VBAC success calculator
This estimates your chance of a vaginal birth after a caesarean using the 2021 MFMU model, the version with race and ethnicity removed. Enter age, pre-pregnancy weight, height, why your caesarean happened and your birth history. Every coefficient and every step of the arithmetic is shown alongside the answer.
VBAC success calculator
No sign-up · PrivateIn years, at this pregnancy. The model was built on a cohort averaging 28.6 years.
Pre-pregnancy weight in kilograms. The model uses weight and height separately, not BMI.
Pre-pregnancy weight in pounds. 10 stone is 140 lb.
In centimetres. Height carries the largest positive coefficient in the model.
Whole feet.
The remaining inches. Leave at 0 for an exact number of feet.
A previous VBAC is by some distance the strongest single factor in the model, and in RCOG’s guidance too.
The model counts arrest of dilation, arrest of descent and failed induction together. A caesarean for breech, for fetal distress or for placenta praevia is a “no”.
Chronic hypertension treated with medication. This is the one variable the 2021 model added that the 2007 model did not have. Pregnancy-induced high blood pressure is not the same thing.
Enter your age to see an estimate.
Nothing you type leaves your device. The whole calculation runs in your browser.
How this is calculated
FormulaHow this is calculated
The equation, printed as the paper prints it
From Grobman WA, Sandoval G, Rice MM and colleagues, “Prediction of vaginal birth after cesarean delivery in term gestations: a calculator without race and ethnicity”, American Journal of Obstetrics and Gynecology 2021;225(6):664.e1–664.e7.
Predicted probability (%) of VBAC = (exp(w) / [1 + exp(w)]) × 100, where
w = −5.952 − 0.023(age in years) − 0.024(pre-pregnancy weight in kg) + 0.056(height in cm) − 0.597(arrest indication) + 0.868(previous vaginal delivery only before the prior caesarean) + 1.869(previous VBAC) − 0.966(treated chronic hypertension)
The last four terms are coded 0 for no and 1 for yes. The model was fitted on 7,712 women from the MFMU Cesarean Registry and has an area under the receiver operating characteristic curve of 0.75 (95% CI 0.74–0.77). The calculator prints every term and its contribution to w, so the arithmetic can be checked rather than trusted.
Why the 2007 version is not used here
The original 2007 MFMU calculator, still running on a number of sites, included race and ethnicity as model variables, so two otherwise identical women received different predicted chances. The 2021 paper states the problem in its own introduction: that tool “included the socially-constructed variables of race and ethnicity, and there is concern that their inclusion may reify a biologic construct of race/ethnicity and perpetuate health disparities.” A lower predicted chance can be used to steer a woman towards a repeat caesarean. The 2021 model was derived from the same registry by the same method with those variables removed. Every other variable was retained, treated chronic hypertension was added, and the classification performance is identical at 0.75.
Verification
The paper publishes twelve worked examples in its Table 3. This implementation reproduces all twelve to within 0.4 of a percentage point — example 1 (30 years, 71 kg, 171 cm, previous VBAC) gives 95.7% against a published 95.6%; example 7 gives 59.6% against 59.4%; example 12 gives 32.4% against 32.1%. The residual is the paper's own rounding of the coefficients to three decimal places.
Where the model is weak, in its authors' own words
The calibration curve “only begins to deviate to any degree when the chance of VBAC is less than 40%”. An Australian validation of 541 women found an area under the curve of 0.707 and concluded that the nomogram “performed poorly in those predicted to have a <40% chance of successful VBAC”. Below 40% this calculator says so on the result itself. The registry data were collected between 1999 and 2002; the authors chose the older dataset deliberately so that the only change from the validated 2007 model would be the removal of race and ethnicity.
The context figures
RCOG Green-top Guideline No. 45 puts the success rate of planned VBAC at 72–75% overall and 85–90% for women with a previous vaginal birth, and the risk of uterine scar rupture at approximately 1 in 200 (0.5%) against under 0.02% for a planned repeat caesarean.
Two calculators, one of which should not still be in use
If you search for a VBAC calculator you will find several, and a number of them are still running a model published in 2007 that used race and ethnicity as inputs. Feed it two women with identical ages, weights, heights and obstetric histories, and it returned a lower chance of vaginal birth for the one recorded as Black or Hispanic. That number then went into a conversation about whether to attempt a vaginal birth at all.
In 2021 the same research team rebuilt the model. Same registry, same method, race and ethnicity removed from consideration entirely. The result classifies just as well — an area under the curve of 0.75, identical to the old model — and every other variable survived. One new one, chronic hypertension treated with medication, was added. This calculator runs that version, and it will not run the other one.
What the model actually uses
Six things, all knowable at a first antenatal appointment: your age, your pre-pregnancy weight, your height, whether your caesarean was for arrest of dilation or descent (or a failed induction), whether you have given birth vaginally before and when, and whether you take medication for chronic high blood pressure.
Height carries the largest positive coefficient per unit — every centimetre adds 0.056 to the log-odds — and a previous VBAC carries the largest single jump at 1.869, which for most women outweighs everything else in the equation put together. RCOG says the same thing in words: previous vaginal delivery, particularly a previous VBAC, is the single best predictor of success and is associated with a planned VBAC success rate of 85 to 90%.
The calculator shows every term and what yours contributes, because a probability with no working shown is a black box, and this is not a field that needs more of those.
The numbers that matter more than the percentage
The chance of a vaginal birth is only half the decision. The other half is what the risks actually are, and RCOG gives them in absolute terms rather than relative ones.
Planned VBAC carries an approximately 1 in 200, or 0.5%, risk of the caesarean scar rupturing. With a planned repeat caesarean the risk is under 0.02%. That difference is real, and it is the reason VBAC labour happens where there is immediate access to an operating theatre and on-site blood transfusion, with continuous monitoring.
The risk of delivery-related death of the baby with planned VBAC is 4 per 10,000, or 0.04% — which RCOG describes as comparable to the risk for a woman labouring with her first baby. The risk of hypoxic ischaemic encephalopathy is 8 per 10,000. Transient breathing problems affect 2 to 3% of babies born by VBAC and 4 to 5% of those born by planned repeat caesarean. Maternal death is 4 per 100,000 with planned VBAC and 13 per 100,000 with a planned repeat caesarean.
Both options are safe. They are safe in slightly different directions, which is exactly why the decision is yours.
What the model cannot see
Everything about how labour starts. RCOG puts the risk of uterine rupture at 0.15 to 0.4% in spontaneous labour, 0.54 to 1.4% if labour is induced and 0.9 to 1.91% if it is augmented with oxytocin, and induced or augmented labour carries roughly a 1.5-fold higher chance of ending in caesarean. None of that is in the equation, because none of it is knowable in the first trimester.
Nor is cervical favourability at admission, the baby's station, or estimated birth weight. RCOG notes that when induced labour, no previous vaginal birth, a BMI over 30 and a previous caesarean for labour dystocia are all present together, successful VBAC is achieved in about 40% of cases.
Who the model is for
It was built on women with one previous lower-segment caesarean, a single baby, head down, at 37 weeks or later — which is exactly the group RCOG describes as suitable for planned VBAC. It does not apply after a previous uterine rupture or a classical vertical scar, both of which RCOG lists as contraindications. It does not apply to twins, a breech baby or major placenta praevia.
With two or more previous caesareans, VBAC may still be offered after counselling by a senior obstetrician; a systematic review puts success at 71.1% and uterine rupture at 1.36% in that group, and this model does not cover it.
A number is not a decision
The authors are unusually direct about this. The model, they write, “is not designed to uncover individual factors or produce a summary probability estimate that indicates someone should or should not undergo a TOLAC”. It estimates the chance of one outcome. How much that outcome matters to you — against recovery time, against future pregnancies, against how your last birth felt — is not something a logistic regression has any view on.
NICE expects the decision to be made with an obstetrician and documented, in most cases by 36 weeks. Take this number to that conversation. Do not let it end one.
Sources
- Prediction of vaginal birth after cesarean delivery in term gestations: a calculator without race and ethnicity (Am J Obstet Gynecol 2021;225:664.e1-664.e7) — Grobman WA, Sandoval G, Rice MM, et al - NICHD Maternal-Fetal Medicine Units Network, accessed
- Validation of updated antenatal vaginal birth after caesarean section prediction model without race and ethnicity in Australia (ANZJOG 2023;63:314-320) — Liu CZ, Mahomed K, accessed
- Birth After Previous Caesarean Birth - Green-top Guideline No. 45 — Royal College of Obstetricians and Gynaecologists, accessed
- Caesarean birth - NICE guideline NG192, section 1.8, planned birth after a previous caesarean birth — National Institute for Health and Care Excellence, accessed