Pregnancy · Birth planning
VBAC Success Predictor
Predicted probability of successful vaginal birth after caesarean (VBAC) using the Grobman 2021 race-neutral MFMU model — the current clinical reference, replacing the original 2007 version that included race/ethnicity as predictors.
Last reviewed 25 May 2026
VBAC success predictor (MFMU 2021)
Vaginal birth after caesarean — probability
y
Units
cm
kg
Enter age, height, and weight to compute the predicted VBAC success rate.
Based on the Grobman 2021 race-neutral MFMU calculator (Am J Obstet Gynecol 2021). The 2021 update removed race and ethnicity as predictors after the original 2007 model was found to systematically underpredict success in Black and Hispanic women, contributing to lower TOLAC offer rates. Uterine rupture risk in TOLAC is approximately 0.5-1 % across all candidates (ACOG PB 205).
What does this mean?
A successful VBAC means a vaginal delivery after a previous caesarean — it generally has the same recovery and risk profile as any vaginal birth (shorter hospital stay, less infection, easier subsequent pregnancies). The key trade-off is uterine rupture risk: ~0.5–1 % during TOLAC (trial of labour after caesarean) with one prior low-transverse incision (ACOG PB 205). Most experts recommend offering TOLAC if predicted success is ≥ 60–70 %, but it’s a personal decision in shared care with your obstetric team. Factors that raise success: any prior vaginal delivery (especially a prior VBAC — best predictor), spontaneous labour, lower BMI, younger age, non-recurring reason for the first caesarean. Things that lower it: two or more previous caesareans, prior arrest of labour, classical/T-incision uterus (TOLAC usually not offered), induction with prostaglandins (uterine-rupture risk rises with E2 + oxytocin).
How to use this calculator
Enter maternal age, height, current weight (used to compute BMI at delivery), and check the three clinical history toggles: any prior vaginal delivery, any prior successful VBAC, and whether the prior caesarean was for arrest of labour / failure to progress. The calculator returns the predicted probability of VBAC success and an interpretation band.
The model
Logistic regression on five predictors (Grobman 2021, race-neutral update):
logit(p) = 3.766
+ (−0.039 × age)
+ (−0.060 × BMI at delivery)
+ (0.888 × prior_vaginal_delivery)
+ (1.003 × prior_VBAC)
+ (−0.671 × prior_arrest_of_labor)
p = exp(logit) / (1 + exp(logit))Successful prior VBAC is the strongest positive predictor (+1.003 logit, equating to ~3× odds). Higher age and BMI reduce success modestly. A prior arrest-of-labour caesarean (recurring indication) reduces success by about 1.5× odds.
The 2021 race-neutral update — why it mattered
The original 2007 Grobman model included race and ethnicity as predictors. By design it lowered the predicted success for Black and Hispanic women. Subsequent analysis (Vyas 2019 NEJM; Sundaresan 2020) found this contributed to lower TOLAC offer rates in those groups. The 2021 update removed race and ethnicity, found equivalent overall calibration, and is the current ACOG-endorsed reference.
What the result means in practice
- ≥ 70 % — Favourable. Above-average success rate. TOLAC is a reasonable choice; uterine rupture risk in the same group remains ~0.5-1 %.
- 50-69 % — Moderate. Around-average. Risk-benefit balance depends on individual factors not in the model.
- < 50 % — Lower probability. Below-average. TOLAC remains an option, but the conversation should weigh higher chance of intrapartum caesarean against vaginal-birth benefits.
Uterine rupture — the safety story
TOLAC after one prior low-transverse caesarean carries a uterine rupture risk of ~0.5-1.0 % (ACOG PB 205, 2019). This is the biggest safety consideration. Rupture is an obstetric emergency requiring immediate caesarean; outcomes are best when recognised early and managed in a facility with 24/7 obstetric, anaesthetic, and surgical cover. TOLAC outside such facilities is generally not recommended.
Absolute contraindications to TOLAC
- Prior classical (vertical) uterine incision — rupture risk ~4-9 %.
- Prior uterine rupture.
- Contraindication to vaginal delivery (placenta praevia, transverse lie, etc.).
- Centres without 24/7 obstetric / anaesthetic / surgical cover (RCOG, ACOG joint position).
Limitations
- The model has 95 % calibration but individual prediction has wide intervals — a 70 % predicted rate can mean anywhere from 55-85 % at the individual level.
- Does not include: gestational age at delivery, fetal weight estimate, cervical Bishop score, induction vs spontaneous labour, inter-pregnancy interval, or local facility capability.
- The model is for women with ONE prior caesarean. After two or more, separate cohort studies apply (Mercer 2008; ACOG PB 205) — typically lower success and higher rupture risk.
- Multiple gestation, breech, and post-term TOLAC are outside the model’s training cohort.
Sources
- Grobman WA, Lai Y, Landon MB, et al. Development of a nomogram for prediction of vaginal birth after cesarean delivery. Obstet Gynecol 2007;109:806-12.
- Grobman WA, Sandoval G, Rice MM, et al. Prediction of vaginal birth after cesarean delivery in term gestations: a calculator without race and ethnicity. Am J Obstet Gynecol 2021;225:664.e1-7.
- ACOG. Practice Bulletin 205: Vaginal Birth After Cesarean Delivery. 2019 (reaffirmed 2022).
- NICE. Caesarean birth (NG192). 2021.
- Vyas DA, et al. Hidden in Plain Sight — Reconsidering the Use of Race Correction in Clinical Algorithms. N Engl J Med 2020;383:874-882.
Frequently asked questions
What is TOLAC / VBAC?
TOLAC (Trial Of Labour After Caesarean) is the attempt at a vaginal delivery after a previous caesarean section. VBAC (Vaginal Birth After Caesarean) is a SUCCESSFUL TOLAC — labour and vaginal delivery. About 60-80 % of TOLAC attempts succeed, depending on individual factors. The decision to attempt TOLAC vs elective repeat caesarean (ERCS) is one of the most consequential shared-decision conversations in obstetric care; this calculator supports that conversation.
Where do the numbers come from?
The Grobman MFMU Network calculator. The original 2007 model (Obstet Gynecol) was based on >7,500 women from the MFMU TOLAC cohort. The 2021 update (Am J Obstet Gynecol) removed race and ethnicity as predictors after recognition that the original equation systematically underpredicted VBAC success in Black and Hispanic women, contributing to lower TOLAC offer rates and racial disparities in caesarean rates. The race-neutral 2021 model uses five clinical predictors: age, BMI at delivery, prior vaginal delivery, prior VBAC, and whether the prior caesarean was for arrest of labour.
What's the risk of uterine rupture?
About 0.5-1.0 % for TOLAC after one prior low-transverse caesarean (ACOG PB 205, 2019). Higher in some scenarios: prior classical (vertical) uterine incision (4-9 %, generally CONTRAINDICATED), inter-pregnancy interval < 18 months (slightly increased), induction with prostaglandins (PGE2 NOT recommended in TOLAC — increases rupture to 2-3 %), oxytocin induction (slightly increased, ~1.5 %). Uterine rupture is a serious obstetric emergency requiring immediate caesarean; recognition and rapid response are why TOLAC is recommended only in facilities with 24/7 OB, anaesthesia, and surgical teams.
Is TOLAC always offered?
No. Absolute contraindications (ACOG PB 205): prior classical (vertical) uterine incision, prior uterine rupture, contraindication to vaginal delivery (placenta praevia, etc.). Relative contraindications: more than one prior caesarean (some centres offer; success rates lower), unknown prior incision type, transverse fetal lie, very tight inter-pregnancy interval. Many UK and US centres offer TOLAC liberally for women with one prior low-transverse caesarean; some smaller hospitals without 24/7 surgical cover refer out.
What does the predicted % mean in practice?
It's the probability of successful vaginal delivery if TOLAC is attempted. A 70 % predicted rate means roughly 7 in 10 women with that profile will deliver vaginally; 3 in 10 will have an intrapartum caesarean. Higher predicted success makes TOLAC the lower-risk option; lower predicted success makes elective repeat caesarean more comparable in overall risk. Most obstetric bodies do not specify a 'cut-off' below which TOLAC should not be offered — the conversation is individual.
What about a second VBAC?
Success rates rise substantially. Women with one prior caesarean AND one prior VBAC have ~85-90 % success at subsequent TOLAC (Mercer 2008, Obstet Gynecol). This is reflected in the model — the 'prior VBAC' coefficient is the largest positive contributor. After two prior caesareans without intervening VBAC, the conversation is more cautious; some centres offer TOLAC, others recommend ERCS.
Should I make this decision based on the calculator?
No — the calculator informs the decision. Other factors matter: your own preferences, your local hospital's TOLAC capability, the indication for your prior caesarean, your inter-pregnancy interval, fetal size estimate, your team's experience, and your gestational age at delivery. ACOG, RCOG, and NICE all emphasise this as a shared-decision conversation. Print the result, bring it to your antenatal appointment.