VBAC Success Rates and Prediction Models: Understanding Your Chances of Important Outcomes

By Pam Candelaria

This article is part of A Woman's Guide to VBAC: Navigating the NIH Consensus Recommendations, a collection of resources that address the most common and pressing questions women may have about their birth choices. View all sections in the guide, including a link to the authors, on the index page.

If you are choosing VBAC, you probably have questions about how your delivery will go. You may wonder how many VBACs are successful or, perhaps more importantly, if your own planned VBAC will end in a vaginal birth. You may be concerned about whether having a lower chance of VBAC success means you have a higher chance of uterine rupture. You may be worried that you're not an ideal VBAC candidate and wonder if planning a VBAC is still a good idea.

The great news is that most VBACs are successful!

While many factors may influence your personal chances of success, overall rates of VBAC success average 74%, and even in women who are not considered good candidates, the chance of having a vaginal birth is almost always above 50%. It is critical to remember that no single factor can tell you whether your own labor will end with a vaginal birth. Some things, like having a previous vaginal birth, either before or after your previous c-section, are very strongly associated with VBAC success; others, like your height, may not have as much impact; and still others, like the size of your baby, can simply not be known until after delivery. Because your birth can be influenced by so many things, and because those things don't equally affect your chances of success, it is impossible to say before labor begins whether any individual woman will deliver vaginally.

In spite of this inherent uncertainty, there are many ways we can try to estimate an individual woman's chance of VBAC success. The Vaginal Birth After Cesarean calculator developed by Grobman, et al, uses factors that are easily identified at the beginning of pregnancy and offers a statistical glimpse of potential VBAC success. One limitation of calculators like this one is their inability to account for the unique situations of individual women. For example, a previous diagnosis of cephalo-pelvic disproportion (or CPD) could be caused by a baby that is truly too big to fit through a mother's pelvis, but it is also frequently associated with posterior ("sunny side up") positioning, a circumstance that, unlike absolute CPD, need not reoccur (or even preclude a vaginal birth) in a future VBAC attempt. Some maternal illnesses like gestational diabetes can be controlled for some women, potentially reducing any negative impact on a planned VBAC.

You should always discuss what factors may affect your planned VBAC in detail with your provider. A careful review of your unique history is the best way to determine your personal chance of a successful TOL.

Even if you have factors associated with lower rates of VBAC success, remember that an unsuccessful TOL does not mean you will experience uterine rupture! With a single previous low-transverse c-section, the risk of uterine rupture is less than 1% even if you are not an "ideal" VBAC candidate. And remember: if VBAC is important to you, a lower chance of success is still better than no chance of success. Prediction calculators, conversations with care providers, and doing your own research are all valuable, but fallible. The only true way to know if you can VBAC is to give your body, and your baby, a chance to try.