What is the Grobman VBAC model?

The Grobman model is a validated logistic regression formula published in Obstetrics & Gynecology (2007) that estimates the probability of successful vaginal delivery for women undergoing TOLAC. It uses demographic and obstetric factors collected at the first prenatal visit.

What does "prior vaginal delivery" mean in this context?

It refers to any vaginal birth — either before or after the previous cesarean. A prior vaginal delivery is one of the strongest positive predictors of VBAC success.

Is a high score a guarantee of VBAC success?

No. The score provides an estimated probability based on population-level data. Individual outcomes depend on many additional intrapartum factors. Always discuss results with a qualified obstetric provider.

What BMI should I enter?

Enter the pre-pregnancy BMI, not the BMI at delivery. Higher pre-pregnancy BMI is associated with lower VBAC success probability in the Grobman model.

Which indication for prior cesarean lowers the score most?

A non-recurring indication (e.g., breech, fetal distress) is more favorable than a potentially recurring indication such as labor dystocia (failure to progress), which may lower the estimated probability.

Does this calculator use race or ethnicity?

This tool calculates probability using the original Grobman et al. (2007) model. In 2021, MFMU and ACOG published a revised race-neutral VBAC calculator to mitigate algorithmic bias; discuss with your obstetric provider which prediction model is preferred in your clinical care.

VBAC Success Score

Enter maternal and obstetric factors to estimate the likelihood of a successful vaginal birth after cesarean (VBAC) using the validated Grobman scoring model.

Patient Factors

Maternal age at delivery (years)

Pre-pregnancy BMI (kg/m²)

Ethnicity

Prior vaginal delivery (before or after cesarean)?

Yes No

Prior successful VBAC?

Yes No

Indication for most recent cesarean

Fill in all fields and press Calculate Score.

Summary

The VBAC Success Score uses the Grobman logistic regression model to estimate the probability of a successful vaginal birth after cesarean (VBAC) for women planning a trial of labor (TOLAC). Inputs include maternal age, pre-pregnancy BMI, ethnicity, prior vaginal delivery, prior VBAC, and the indication for the previous cesarean section. The result is a percentage probability that helps guide patient counseling and clinical decision-making.

How it works

Enter maternal age at delivery.
Enter pre-pregnancy BMI (kg/m²).
Select ethnicity and indicate prior vaginal delivery history.
Specify whether a prior VBAC was achieved.
Select the indication for the most recent cesarean.
The calculator applies the Grobman logistic regression formula and displays the estimated VBAC success probability.

Use cases

Counsel patients considering trial of labor after cesarean (TOLAC).
Support shared decision-making between patient and obstetrician.
Assess candidacy for VBAC during prenatal care planning.
Compare risk profiles across different clinical scenarios.
Educate patients on factors that influence VBAC outcomes.
Document predicted success rate in the clinical record.

Frequently Asked Questions

Last updated: 2026-06-15 · Reviewed by Nham Vu

VBAC Success Score

Patient Factors

Estimated VBAC Success Probability

Score Components

Summary

How it works

Use cases

Frequently Asked Questions

VBAC Success Score

Use the VBAC Success Score

Patient Factors

Estimated VBAC Success Probability

Score Components

Summary

How it works

Use cases

Frequently Asked Questions