of Pana chocolate when you do the number-crunching!