VBAC Series: VBAC Calculator, is it accurate?

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If you are planning a vaginal birth after cesarean (VBAC), you may have come across a VBAC Calculator.  The VBAC calculator attempts to take variables* into account and predict the likelihood of vaginal delivery after a cesarean. 

Before you break down in tears, especially if you have never had a previous vaginal delivery, lets consider a few things.  A recent Letter to the Editor in Birth: Issues in Perinatal Care critiqued the use of this calculator.

An interesting point to consider is that the risk of perinatal death or hypoxic-ischemic encephalopathy in the newborn is 1 in 2000 for a woman attempting a VBAC.  Even if a women’s likelihood of successful vaginal delivery is low, her risk of poor outcomes for her baby is extremely low if she does not have a VBAC in the end.  Additionally, the authors pointed out that we don’t use a calculator to determine if a woman with no prior deliveries will have a successful vaginal delivery, though there are certainly factors that influence this. 

Multiple studies show that the actual success rate of a VBAC is somewhere around 74%, and this increases to 94% if the woman has had a prior vaginal delivery.  VBAC calculators are more accurate the closer to delivery used, rather than the start of a pregnancy.  There is a second calculator that takes into account hospital admission factors, gestational age, cervical exam, etc.  I was unable to find a link to a public website containing this calculator, but will update this page if I do. 

I think the VBAC calculator fails to take into account a variable that I personally believe has a huge impact, the support of your provider.  While some providers may initially promise the backing for a VBAC, their support may wane over the course of your pregnancy.  A gestational age or predicted estimated fetal weight cut-off, strict monitoring in early labor or an epidural “just in case” may be signs your provider isn’t as supportive as they may say.  Similar, a hospital setting that is not supportive of VBAC can also deter a woman’s outcome of VBAC.   A woman in a supportive environment with a low prediction of vaginal delivery might just be more successful than another in an unsupportive environment with a higher prediction of vaginal delivery.  Wouldn’t it be interesting if the provider’s and hospital’s VBAC rate was added to the calculator!

When I use the VBAC calculator to predict my VBAC a few years ago, I received a likelihood of 65% and without the patience of my providers it probably would have been 0%.  This time the calculator predicts 91%, slightly less than what research shows.  But with the same supportive providers, barring an emergency, I think the likelihood is much higher. Only time will tell!

Curious if anyone else received a low likelihood of success and went on to have a vaginal delivery.  What did you find to be the biggest impact on your outcome?

* BMI is the pre-pregnancy BMI, not current.  The second hospital admission calculator takes into account the current BMI