VBAC probability calculator
Valid range 15–50 years.
BMI is computed automatically from weight and height. See also our BMI calculator.
Categories exactly as in the original study. The 2021 model does not use this variable.
Estimated results
Each factor's contribution to the score
| Factor | Contribution to score (w) |
|---|
Total score w is converted to probability with the formula P = ew / (1 + ew). Positive values push the odds up, negative values pull them down.
Success-likelihood categories
| Category | Range | Explanation |
|---|---|---|
| High | ≥80% | High likelihood of success. Counseling literature reports that mothers in this range generally experience equivalent or lower morbidity with a trial of labor than with elective repeat cesarean. |
| Moderate | 60–79% | Moderate likelihood of success. At the 60–70% threshold, evidence shows the TOLAC risk balance is comparable to elective repeat cesarean — an individual discussion with your doctor is strongly recommended. |
| Low | <60% | Low likelihood of success. This does not mean VBAC is impossible, but repeat cesarean is more probable — consider a thorough clinical evaluation with your obstetrician. |
Explanation of each risk factor
2021 model (default)
| Factor | Effect |
|---|---|
| Model constant The equation's starting point before personal factors are entered. Its value is 3.302 in the 2021 model. | ▪ neutral |
| Maternal age Each additional year lowers the score by 0.043. Younger age is associated with a higher likelihood of success. | ↓ lowers |
| Pre-pregnancy BMI Each additional 1 kg/m² lowers the score by 0.052. Lower BMI is associated with a higher likelihood of success. | ↓ lowers |
| Prior vaginal birth Adds 0.832 to the score. A prior vaginal birth is a strong positive predictor. | ↑ raises |
| Prior VBAC Adds 0.489 to the score. A prior successful VBAC is the second-strongest positive predictor after vaginal history. | ↑ raises |
| Recurring cesarean indication Subtracts 0.632 from the score. An indication that may recur (e.g., arrest of labor progress) lowers the odds. | ↓ lowers |
| Treated chronic hypertension Subtracts 0.513 from the score. The clinical, objective replacement for the race/ethnicity variable in the 2021 model. | ↓ lowers |
2007 model (historical comparison)
| Factor | Effect |
|---|---|
| Model constant The equation's starting point before personal factors are entered. Its value is 3.766 in the 2007 model. | ▪ neutral |
| Maternal age Each additional year lowers the score by 0.039. | ↓ lowers |
| Pre-pregnancy BMI Each additional 1 kg/m² lowers the score by 0.060. | ↓ lowers |
| Race/ethnicity (2007 model) The African-American category subtracts 0.671 and Hispanic subtracts 0.680 from the score. This variable was removed in the 2021 update because race is a social construct, not a biological predictor. | ↓ lowers |
| Prior vaginal birth Adds 0.888 to the score. | ↑ raises |
| Prior VBAC Adds 1.003 to the score — the largest positive coefficient in this model. | ↑ raises |
| Recurring cesarean indication Subtracts 0.632 from the score. | ↓ lowers |
Full 2021 model equation: x = 3.302 − 0.043·age − 0.052·BMI + 0.832·(vaginal history) + 0.489·(VBAC history) − 0.622·(recurring indication) − 0.513·(chronic hypertension). 2007 model: w = 3.766 − 0.039·age − 0.060·BMI − 0.671·(African-American) − 0.680·(Hispanic) + 0.888·(vaginal history) + 1.003·(VBAC history) − 0.632·(recurring indication). Probability = ew/(1+ew).
Verified worked examples
The figures below are computed directly from the model equations (verified against the reference implementation), not invented illustrations.
Lowest-risk profile
Age 25, BMI 22, prior VBAC, prior cesarean indication not recurring.
- 2021 model (current SMFM): 91.7% High
- 2007 model (original): 96.6% High
Highest-risk profile
Age 42, BMI 42, no prior vaginal birth, recurring cesarean indication (the 2021 model also uses chronic hypertension).
- 2021 model (current SMFM): 13.9% Low
- 2007 model (original): 26.4% Low
Neutral profile
Age 30, BMI 24, no vaginal birth history.
- 2021 model (current SMFM): 68.2% Moderate
- 2007 model (original): 76.1% Moderate
Comparison: trial of labor vs elective repeat cesarean
| Aspect | Trial of labor (TOLAC) | Elective repeat cesarean |
|---|---|---|
| Likelihood of successful trial of labor | 60–80% in the general population (obstetrics literature review); this calculator gives a personal estimate. | Not applicable — scheduled birth. |
| Uterine rupture risk | About 0.7% for one prior low-transverse cesarean (ACOG Practice Bulletin No. 205). | Very low without labor; surgical and placenta risks (e.g., accreta) rise with the number of cesareans (Silver et al., Obstet Gynecol 2006). |
| Maternal morbidity | Lowest with successful VBAC; highest with failed TOLAC ending in emergency cesarean. | Planned elective surgery risk; no labor-related rupture risk. |
| Recovery | Generally faster when VBAC succeeds. | Longer post-surgery recovery. |
| Future pregnancies | One prior cesarean; VBAC remains an option. | Each additional cesarean raises the risk of placenta previa/accreta and surgical complications. |
Attribution: uterine rupture risk ~0.7% from ACOG Practice Bulletin No. 205 (one prior low-transverse cesarean); placenta accreta risk rises with cesarean count per Silver et al., Obstet Gynecol 2006; population TOLAC success 60–80% from obstetrics literature reviews. Population figures for counseling — not personal predictions.
TOLAC contraindication checklist
Based on ACOG Practice Bulletin No. 205. Check if it matches your condition (per medical records) — this list is for discussion with your doctor, not self-diagnosis.
Frequently asked questions
What is the VBAC risk score (Grobman)?
The VBAC risk score is a statistical model that estimates the likelihood of successful vaginal birth after cesarean (VBAC) when a mother undergoes a trial of labor after cesarean (TOLAC). The model used by this calculator was developed by Grobman and colleagues from a prospective cohort of 7,660 mothers at 19 academic hospitals in the United States and published in Obstetrics & Gynecology in 2007, then updated without the race/ethnicity variable in 2021 in the American Journal of Obstetrics and Gynecology. The model uses factors known from early pregnancy — maternal age, body mass index (BMI), birth history, and prior cesarean indication — to compute a personal probability, not a population average.
Which variables does the 2007 Grobman model use?
The original 2007 model uses six variables: (1) maternal age, (2) pre-pregnancy BMI, (3) race/ethnicity (the African-American and Hispanic categories in the original study), (4) prior vaginal birth history, (5) prior VBAC history, and (6) whether the prior cesarean indication was recurring — e.g., arrest of labor progress that may recur. All six go into a logistic regression equation producing a success probability between 0 and 100 percent.
What is the difference between the 2007 model and the 2021 race-free model?
The 2021 model is the official update from the same research team (Grobman et al., American Journal of Obstetrics and Gynecology 2021) that removed the race/ethnicity variable and replaced it with an objective clinical variable: treated chronic hypertension. According to its developers, its predictive accuracy is equivalent to the 2007 model, and the update is endorsed by the Society for Maternal-Fetal Medicine (SMFM). This calculator uses the 2021 model as the default and provides the 2007 model as a clearly labeled historical comparison.
How do I read this calculator's percentage result?
The result is read as a likelihood: for example, 75% means that of 100 mothers with the same profile undergoing a trial of labor, about 75 are estimated to deliver vaginally and about 25 to end with repeat cesarean. This is a population estimate applied to a personal profile — not a definite forecast for one pregnancy, and not a recommendation to choose one mode of birth.
What do the low, moderate, and high categories mean?
The calculator groups results into three categories: low (below 60%), moderate (60–79%), and high (80% and above). The 60% cutoff is used because obstetric counseling literature reports that mothers with a 60–70% or higher likelihood of success experience equivalent or lower maternal morbidity with a trial of labor than with elective repeat cesarean. These categories aid discussion with healthcare providers, not unilateral decision-making.
How accurate is the Grobman model?
In its initial development, the 2007 model had an area under the ROC curve (AUC) of about 0.75 on internal validation — meaning moderate-to-good discrimination, not perfect. Validations in other populations (United States, Japan, Spain, Italy, China, Jordan) reported AUCs between 0.67 and 0.80. The 2021 model is reported by its developers to have accuracy equivalent to the 2007 model. These figures confirm that the calculator's output is a helpful estimate, not certainty.
What is a recurring cesarean indication?
A recurring indication is a reason for the prior cesarean that may occur again in a subsequent birth, e.g., arrest of labor progress (arrest of dilation/descent) or failed induction. Conversely, non-recurring indications include situational ones like breech position or fetal distress. In the Grobman model, a recurring indication lowers the likelihood of success because the underlying cause may still be present.
Who is generally advised against a trial of labor after cesarean?
According to the American College of Obstetricians and Gynecologists (ACOG, Practice Bulletin No. 205), TOLAC is generally not advised with: a prior classical/vertical or T uterine incision; prior uterine rupture; extensive transfundal uterine surgery; conditions requiring cesarean such as placenta previa; or a facility unable to perform emergency cesarean. The contraindication checklist on this page can be used as discussion material, not for self-diagnosis.
How large is the uterine rupture risk during TOLAC?
For mothers with one prior low-transverse cesarean undergoing a trial of labor, ACOG (Practice Bulletin No. 205) reports a uterine rupture risk of about 0.7%. This risk is higher with classical/vertical incisions, so the prior incision type is important information to confirm from medical records. This success-likelihood calculator does not compute personal rupture risk — the 0.7% figure is a population number for counseling.
Can this calculator replace a discussion with my doctor?
No. This calculator is an educational tool that translates a research model into easy-to-understand numbers. The decision between attempting vaginal birth or planning a repeat cesarean involves many factors not covered by this model — the prior uterine incision type, current pregnancy conditions, estimated fetal weight, interpregnancy interval, and maternal preference. Bring this calculator's results as discussion material with your obstetrician or midwife.
Why did the 2021 model remove the race/ethnicity variable?
The model's lead researcher, William Grobman, explained that race is a social construct, not a biological variable, and keeping it in a predictive model risks reinforcing care inequities — e.g., steering Black and Hispanic mothers away from VBAC without clinical basis. The update analysis showed the race-free model has equivalent accuracy, so the race variable was replaced with treated chronic hypertension as an objective clinical predictor.
Does a prior VBAC guarantee the next one will succeed?
Nothing guarantees it, but prior VBAC is the strongest positive predictor in the Grobman model: its coefficient (+1.003 in the 2007 model and +0.489 in the 2021 model) is the largest among the factors that raise the odds. In the obstetrics literature, prior vaginal birth is consistently cited as the strongest predictor of VBAC success. Each pregnancy is still reassessed because conditions can change.