VBAC Calculator | Probability of Successful Vaginal Birth After Caesarean

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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.

Age (years)
Enter your age in years. The starting value is 30.
Ethnicity (optional)
This field is used only for the earlier equation comparison.
Pre-pregnancy BMI (optional)
If left blank, we’ll calculate BMI from height/weight below (or assume BMI 26 if nothing is entered).
Pre-pregnancy Height & weight (optional)
Height (ft + in)
Weight (lb)
Any prior vaginal delivery? (optional)
Vaginal birth since the prior cesarean? (optional)
Recurring reason for cesarean delivery (labor stopped progressing)? (optional)

Choose Yes if you have chronic hypertension treated with medication.

Probability of successful vaginal birth:
—
VBAC Score (Grobman model): —

For counseling only. Talk with your clinician about your situation.

* The current equation uses age, height, pre-pregnancy weight, prior birth history, arrest of labor as the previous cesarean reason, and treated chronic hypertension. The earlier equation comparison also uses ethnicity and BMI. If measurements are incomplete, its estimate remains available. These estimates support a discussion about your birth options; they do not determine eligibility for VBAC.

Peer-Reviewed Science
Updated September 2026
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How to use this calculator

  • Enter age.
  • Enter your pre-pregnancy BMI, or use height and weight to calculate it. The current equation also needs height.
  • 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 2026

Methodology and sources

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

  • Published Grobman antenatal VBAC equations: the current 2021 estimate and a separate 2007 comparison.
  • The current equation uses maternal age, height, pre-pregnancy weight, birth history, arrest of labor as the prior cesarean indication, and treated chronic hypertension. BMI and ethnicity are used for the earlier comparison.
  • 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.
  • The main estimate uses the published 2021 coefficients when height and weight are available. The earlier comparison uses the published 2007 coefficients. BMI and height can supply weight when weight is not entered.

Notes on ethnicity variables

  • Ethnicity does not affect the current estimate. The earlier comparison retains the ethnicity terms from the 2007 equation, so you can see both estimates separately.

Scope and limits

  • The source population had one prior low-transverse cesarean and a singleton, head-down pregnancy at term.
  • Your clinician assesses eligibility for labor separately, including uterine scar type and any history of uterine rupture.
  • The estimate does not replace shared decision making, informed consent, or intrapartum clinical assessment.

Sources

Last updated: September 2026

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. Age, pre-pregnancy measurements, previous birth history and the reason for the cesarean affect the estimate. The current equation does not use ethnicity.

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. Leave BMI blank 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.

Grobman et al., 2021 supplies the current equation: log-odds = −5.952 − 0.023 × age − 0.024 × weight in kg + 0.056 × height in cm − 0.597 for prior arrest indication + 0.868 for vaginal birth only before cesarean, or +1.869 for prior VBAC − 0.966 for treated chronic hypertension. Convert log-odds with 1/(1+exp(−x)). The 2007 comparison uses its original published coefficients and BMI; ethnicity affects that comparison only. Prior VBAC also counts as a prior vaginal delivery in the 2007 comparison. Source population: one prior low-transverse cesarean, singleton cephalic pregnancy at term considering labor.