VBAC Calculator · Vaginal Birth After Cesarean

VBAC Calculator

Estimate your chance of a successful vaginal birth after cesarean using the 2021 Grobman model

About you
years
cm
kg
Obstetric history
Arrest of dilation / descent means labour did not progress normally. This is the most common recurring indication and lowers VBAC success.
Current pregnancy
VBAC Prediction Result
Chance of successful VBAC
— %
— per 100 — baseline —
💚—
VBAC chance
— %
Uterine rupture risk
— %
Maternal morbidity
— %
Neonatal morbidity
— %
VBAC chance interpretation
VBAC chanceCategoryClinical implication
TOLAC vs elective repeat cesarean
OutcomeTOLAC (attempt VBAC)Elective repeat cesarean
Clinical notes
About the model: This calculator uses the 2021 race‑neutral Grobman model, which predicts VBAC success using maternal age, pre‑pregnancy weight, height, prior vaginal delivery, prior VBAC, chronic hypertension, and the indication for the prior cesarean. The original 2007 model also included race and ethnicity; the 2021 version removed these to reduce racial inequities in care.
Important: A VBAC success rate of 60–70% or higher is generally considered favourable. A trial of labour after cesarean (TOLAC) carries a small risk of uterine rupture (about 0.5–0.9% with one prior low‑transverse incision). Always discuss the risks and benefits of TOLAC versus elective repeat cesarean with your obstetrician.
When TOLAC is not recommended: Prior classical (vertical) incision, prior uterine rupture, more than two prior cesareans, placenta previa, or any absolute contraindication to vaginal delivery.
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Faiq Ur Rahman

Founder & CEO, Toolraxy

Faiq Ur Rahman is a web designer, digital product developer, and founder of Toolraxy, a growing platform of web-based calculators and utility tools. He specializes in building structured, user-friendly tools focused on health, finance, productivity, and everyday problem-solving.

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Introduction

Choosing between a trial of labor after cesarean and a scheduled repeat cesarean is not a coin flip. It’s a decision that hinges on measurable clinical factors, and those factors can be combined into a probability. The VBAC Calculator does exactly that. It uses the 2021 race-neutral Grobman model, a validated prediction tool derived from a large U.S. cohort, and applies it to your specific inputs. The output is a single percentage: your predicted chance of delivering vaginally if you attempt TOLAC. Around that headline number, the tool also estimates uterine rupture risk and gives you comparison figures for maternal and neonatal morbidity. It’s built for anyone with one prior low-transverse cesarean who is trying to understand what the odds actually look like before their next delivery conversation.

 

How to Use the VBAC Calculator

  1. Enter your age in years. The field accepts values from 15 to 55.

  2. Type your height in centimeters. The calculator expects 130 to 200 cm.

  3. Add your pre-pregnancy weight in kilograms. Range is 30 to 200 kg.

  4. Select whether you have chronic hypertension that requires treatment.

  5. Choose your prior vaginal delivery history: none, before the prior cesarean, or a VBAC after the prior cesarean.

  6. Pick the interval since your last cesarean: under 18 months, 18 to 36 months, or over 36 months.

  7. Select the indication for the prior cesarean — arrest of dilation or descent, or another non-recurring reason.

  8. Choose the gestational age at delivery and estimated fetal weight.

  9. Hit Calculate. The result updates live as you adjust inputs, but the button forces a refresh.

 

How the VBAC Calculator Formula Works

The core of the tool is the 2021 Grobman logistic regression equation. It’s a validated prediction model, not a formula anyone invented for a website.

Formula: VBAC probability = e^w ÷ (1 + e^w) × 100

Where w is:

w = -5.952

  • 0.023 × age (years)

  • 0.024 × pre-pregnancy weight (kg)

  • 0.056 × height (cm)

  • 0.597 × (arrest of dilation/descent = 1, otherwise 0)

  • 0.868 × (prior vaginal delivery only before the cesarean = 1)

  • 1.869 × (prior VBAC after the cesarean = 1)

  • 0.966 × (treated chronic hypertension = 1)

That gives a base probability as a decimal. The calculator then multiplies it by three adjustment factors that the Grobman model doesn’t include but that clinicians routinely consider: interval since prior cesarean (0.85× under 18 months, 1.00× at 18–36 months, 1.05× over 36 months), gestational age at delivery (1.05× at 37–38 weeks, 1.00× at 39–40, 0.85× at 41+), and estimated fetal weight (1.05× under 3,500 g, 1.00× at 3,500–4,000 g, 0.75× above 4,000 g). The final figure is clamped between 1% and 99%.

The race-neutral version dropped race and ethnicity from the equation. The original 2007 model included them. Removing race reduces the model’s accuracy at the population level by a small amount but eliminates a source of unequal care.

 

Worked Example

A 34-year-old woman, 168 cm tall, 72 kg pre-pregnancy, no chronic hypertension. Her first delivery ended in a cesarean for arrest of dilation. She has no prior vaginal delivery. It’s been 22 months.

Plug into w:

w = -5.952 – (0.023 × 34) – (0.024 × 72) + (0.056 × 168) – (0.597 × 1) + 0 + 0 – 0
w = -5.952 – 0.782 – 1.728 + 9.408 – 0.597
w = 0.349

Probability = e^0.349 ÷ (1 + e^0.349) = 1.4176 ÷ 2.4176 = 0.5864 = 58.6%

Now apply the adjustments. Interval of 22 months falls in the 18–36 month band, so 1.00×. Gestational age at delivery is 39–40 weeks, so 1.00×. Estimated fetal weight under 3,500 g, so 1.05×. Final: 58.6 × 1.05 = 61.6%.

The calculator labels this Favourable. The verdict banner suggests TOLAC is generally considered a reasonable option. Uterine rupture risk would show around 0.70%, maternal morbidity around 2.0%, neonatal morbidity around 5.0%.

Now flip one input: change the indication to “other / non-recurring.” The arrest term drops to 0, w rises to 0.946, base probability jumps to 72.0%, and after the 1.05× EFW adjustment the final reads 75.6%. That’s a 14-point swing from a single dropdown. This is why the model uses logistic regression rather than a fixed success-rate table.

Frequently Asked Questions

What is a good VBAC success rate?

Above 70% is generally considered favourable, and above 60% is often framed as reasonable. Below 50% tips the balance toward elective repeat cesarean for many providers. The calculator labels 60–69% Favourable and 70%+ High or Very high.

 

Can I have a VBAC after two C-sections?

Some providers will offer TOLAC after two prior low-transverse cesareans, but the uterine rupture risk is higher and hospital policies vary. This calculator only models one prior cesarean. Discuss two-prior TOLAC with your obstetrician directly.

 

What disqualifies you from a VBAC?

Prior classical or T-incision, prior uterine rupture, placenta previa, more than two prior cesareans at most institutions, and any absolute contraindication to vaginal delivery. The calculator doesn’t screen for these, so check with your provider before relying on the result.

 

Can you be induced for a VBAC?

Yes, but induction for TOLAC carries a slightly lower success rate than spontaneous labor, and some methods (especially prostaglandins) are avoided due to rupture concerns. The Grobman model doesn’t include induction as a variable, so this calculator doesn’t adjust for it.

 

What is the risk of uterine rupture with VBAC?

About 0.5% to 0.9% with one prior low-transverse incision. The calculator’s estimate starts at 0.7% and adjusts upward slightly for short inter-delivery intervals, chronic hypertension, and predicted VBAC below 50%. It never exceeds 2.5% in this model.

 

Is a VBAC more painful than a C-section?

Labor itself is usually more physically demanding than a scheduled cesarean, but recovery from a vaginal delivery is typically faster. That’s a real tradeoff many women weigh. The calculator doesn’t model pain or recovery time, it models the probability of achieving vaginal delivery if you attempt it.

 

What happens if my VBAC attempt fails?

You have a repeat cesarean, usually within 30 minutes if it’s a non-emergent situation. The calculator’s maternal and neonatal morbidity estimates account for the possibility of a failed TOLAC by including the elevated risks in the aggregate figures. A failed TOLAC is not a rare event, it happens in about 20% to 40% of attempts depending on the input profile.

 

How accurate is the VBAC calculator?

The 2021 Grobman model has an area under the ROC curve around 0.75 in external validation, which is considered good for a clinical prediction tool. It’s more accurate than eyeballing a general success rate but far from perfect. It doesn’t capture hospital-level factors or individual anatomy.

 

How long after a C-section can you have a VBAC?

Most providers recommend at least 18 months between deliveries. The calculator applies a 15% success penalty if the interval is under 18 months. It applies a slight 5% bonus if the interval is over 36 months. Talk to your obstetrician about the optimal spacing for you.

 

Does age affect VBAC success?

Yes. Each additional year reduces the logit by 0.023, which compounds. A 42-year-old with all other factors at baseline would land roughly 15 points below a 25-year-old with the same profile. Age 35 and 40 are not hard cutoffs, they’re continuous variables in the model.

 

What is TOLAC?

TOLAC stands for Trial of Labor After Cesarean. It’s the attempt to deliver vaginally after a prior cesarean. VBAC is the successful outcome of a TOLAC. The terms get used interchangeably, but technically TOLAC is the attempt and VBAC is the result.

 

Can I use this calculator without knowing my prior cesarean details?

You need to know the indication for your prior cesarean and whether you’ve had any prior vaginal deliveries. Those two inputs move the prediction by more than 20 points. If you don’t know the indication, ask your obstetrician, it’s in your operative report.

Disclaimer

This calculator uses the 2021 Grobman model as published and applies additional adjustment factors for interval since prior cesarean, gestational age at delivery, and estimated fetal weight. It’s a prediction tool, not a medical device. It cannot account for hospital-level factors, individual anatomy, prior surgical details beyond what you select, or any absolute contraindication to TOLAC. The uterine rupture, maternal morbidity, and neonatal morbidity estimates are approximations, not validated risk scores. Do not use this output to make a delivery decision on your own. Bring the result to your obstetrician and discuss it in the context of your full clinical picture. If you experience severe abdominal pain, heavy bleeding, or decreased fetal movement during a TOLAC, seek immediate medical care.

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