VBAC Calculator
Use this VBAC calculator to estimate your chance of a vaginal birth after cesarean if you try labor, using the race-free 2021 MFMU model.
Updated October 20, 2026
Medical Disclaimer: This calculator provides a general estimate based on established research and is for educational purposes only, not a substitute for medical advice. Individual results can vary based on health conditions, so consult a qualified healthcare professional or registered dietitian before making significant changes to your diet, exercise, or health routine. Read our full Medical Disclaimer.
How Your Chance of VBAC Is Calculated
VBAC means vaginal birth after cesarean. Choosing to go into labor after a past cesarean is called a trial of labor after cesarean (TOLAC), and most people who try it give birth vaginally⁴. Your chance of VBAC is the share of people like you who would have a vaginal birth if they tried labor.
What the VBAC Calculator Measures
It estimates the chance that labor ends in a vaginal birth rather than a cesarean during labor. It does not estimate the risk of the uterine scar tearing (uterine rupture) or other complications. Its authors say it supports a shared decision with a clinician and does not show whether someone should try labor¹.
The Formula
This calculator uses the 2021 model from the Maternal-Fetal Medicine Units (MFMU) Network, based on 11,687 people who tried labor after one low transverse cesarean at US hospitals from 1999 to 2002; the final formula used the 7,712 with complete data¹. Yes or no answers count as 1 for yes and 0 for no:
w = -5.952 - 0.023 x age - 0.024 x pre-pregnancy weight (kg) + 0.056 x height (cm) - 0.597 x arrest disorder + 0.868 x vaginal birth only before the cesarean + 1.869 x previous VBAC - 0.966 x treated chronic high blood pressure
Chance of VBAC (%) = 100 x e^w / (1 + e^w)
"Arrest disorder" means the past cesarean was done because the cervix stopped opening or the baby stopped moving down, including an induction that did not work¹.
How This Calculator Computes Your Result
Three questions first check that the model fits you: one previous cesarean, a low transverse cut in the uterus, and one baby. You then enter age, height, pre-pregnancy weight, vaginal birth history, the reason for your cesarean, and whether you take medicine for long-term high blood pressure. Pounds convert at 0.45359237 kg and inches at 2.54 cm, and the result is rounded to a whole percent. If you are not sure why your cesarean was done, you get a range: your chance with and without that reason.
What Your Result Means
A result of 70% means that out of 100 people with your answers who try labor, about 70 would be expected to give birth vaginally and about 30 would have a cesarean during labor. There is no official pass mark. In the study behind the calculator, 74% of people who tried labor had a vaginal birth¹. These published examples are for a 30-year-old who weighed 71 kg (157 lb) before pregnancy¹:
Example | Chance of VBAC |
Base: 156 cm (5 ft 1 in), no vaginal birth, no other factors | 59.4% |
Base, but 37 years old | 55.4% |
Base, but 171 cm (5 ft 7 in) tall | 77.2% |
Base, but a vaginal birth only before the cesarean | 77.7% |
Base, but a previous VBAC | 90.5% |
Base, but the cesarean was for labor not progressing | 44.6% |
Base, but treated long-term high blood pressure | 35.8% |
Our tool may read up to half a point higher, because the published formula is rounded to three decimals.
What Raises or Lowers the Estimate
A previous VBAC is the strongest factor. It multiplies the odds of a vaginal birth by about 6.5, and a vaginal birth only before the cesarean multiplies them by about 2.4¹. Each extra centimeter of height raises the odds by about 6%, and each extra year of age or kilogram of pre-pregnancy weight lowers them by about 2%¹. A past cesarean for labor not progressing lowers the odds by about 45%, and treated long-term high blood pressure lowers them by about 62%¹.
Because height and weight count separately, the same BMI can give different results. At age 30 with no vaginal birth, 155 cm and 62.5 kg gives 63%, but 170 cm and 75.1 kg gives 75%, though both are a BMI of 26. Only 144 people in the study (1.2%) had treated long-term high blood pressure, so results that include it are less certain¹.
Why the Calculator No Longer Uses Race
The earlier MFMU calculator, published in 2007, included race and ethnicity¹. People identified as Black or Hispanic were given a lower predicted chance of VBAC³. The MFMU authors noted concern that this could treat race as biology and keep health gaps in place¹. The 2021 version removed race and ethnicity, kept the other factors and added treated long-term high blood pressure¹. It predicts as well as the old model: both scored 0.75 on an accuracy measure called the AUC, where 0.5 is a coin flip and 1.0 is perfect¹.
What the Estimate Leaves Out
The International Federation of Gynecology and Obstetrics (FIGO) puts VBAC success at 60% to 80% and the risk of uterine rupture at about 0.3% to 0.7% after one low transverse cesarean⁴. FIGO advises against labor after a classical, T-shaped or J-shaped cut, or one that reached the upper uterus⁴. It also does not recommend routine use of VBAC prediction models, because many have not been tested in new groups of patients⁴.
This model only uses facts known at the first prenatal visit, so it does not include how labor starts or what a cervical exam shows¹. Induced labor has been linked with lower VBAC success than labor that starts on its own⁴. Our Bishop Score Calculator explains how readiness of the cervix is scored. The MFMU Network also offers a version for use at admission for delivery, which this tool does not include².
Chance of VBAC in the Published Examples
Each bar changes one thing from the base example: a 30-year-old, 156 cm (5 ft 1 in) tall, who weighed 71 kg (157 lb) before pregnancy.
Show data table
| Example | Chance of VBAC (%) |
|---|---|
| Base: no vaginal birth, no other factors | 59.4 |
| Base, but 37 years old | 55.4 |
| Base, but 171 cm (5 ft 7 in) tall | 77.2 |
| Base, but a vaginal birth only before the cesarean | 77.7 |
| Base, but a previous VBAC | 90.5 |
| Base, but the cesarean was for labor not progressing | 44.6 |
| Base, but treated long-term high blood pressure | 35.8 |
A Note on Accuracy
This calculator gives a group average for people with your answers, not a prediction for you. Its estimates matched real outcomes closely, except below about 40%, where few people fell¹. It was built on US hospital data from 1999 to 2002 and tested on a separate part of the same data¹. It applies only to one previous low transverse cesarean and one baby, head down, at 37 weeks or later. It does not weigh the risks of labor against a planned repeat cesarean, so use it to start a conversation with your clinician.
Frequently Asked Questions
What is a good VBAC success rate?
There is no official cutoff, but most people who try labor after one cesarean give birth vaginally: 60% to 80% by FIGO's estimate, and 74% in the study behind this calculator⁴ ¹.
How accurate is the VBAC calculator?
It has an AUC of 0.75, so it ranks people's chances fairly well but not perfectly, and its estimates matched real outcomes well above about 40%¹. It is a group estimate, not a promise about one birth.
Does the VBAC calculator still use race?
No. The MFMU Network removed race and ethnicity in 2021, and the new version predicts as well as the old one¹.
What is the risk of uterine rupture with VBAC?
FIGO puts it at about 0.3% to 0.7%, or 3 to 7 in 1,000, for people with one previous low transverse cesarean⁴. This calculator does not estimate that risk.
Sources
Prediction of VBAC in Term Gestations: A Calculator Without Race and Ethnicity, Grobman et al., 2021
Vaginal Birth After Cesarean Calculator, NICHD MFMU Network
Evaluating the Elimination of Race from VBAC Calculators, NYC Health Department
FIGO Good Practice Recommendations for Vaginal Birth After Cesarean Section, Barnea et al., 2025