# What is my chance of a VBAC?

Estimates the chance of a vaginal birth after cesarean (VBAC) with a trial of labor from the MFMU Network model without race and ethnicity (Grobman 2021).

- Page: https://www.acalculator.org/health/vbac-calculator
- JSON spec: https://www.acalculator.org/health/vbac-calculator.json
- Version: 8b96e8e93e89

## Default answer

Example with the default inputs (Your age 30, Height 5 ft 1.42 in, Weight before pregnancy 156.528206151263 lb, Earlier vaginal birth None, Cesarean for labor that stopped? No, Treated chronic high blood pressure? No): The predicted chance of a vaginal birth after cesarean is 59.6%.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| age | Your age | Your age in whole years. |
| height | Height | Your height. |
| weight | Weight before pregnancy | Your weight before this pregnancy. |
| vag | Earlier vaginal birth | Any vaginal birth before this pregnancy: none, only before the cesarean, or a VBAC (after it). |
| arrest | Cesarean for labor that stopped? | Whether the cesarean was done because labor stopped progressing (an arrest of dilation or descent). |
| htn | Treated chronic high blood pressure? | Whether you have high blood pressure from before pregnancy that is treated with medicine. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| chance | Chance of VBAC | The predicted chance of a vaginal birth with a trial of labor, as a percent, to 1 decimal. |

## Method

Chance (%) = 100 × e^w ÷ (1 + e^w), with w = −5.952 − 0.023 × age − 0.024 × weight (kg) + 0.056 × height (cm) − 0.597 × arrest + 0.868 × vaginal birth only before the cesarean + 1.869 × earlier VBAC − 0.966 × treated chronic hypertension.

## Assumptions

- For people with one earlier low-transverse cesarean, one baby head-down, at 37 weeks or later, who have a trial of labor.
- The equation uses the coefficients printed in Grobman 2021, so results can differ from the paper’s own table by a few tenths of a percent.
- The chance is rounded to 1 decimal, halves up.

## Worked examples

1. age = 28, weight = 68.038856, height = 1.6256, vag = before, arrest = no, htn = no gives chance = 85.1%. Source: Grobman 2021 equation: 68.039 kg, 162.56 cm.
2. age = 30, weight = 71, height = 1.71, vag = vbac, arrest = no, htn = no gives chance = 95.7%. Source: Grobman 2021, equation in Results: w = 3.0955, 95.69%.
3. age = 30, weight = 71, height = 1.56, vag = before, arrest = no, htn = no gives chance = 77.9%. Source: Grobman 2021 equation: 77.87%.
4. age = 30, weight = 71, height = 1.56, vag = none, arrest = no, htn = no gives chance = 59.6%. Source: Grobman 2021 equation: 59.63%.
5. age = 30, weight = 71, height = 1.56, vag = none, arrest = yes, htn = no gives chance = 44.8%. Source: Grobman 2021 equation: 44.84%.
6. age = 37, weight = 71, height = 1.56, vag = none, arrest = no, htn = yes gives chance = 32.4%. Source: Grobman 2021 equation: 32.37%.

## FAQ

### What is a VBAC?

A vaginal birth after cesarean (VBAC) is a vaginal birth for someone who had a cesarean before. Trying for one is called a trial of labor after cesarean (TOLAC). In the study behind this calculator, 74% of people who had a trial of labor gave birth vaginally.

### Why does this calculator leave out race?

An older MFMU Network calculator used race and ethnicity as predictors, and concern was raised about using these social categories. Grobman and others (2021) built a new model from the same registry without race and ethnicity. It predicts as well (area under the curve 0.75) and adds treated chronic high blood pressure.

### What raises the chance of a VBAC?

In the model, an earlier vaginal birth raises the chance, most of all a vaginal birth after the cesarean (a VBAC). Being taller raises it. Being older, weighing more, a cesarean done because labor stopped, and treated chronic high blood pressure lower it.

### Who is the model for?

People with one earlier cesarean with a low-transverse cut on the uterus, one baby in the head-down position, at 37 weeks or later, who have a trial of labor. It was built with facts known at the first prenatal visit.

## Sources

- Grobman WA, Sandoval G, Rice MM, et al. Prediction of vaginal birth after cesarean delivery in term gestations: a calculator without race and ethnicity. Am J Obstet Gynecol. 2021;225(6):664.e1-664.e7. https://doi.org/10.1016/j.ajog.2021.05.021
- NICHD Maternal-Fetal Medicine Units Network. Vaginal Birth After Cesarean Calculator. https://mfmunetwork.bsc.gwu.edu/web/mfmunetwork/vaginal-birth-after-cesarean-calculator (retrieved 2026-09-29)