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
| Field | What to enter |
|---|---|
| Age | In years |
| Height | Units: cm, in, m, ft, mm |
| Weight | Units: kg, lb, st, g |
| Ethnicity (original model) | Choose one: White or other, African-American, Hispanic |
| Any previous vaginal birth? | Choose one: No, Yes |
| Vaginal birth since the caesarean? | Choose one: No, Yes |
| Previous 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 (%) |
|---|---|
| 16 | 83 |
| 24 | 78 |
| 32 | 73 |
| 40 | 66 |
| 48 | 59 |
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.
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)
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.
Frequently asked questions
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.