Estimate your chance of a successful vaginal birth after cesarean using the 2021 Grobman model
| VBAC chance | Category | Clinical implication |
|---|
| Outcome | TOLAC (attempt VBAC) | Elective repeat cesarean |
|---|
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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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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.
Enter your age in years. The field accepts values from 15 to 55.
Type your height in centimeters. The calculator expects 130 to 200 cm.
Add your pre-pregnancy weight in kilograms. Range is 30 to 200 kg.
Select whether you have chronic hypertension that requires treatment.
Choose your prior vaginal delivery history: none, before the prior cesarean, or a VBAC after the prior cesarean.
Pick the interval since your last cesarean: under 18 months, 18 to 36 months, or over 36 months.
Select the indication for the prior cesarean — arrest of dilation or descent, or another non-recurring reason.
Choose the gestational age at delivery and estimated fetal weight.
Hit Calculate. The result updates live as you adjust inputs, but the button forces a refresh.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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