VBAC Calculator – Vaginal Birth After Cesarean
Estimate the chance of a successful vaginal birth after cesarean with the published Grobman 2021 (MFMU) model — the race-free update of the standard NICHD calculator — including a factor-by-factor breakdown.
Enter age, height, weight, and three yes/no history items; the predicted probability, its drivers, and the model's calibration limits are shown instantly.
Estimate VBAC success probability
The published Grobman 2021 model: age, height, weight, obstetric history, prior-cesarean indication, and treated chronic hypertension.
No vaginal birth so far.
Also called failure to progress / labor dystocia.
Educational tool only — not medical advice. Decisions about trial of labor after cesarean belong in a shared discussion with your obstetric care team.
About the VBAC calculator
Choosing between a planned repeat cesarean and a trial of labor after cesarean (TOLAC) is one of the most common shared decisions in obstetrics, and a numeric estimate of the chance that a trial of labor ends in a vaginal birth (VBAC) is a standard part of that conversation.
This tool implements the published Grobman 2021 model from the NICHD Maternal-Fetal Medicine Units Network — the updated version of the widely used 2007 calculator, rebuilt without race and ethnicity variables and with treated chronic hypertension added as an objective clinical predictor. It is a logistic regression: the calculator sums a weight for each factor (age, pre-pregnancy weight, height, obstetric history, whether the prior cesarean was performed for an arrest of dilation or descent, and medically treated chronic hypertension) and converts the total log-odds into a probability. The model's discrimination in its derivation cohort was an AUC of 0.75.
The factor breakdown shown under the result is the model itself, made visible: previous VBAC is by far the strongest favorable predictor, a prior vaginal delivery before the cesarean also helps substantially, while an arrest-of-labor indication and treated chronic hypertension lower the estimate. Height enters positively and age and weight negatively, each proportionally to the published coefficients — nothing here is invented or adjusted.
Two limits matter when reading the number. First, calibration: ACOG's practice advisory notes that predictions are more likely to be accurate when the estimated chance is roughly 60% or higher, and deviate substantially below 40% — a low estimate should never by itself be a barrier to attempting labor. Second, scope: the model does not include every clinically relevant factor (gestational age, fetal size, cervical status, and institutional factors all matter), so the output is a conversation starter for shared decision-making with an obstetric team, not a verdict.
VBAC probability examples
Click any example button to load a preset scenario into the calculator.
| Patient profile | Estimated probability | Interpretation |
|---|---|---|
| Age 30, 156 cm, 71 kg, no history factors | About 60% | A published validation example of the model (example 7 in the paper). |
| Same, but 171 cm with a previous VBAC | About 96% | Previous VBAC is the strongest favorable predictor (also a published example). |
| Age 30, 156 cm, 71 kg, vaginal delivery before the cesarean | About 78% | A prior vaginal birth helps even when it predates the cesarean. |
| Arrest indication + treated hypertension | Substantially lower | Both factors carry negative weights in the model (−0.597 and −0.966 log-odds). |
How to use the VBAC calculator
- Pick your preferred units — imperial (feet/inches, pounds) or metric (centimeters, kilograms).
- Enter maternal age, height, and pre-pregnancy weight; the calculated pre-pregnancy BMI appears underneath.
- Select the obstetric history that fits: no prior vaginal delivery, a vaginal delivery only before the prior cesarean, or a previous successful VBAC.
- Answer the two clinical questions: whether the prior cesarean was for arrest of dilation or descent, and whether chronic hypertension is being treated with medication.
- Read the probability, the factor breakdown that explains it, and the calibration caveat — then discuss the result with your obstetric care team.
VBAC calculator FAQ
Which model does this calculator use?
The Grobman 2021 logistic-regression model from the NICHD MFMU network — the update of the standard 2007 VBAC calculator that removed race and ethnicity and added medically treated chronic hypertension. The coefficients are used exactly as published.
Why doesn't it ask about race or ethnicity?
The 2021 revision deliberately removed those variables: outcome differences by race reflect social factors rather than biology, and including them risked steering patients away from a trial of labor without a biological basis. The updated model performs as accurately as the old one.
How accurate is the estimate?
The model's AUC is about 0.75, and calibration is best for predictions above roughly 60%. Below about 40% the estimates deviate substantially from observed outcomes, so low numbers should be treated with particular caution — and never used alone to rule out a trial of labor.
What counts as an arrest indication?
A prior cesarean performed because labor stopped progressing — arrest of dilation or arrest of descent, often documented as failure to progress or labor dystocia. Cesareans for breech position, fetal heart-rate concerns, or placenta previa are not arrest indications.
Why do height and weight enter separately instead of BMI?
The published 2021 model was built on pre-pregnancy weight and height as separate predictors (taller stature predicts success; higher weight predicts the opposite). The calculator shows the derived BMI for reference but the math follows the published coefficients.
Does the calculator decide whether I should attempt a VBAC?
No. Professional guidance is explicit that a calculator score should not be a barrier to a trial of labor. It is one input into shared decision-making that also weighs rupture risk, future family plans, facility capabilities, and personal preference.