acalculator

What is my chance of a VBAC?

Enter your age, height, weight before pregnancy, and birth history to estimate the chance of a vaginal birth after cesarean (VBAC) if you have a trial of labor, by the 2021 MFMU Network model. For information only. This is not medical advice or a diagnosis. Do not use it to make treatment decisions without a clinician. No licensed clinician has reviewed this page.

Your numbers

Units
Earlier vaginal birth
Cesarean for labor that stopped?
Treated chronic high blood pressure?
Chance of VBAC
59.6%

The predicted chance of a vaginal birth after cesarean is 59.6%.

Chance of VBAC: 59.6%. The predicted chance of a vaginal birth after cesarean is 59.6%.

This calculator gives general information only. It is not medical advice and does not replace a doctor or other health care provider. Talk with a professional about your results. Terms of use

How to calculate

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).

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%.

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.

  • 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.

Machine-readable copies: Markdown, JSON.

Worked examples

Each example is checked against the calculator on every build.

  1. Your age 28, Weight before pregnancy 150 lb, Height 5 ft 4 in, Earlier vaginal birth Only before the cesarean, Cesarean for labor that stopped? No, Treated chronic high blood pressure? No gives Chance of VBAC 85.1%.Source: Grobman 2021 equation: 68.039 kg, 162.56 cm
  2. Your age 30, Weight before pregnancy 156.5 lb, Height 5 ft 7.3 in, Earlier vaginal birth A VBAC, Cesarean for labor that stopped? No, Treated chronic high blood pressure? No gives Chance of VBAC 95.7%.Source: Grobman 2021, equation in Results: w = 3.0955, 95.69%
  3. Your age 30, Weight before pregnancy 156.5 lb, Height 5 ft 1.4 in, Earlier vaginal birth Only before the cesarean, Cesarean for labor that stopped? No, Treated chronic high blood pressure? No gives Chance of VBAC 77.9%.Source: Grobman 2021 equation: 77.87%
  4. Your age 30, Weight before pregnancy 156.5 lb, Height 5 ft 1.4 in, Earlier vaginal birth None, Cesarean for labor that stopped? No, Treated chronic high blood pressure? No gives Chance of VBAC 59.6%.Source: Grobman 2021 equation: 59.63%
  5. Your age 30, Weight before pregnancy 156.5 lb, Height 5 ft 1.4 in, Earlier vaginal birth None, Cesarean for labor that stopped? Yes, Treated chronic high blood pressure? No gives Chance of VBAC 44.8%.Source: Grobman 2021 equation: 44.84%
  6. Your age 37, Weight before pregnancy 156.5 lb, Height 5 ft 1.4 in, Earlier vaginal birth None, Cesarean for labor that stopped? No, Treated chronic high blood pressure? Yes gives Chance of VBAC 32.4%.Source: Grobman 2021 equation: 32.37%

Medical disclaimer

This calculator is for information and education only. It is not medical advice, and its result is not a diagnosis. Do not use it to start, stop, or change a treatment without a clinician who knows your health history. No licensed clinician has reviewed this page. This site's editors checked the formulas and worked examples against published sources. If you think you have a medical emergency, call your local emergency number.

When this does not apply. The model is for people with one earlier low-transverse cesarean, one live baby head-down, at 37 weeks or later, who have a trial of labor (Grobman 2021). It does not apply after more than one cesarean, with another kind of uterine cut, with twins or a breech baby, or before 37 weeks.

How the chance is worked out

Grobman and others (2021) give the model as:

chance of VBAC (%) = 100 × e^w ÷ (1 + e^w)

w = −5.952 − 0.023 × age − 0.024 × weight before pregnancy (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

  • Age in years; weight in kilograms (pounds × 0.45359237); height in centimeters (inches × 2.54).
  • Arrest, vaginal birth only before the cesarean, earlier VBAC, and treated chronic hypertension are 1 for yes and 0 for no. "Earlier vaginal birth" has three answers, so at most one of its two terms is 1.
  • The chance is rounded to 1 decimal place, halves up, read from the decimal value (within 1e-9, relative, of a half counts as the half).

The paper prints the coefficients to 3 decimals. Its own table of examples was made with the unrounded coefficients, so the page can differ from that table by a few tenths of a percent (for example 95.7 here, 95.6 in the paper).

Who the model is for

People with one earlier low-transverse cesarean, one live baby head-down, at 37 weeks or later, who have a trial of labor (MFMU Cesarean Registry, 11,687 people). It does not apply to more than one earlier cesarean, other uterine cuts, twins, or breech babies.

Inputs the calculator accepts

Age 16 to 50 years; height 140 to 195 cm; weight before pregnancy 35 to 180 kg.

Worked examples by hand

Age 28, 5 ft 4 in (162.56 cm), 150 lb (68.039 kg), a vaginal birth only before the cesarean. w = −5.952 − 0.644 − 1.633 + 9.103 + 0.868 = 1.742; e^1.742 = 5.71; 5.71 ÷ 6.71 = 85.1%.

Age 30, 71 kg, 171 cm, an earlier VBAC, no arrest, no hypertension (paper example 1). w = −5.952 − 0.69 − 1.704 + 9.576 + 1.869 = 3.099; e^3.099 = 22.18; 22.18 ÷ 23.18 = 95.7% (paper 95.6).

Age 30, 71 kg, 156 cm, vaginal birth only before the cesarean (example 3). w = −5.952 − 0.69 − 1.704 + 8.736 + 0.868 = 1.258; 77.9% (paper 77.7).

Age 30, 71 kg, 156 cm, no earlier vaginal birth (example 7). w = 0.39; e^0.39 = 1.477; 1.477 ÷ 2.477 = 59.6% (paper 59.4).

The same with a cesarean for arrest (example 10). w = 0.39 − 0.597 = −0.207; 44.8% (paper 44.6).

Age 37, 71 kg, 156 cm, treated chronic hypertension (example 12). w = −5.952 − 0.851 − 1.704 + 8.736 − 0.966 = −0.737; 32.4% (paper 32.1).

Other questions people ask

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.