Enter age, BMI or height and weight, ethnicity, prior vaginal birth, any vaginal birth since the caesarean, and whether the prior indication is recurrent. The result shows a VBAC probability and a simple score for counselling.
What this tool calculates
- An estimated probability of successful vaginal birth after a prior caesarean.
- A simple score derived from the probability for quick reading.
Model used
- Logistic regression in the style of the Grobman antenatal VBAC model.
- Inputs: maternal age, pre-pregnancy BMI, ethnicity group, any prior vaginal birth, vaginal birth since the prior caesarean, and a recurring indication for caesarean.
- Form: p equals 1 divided by 1 plus exp of the negative linear predictor. The linear predictor is the intercept plus the sum of each coefficient times its variable. The calculator presents p as a percentage.
- Coefficients are tuned to typical published ranges so outputs align with widely used calculators for the same inputs.
Notes on ethnicity variables
- Some legacy VBAC models include race or ethnicity terms. Several organisations discuss removing race terms from prediction tools. Local practice varies. Use clinical judgement and local policy.
Scope and limits
- Use for singleton, cephalic pregnancies considering trial of labour after caesarean.
- Common exclusions include classical or unknown uterine scar and prior uterine rupture.
- The estimate does not replace shared decision making, informed consent, or intrapartum clinical assessment.
Sources
- Grobman WA, et al. Development of a nomogram for prediction of vaginal birth after caesarean delivery. Obstet Gynecol. 2007. pubmed.ncbi.nlm.nih.gov/17906012/
- Grobman WA, et al. Development of a VBAC prediction model. Validation and refinement. Am J Obstet Gynecol. 2009. pubmed.ncbi.nlm.nih.gov/19683691/
- ACOG. Vaginal birth after cesarean (VBAC), patient guidance. acog.org/womens-health/faqs/vaginal-birth-after-cesarean-vbac
- RCOG. Birth after previous caesarean birth. Green-top Guideline No. 45. rcog.org.uk
Last updated: 26 Sep 2025
How to use this calculator
- Enter age.
- Tick BMI known, then enter BMI. Or untick and enter height and weight to compute BMI.
- Select ethnicity.
- Answer: any prior vaginal delivery, any vaginal birth since the caesarean, and whether the prior indication is recurrent.
- Press Calculate to see the VBAC probability and a simple score.
For counselling only. Use with clinical review and local guidelines.
Last updated: September 2025
Frequently asked questions
What is a VBAC calculator?
A tool that estimates the chance of successful vaginal birth after caesarean using factors such as age, BMI, ethnicity, prior vaginal birth, and the reason for the prior caesarean.
Which factors increase the chance of VBAC?
Common positive factors include a prior vaginal birth and a vaginal birth after the caesarean. Higher age, higher BMI, a recurrent indication, and some ethnicity categories are often associated with lower chances in published models.
Does this replace clinical advice?
No. It supports counselling. Decisions depend on scar type, pregnancy details, facility resources, and clinician review.
Can I use height and weight instead of BMI?
Yes. Untick BMI known and enter height and weight. The tool will calculate BMI.
Is induction compatible with VBAC?
Induction may be considered in some settings. Risks and methods vary. Discuss the plan with a clinician familiar with your history.