This calculator estimates the probability of a successful vaginal birth after cesarean (VBAC) using a published logistic-regression model from the Maternal-Fetal Medicine Units (MFMU) Network. It's meant for patients and clinicians discussing whether to attempt a trial of labor after cesarean (TOLAC), using information available early in pregnancy. This article explains how the model works, what each input means, and where its limits lie — including why it deliberately excludes race and ethnicity.
How the VBAC success model works
The calculator implements a logistic regression model: seven weighted terms (an intercept plus six clinical factors) are summed into a 'linear predictor,' which is then converted into a probability between 0% and 100% using the standard logistic function. Each factor's coefficient reflects how strongly, and in which direction, it moves the estimate — a prior successful VBAC has the single largest positive effect, while treated chronic hypertension and an arrest-of-labor indication for the prior cesarean both pull the estimate down.
This is the antepartum version of the MFMU model, meaning it only uses information available at an early prenatal visit — maternal age, pre-pregnancy weight and height, and obstetric history. A separate, later MFMU model adds labor-time findings such as the cervical exam and is meant for use once a patient is in labor; that 'at admission for delivery' model is not what this calculator implements.
Why race and ethnicity were removed
The original 2007 MFMU VBAC calculator (Grobman et al., Obstet Gynecol 2007) included patient race and ethnicity as predictors. Research subsequently found that this produced systematically lower predicted VBAC probabilities for Black and Hispanic patients compared to White patients with otherwise identical clinical factors — a gap that lowered TOLAC counseling and offering rates for those patients without a biological basis for the difference.
Because studies also showed that removing race and ethnicity did not meaningfully reduce the model's overall predictive accuracy, the same MFMU authors published a race-neutral revision in 2021 (Grobman WA, Sandoval G, Rice MM, et al. Am J Obstet Gynecol. 2021;225:664.e1-664.e7). This calculator implements that 2021 revision, which the American College of Obstetricians and Gynecologists (ACOG) has since endorsed in its counseling guidance on VBAC calculators.
Inputs and what they mean
Maternal age and pre-pregnancy weight/height are continuous predictors — the model uses raw weight (kg) and height (cm) rather than a combined BMI figure, so the two act somewhat independently in the equation.
Prior vaginal delivery history is the strongest lever in the model. A prior vaginal delivery that happened only before the cesarean adds a moderate positive effect; a prior VBAC (a vaginal delivery that happened after the cesarean) adds more than double that effect and is the single biggest positive predictor.
Indication for the prior cesarean distinguishes an arrest-of-labor disorder (which lowers the predicted probability) from other indications such as breech presentation, non-reassuring fetal status, or an elective repeat cesarean (which don't carry that penalty).
Treated chronic hypertension lowers the predicted probability, reflecting its association with a higher likelihood of needing a repeat cesarean.
Limits and edge cases
This model was developed and validated for patients with one prior low-transverse cesarean who are otherwise appropriate TOLAC candidates. It was not built for patients with multiple prior cesareans, a classical or T-shaped uterine incision, a prior uterine rupture, or other recognized contraindications to TOLAC — those situations require individualized clinical assessment, not this calculator.
The output is a probability, not a threshold or a recommendation. The calculator groups results into descriptive bands (lower, moderate, favorable, high) purely for readability; the source paper does not define official cutoffs. A lower predicted probability does not rule out attempting TOLAC, and a higher one does not guarantee success — the estimate is one input among many (including the patient's own priorities and the risks of a repeat cesarean) that should be discussed with the treating obstetric provider.