# VBAC Calculator (Vaginal Birth After Caesarean)

URL: https://speedycalculator.com/calculators/health/vbac-risk-score/
Last updated: 2026-10-30

> The VBAC calculator estimates the chance of a successful vaginal birth after a previous caesarean (trial of labour) with the Grobman MFMU nomogram, from age, BMI, ethnicity, previous vaginal births and the reason for the earlier caesarean.

## Inputs

- Age (years)
- Height (cm, in, m, ft, mm)
- Weight (kg, lb, st, g)
- Ethnicity (original model)
- Any previous vaginal birth?
- Vaginal birth since the caesarean?
- Previous caesarean for failure to progress (arrest disorder)?

## Results

- BMI
- Model score
- Chance of successful VBAC

## Formula

```
Probability = eʷ ÷ (1 + eʷ)
w = 3.766 − 0.039 × age − 0.060 × BMI − 0.671 × African-American − 0.680 × Hispanic + 0.888 × any previous vaginal birth + 1.003 × vaginal birth after the caesarean − 0.632 × recurring indication (arrest of labour)
```

## Explanation

Most women with one previous low transverse caesarean can safely try for a vaginal birth, and 60–80% succeed. A successful VBAC avoids surgery and its risks in future pregnancies. The 2021 updated MFMU calculator removes race and adds chronic hypertension; this calculator uses the original published model.

### How to use the VBAC Calculator

- **Age**
- **Height**
- **Weight**
- **Ethnicity (original model)**
- **Any previous vaginal birth?**
- **Vaginal birth since the caesarean?**
- **Previous caesarean for failure to progress (arrest disorder)?**

Results update as you type, and every field accepts the units shown in its menu.

### How it works

Enter your details. BMI is calculated from pre-pregnancy height and weight.

### Inputs explained

FieldWhat to enterAgeIn yearsHeightUnits: cm, in, m, ft, mmWeightUnits: kg, lb, st, gEthnicity (original model)Choose one: White or other, African-American, HispanicAny previous vaginal birth?Choose one: No, YesVaginal birth since the caesarean?Choose one: No, YesPrevious caesarean for failure to progress (arrest disorder)?Choose one: No, Yes

### What each result means

- **BMI** = [Weight]÷([Height]÷100)^2
- **Chance of successful VBAC** = exp([Model score])÷(1 + exp([Model score]))×100

### Example values

How the main result changes when **Age** changes and everything else stays at the example values above:

Age (years)Chance of successful VBAC (%)16832478327340664859
*Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.*

## Worked example

Inputs — Age: 32 years; Height: 165 cm; Weight: 70 kg; Ethnicity (original model): White or other; Any previous vaginal birth?: No; Vaginal birth since the caesarean?: No; Previous caesarean for failure to progress (arrest disorder)?: No.

Results — BMI = 25.7; Model score = 0.98; **Chance of successful VBAC = 73 %**.

## Key terms

- **VBAC**: Vaginal birth after caesarean.
- **TOLAC**: Trial of labour after caesarean.

## FAQ

**Is VBAC safe?**
For most women with one previous low transverse caesarean, yes; uterine rupture occurs in about 0.5–1% of trials of labour.

**What improves the chance of success?**
A previous vaginal birth, spontaneous labour and a non-recurring reason for the first caesarean.

**Who should not attempt VBAC?**
Women with a previous classical (vertical) uterine incision or prior uterine rupture.


## Sources

- [Grobman WA et al. Development of a nomogram for prediction of vaginal birth after cesarean delivery. Obstet Gynecol, 2007](https://pubmed.ncbi.nlm.nih.gov/17400861/)
- [Grobman WA et al. Prediction of vaginal birth after cesarean delivery in term gestations: a calculator without race and ethnicity. Am J Obstet Gynecol, 2021](https://pubmed.ncbi.nlm.nih.gov/33831382/)