The VBAC Calculator and Why it's Bulls$&*!

I am a VBAC candidate for my next pregnancy. I am not currently pregnant, but plan to become pregnant in about a year, which means by the time I have another baby, I will be 29 years old. I am of hispanic descent, but identify as white. My weight fluctuates, and though I can't tell what my weight will be when I become pregnant next time, I took a guesstimate. This is what my VBAC calculator looks like:

vbac calc.png

Notice how the text appears in red, which indicates basically a non-starter for attempting a VBAC. For reference, here is the calculator when the factors are more 'VBAC-conducive:'

vbac calc2.png

Before I point out some of the flaws in each of these criteria, I want to discuss using a 'calculator' on the whole as a decision-making tool.

This is not a real reflection of your personal actual chances of success. It is a statistical reflection of population-based studies that looked at only certain factors within the overall maternal health of VBAC candidates. That means that the studies on which these calculators are based do not take into account yourself, your baby, your doctor, your preferences, your life. Some things they do not account for include:

  1. Your environment: the hospital/home/center in which you are giving birth and their cesarean rate, your doctor's or midwife's own cesarean rate, the common practices on inductions and cesareans and other interventions in your area, whether or not and what kind of support you have, the personality of your labor nurse.

  2. Your risk of uterine rupture, which is affected by: the number of cesareans you’ve had, whether or not you are induced, and which method was used, the shape and suturing of your scar, and whether or not that scar had enough time to heal completely, and how the suturing was done to prevent thinning of the uterine lining, called a ‘window.’

  3. Your race if you are anything other than Black, White, or Hispanic, and what it means to be any of those categories, since they are self-reported, and there are no clear, measurable delineations between any of them.

  4. Other pregnancy complications that lead to a cesarean, like: the size and position of your baby and your body, infection risks, mode of monitoring the baby (yes, that’s a risk factor), length of the pregnancy and overall risk of stillbirth, gestational diabetes, risk of preeclampsia and eclampsia, placental issues like previa or accreta, and a number of other variables that could cause a non-scarred pregnant person to need a cesarean.

Another important thing to consider here is that the evaluation of 'success' in this situation is whether or not the birth was vaginal. Though your personal mode of delivery may affect your emotions, it does not determine for all women all the time whether or not they will be satisfied with their birth experience, and whether or not YOU will consider it a success.

So, knowing this, the factors that the calculator does consider are based on the information and studies we currently have available. There are, in fact, levels of quality of evidence: some of the studies that have been done are good, and some are shite. Not ALL of the evidence available on VBAC success is high quality evidence. Since you can't usually 'randomize' a woman to VBAC success, these are mostly observational studies, that look back at women who were successful already, and determining which parts of her made her a 'success.' So, keep in mind that the number of women who even attempt a VBAC is very small in relation to the number of women who might be eligible, and that may, in itself, confound some of the results.

Ok, so some issues with the factors themselves:

  1. Maternal age and BMI are factors that affect a pregnancy in general, not VBAC success per se. These are things that lead to other complications, which lead to a cesarean. So 2 layers of risk.

  2. BMI isn’t clear on whether that is when the person become pregnant, or during the third trimester? I’m going to assume that it’s at conception, because that is usually what is measured against health outcomes in the studies I’ve seen. But that can easily be miscalculated.

  3. Race: certain populations are at higher risk of bad health outcomes. But as THIS Times article describes, that may mainly be due to the lived experience of being a person of color, and not genetic, and can therefore be positively impacted with active social change. Not to mention how impossible it is to measure whether someone is Hispanic or not.

  4. Previous vaginal delivery: while having had a previous vaginal delivery before DOES increase the likelihood that you will have another vaginal delivery, not having had one does NOT necessarily mean that you are LESS likely to have one. It just means that you haven’t identified yet whether you have the advantage of a ‘proven’ uterus or not. An unknown variable doesn’t automatically mean that it’s a variable that works against you.

  5. Indication for prior cesarean for arrest: The diagnosis of ‘arrest’ or ‘labor dystocia’ (Not to be confused with shoulder dystocia), is soooo subjective. It basically depends on how patient your doctor is. Outdated studies from the 50s have been used to measure how long an ‘average’ labor length is, and is therefore applied to how long it ‘should’ be. Inductions that take extra time are measured by a normal labor length, people arrive at the hospital too early in labor, and VBACs, which have been shown to progress in a similar pattern to a first-time mother, are measured against the labor length of other women who have had children before vaginally. These are just some of the reasons why it is the NUMBER ONE reason for emergency cesareans, and is terribly over diagnosed. ALSO: whether or not you had a diagnosis of ‘arrest’ that led to your last cesarean hardly has bearing on whether or not you will have a diagnosis of arrest with your Labor after cesarean, and may not necessarily lead to a cesarean either, depending on your doctor’s practices.

So basically this is it. If you come to your doctor, and the first thing they do is run a ‘calculator’ on you, and then use that calculator to tell you whether or not they will support your VBAC, RUN. Or at the very least, show them this blog post, and discuss it together, please. :)