VBAC Calculator (Vaginal Birth After Cesarean)
This calculator estimates the chance of a successful vaginal birth after cesarean (VBAC) using two clinically validated models. The Grobman antenatal model can be used before labour begins and needs only information available at the first prenatal visit. The Flamm intrapartum score is completed at admission for delivery and adds cervical examination findings. Both models produce a probability of successful vaginal delivery; they are counselling tools and do not replace clinical judgement.
What is VBAC and how is this calculator used?
Vaginal birth after cesarean (VBAC) refers to a vaginal delivery by a person who had a previous birth by cesarean section. Most guidelines from organisations such as ACOG, NICE, and SOGC support offering a trial of labour after cesarean (TOLAC) to eligible people, because a successful VBAC avoids major abdominal surgery and its associated risks for both the current and future pregnancies. This calculator presents two evidence-based prediction tools: the Grobman antenatal model, which can be calculated at the first prenatal visit using information the patient already knows, and the Flamm intrapartum score, completed at admission for delivery after a cervical examination. Enter your details on the left to see your estimated probability instantly.
The Grobman antenatal model
The Grobman model was originally developed in 2007 by William Grobman and the NICHD Maternal-Fetal Medicine Units (MFMU) Network from a prospective cohort of 7,660 patients across 19 academic centres in the United States. That 2007 nomogram used maternal age, pre-pregnancy BMI, race and ethnicity, prior vaginal delivery, prior VBAC, and a potentially recurrent indication for the cesarean. In 2021 the MFMU Network published a full refit of the model on 11,687 patients from the same Cesarean Registry, this time without race or ethnicity (Grobman et al., American Journal of Obstetrics and Gynecology 2021). The refit is what this calculator uses. It is important that it is a genuine refit rather than the 2007 equation with the race terms crossed out: dropping variables from a fitted logistic regression without re-estimating the remaining coefficients does not produce a validated model. The 2021 equation is: w = -5.952 - 0.023 x age - 0.024 x pre-pregnancy weight (kg) + 0.056 x height (cm) - 0.597 x (arrest of dilation or descent) + 0.868 x (vaginal delivery only before the prior cesarean) + 1.869 x (previous VBAC) - 0.966 x (medication-treated chronic hypertension); probability = exp(w) / [1 + exp(w)]. Note that height and weight enter separately rather than as BMI, that the indication term is specifically arrest of dilation or descent, and that treated chronic hypertension is a new variable not present in 2007. The refit achieved an area under the ROC curve of 0.75, the same discrimination as the older race-inclusive model.
The Flamm intrapartum score
The Flamm and Geiger (1997) scoring system is designed for use at admission for delivery and adds cervical examination findings that are not available during prenatal counselling. Five factors are scored: maternal age under 40 (2 points), vaginal birth history (0-4 points depending on when the delivery occurred relative to the cesarean), a reason other than failure to progress for the prior cesarean (1 point), cervical effacement (0-2 points), and cervical dilation of 4 cm or more (1 point). Scores range from 0 to 10 and map to success rates from 49% at 0-2 points up to 95% at 8-10 points. An important limitation is that a low score does not reliably predict failure: even patients with a score of 0-2 have a nearly 50% success rate, so the score informs counselling but should not be used alone to deny a trial of labour.
Benefits and risks of VBAC vs. repeat cesarean
A successful VBAC avoids a major abdominal operation and its immediate complications (blood transfusion, bladder injury, surgical infection) as well as long-term risks from repeated uterine scars such as abnormal placentation in future pregnancies. Most people who attempt TOLAC achieve a successful vaginal delivery (approximately 60-80% depending on selection). The primary risk of TOLAC compared with planned repeat cesarean is uterine rupture, which occurs in about 0.5-0.9% of trials of labour in people with one prior low-transverse cesarean and can be life-threatening for both the person in labour and the baby. Rupture risk is higher with classical uterine scars, prior uterine surgery, or a short interval between deliveries. The decision should weigh the probability of success against individual risk factors and the patient's own preferences, with access to emergency cesarean capability as a prerequisite.
Flamm intrapartum score - success rate table (Flamm and Geiger 1997)
| Flamm score | VBAC success rate |
|---|---|
| 0-2 | 49% |
| 3 | 60% |
| 4 | 67% |
| 5 | 77% |
| 6 | 89% |
| 7 | 93% |
| 8-10 | 95% |
Observed VBAC success rates by total score from the original Flamm-Geiger derivation cohort. A low score does not predict failure.
Frequently asked questions
What is a good VBAC success probability?
There is no single threshold that determines whether to attempt a trial of labour. ACOG notes that a predicted success rate of 60-70% or higher is generally considered favourable, and most guidelines suggest discussing TOLAC when the probability is above 50%. However, probability alone does not determine the best decision: a patient with a 55% chance who strongly wants to avoid surgery may choose differently from one with a 75% chance who prioritises certainty. The final decision should integrate the probability estimate, individual risk factors, hospital resources, and personal values.
Why does this calculator use the race-free Grobman model?
The original 2007 Grobman model included race and ethnicity terms that assigned lower predicted success to Black and Hispanic patients. Subsequent analyses showed this reduced the likelihood of offering TOLAC to these groups without a clear biological basis for the adjustment, contributing to inequities in obstetric care. In 2021, Grobman and colleagues re-derived the model on 11,687 MFMU Cesarean Registry patients without race or ethnicity, selecting variables afresh and re-estimating every coefficient. This calculator implements that 2021 equation exactly as published. It does not simply delete the race terms from the 2007 formula, because removing a covariate from a fitted regression without refitting leaves the remaining coefficients biased and produces a model that has never been validated.
When should I use the Grobman model vs. the Flamm score?
Use the Grobman antenatal model during prenatal appointments, ideally at the first visit or any time the birth plan is being discussed, because it only needs information the patient can readily report: age, pre-pregnancy weight and height, and obstetric history. Use the Flamm intrapartum score at hospital admission for delivery, once a cervical examination has been performed. The Flamm score is more reflective of where labour stands at that moment; the Grobman model is better for early counselling.
Does a low score mean I cannot have a VBAC?
No. A low score means the statistical probability from the model is lower, not that vaginal birth is impossible. The Flamm table shows that even at the lowest scores (0-2), about 49% of patients successfully deliver vaginally. Neither model should be used to deny a trial of labour without a full clinical assessment. Many people with unfavourable scores go on to have successful VBACs, and individual factors not captured in the model (such as the reason for the prior cesarean, the course of the current labour, and fetal position) can shift the outcome.
What are the main risk factors for uterine rupture during TOLAC?
The highest risk is associated with a classical (vertical) uterine incision, a prior uterine rupture, or more than two prior cesarean deliveries. Other factors that modestly increase rupture risk include: an interpregnancy interval of less than 18 months, labour induction (especially with prostaglandins), an unknown uterine scar type, and a prior low-vertical or T-shaped incision. Low-transverse uterine scars, which are by far the most common, carry the lowest rupture risk (approximately 0.5-0.9%) during a trial of labour.
Is prior vaginal delivery really important for VBAC prediction?
Yes, it is one of the strongest predictors in both models. In the 2021 Grobman equation, a vaginal delivery that occurred only before the cesarean adds 0.868 to the linear predictor, while a previous VBAC adds 1.869, which is by far the largest coefficient in the formula. In the Flamm model, a history of vaginal birth both before and after the cesarean scores 4 points out of a maximum 10, contributing nearly half the possible total. The biological interpretation is that a proven pelvis and prior experience of successful labour are highly predictive of future success.
Sources
- Grobman WA et al. Prediction of vaginal birth after cesarean delivery in term gestations: a calculator without race and ethnicity. Am J Obstet Gynecol 2021; 225(6):664.e1-664.e7. (Source of the equation used here.)
- Grobman WA et al. Development of a nomogram for prediction of vaginal birth after cesarean delivery. Obstet Gynecol 2007; 109(4):806-12. (Original race-inclusive model, superseded here.)
- Flamm BL, Geiger AM. Vaginal birth after cesarean delivery: an admission scoring system. Obstet Gynecol 1997; 90(6):907-10.
- ACOG Practice Bulletin 205: Vaginal Birth After Cesarean Delivery. Obstet Gynecol 2019; 133(2):e110-e127.