Calculate the probability of successful vaginal birth after cesarean (VBAC). Uses the MFMU prediction model to estimate TOLAC success based on maternal factors.
The VBAC (Vaginal Birth After Cesarean) prediction calculator estimates the probability of successful vaginal delivery during a trial of labor after cesarean (TOLAC). Based on the MFMU (Maternal-Fetal Medicine Units Network) prediction model developed by Grobman et al., it uses readily available maternal characteristics to generate a personalized success probability.
The model considers age, BMI, race/ethnicity, prior vaginal delivery history, prior VBAC success, the indication for the original cesarean, and cervical examination findings at admission. Women with favorable profiles (prior vaginal delivery, non-recurring indication, favorable cervix) may have success rates exceeding 85%, while those with multiple unfavorable factors may have rates below 40%.
This calculator supports shared decision-making between the patient and obstetric team, helping quantify what can otherwise be a difficult and anxiety-laden choice between TOLAC and planned repeat cesarean delivery. Check the example with realistic values before reporting. Use the steps shown to verify rounding and units. Cross-check this output using a known reference case.
The decision between TOLAC and repeat cesarean is one of the most common and consequential in obstetrics. Approximately 30% of US births are cesarean, and with the cesarean rate rising, an estimated 400,000+ women per year face this decision. A successful VBAC avoids major abdominal surgery with its associated complications, while a failed TOLAC may result in emergency cesarean with higher maternal morbidity than a planned repeat.
The MFMU calculator transforms this decision from subjective estimation to evidence-based quantification, allowing patients to make informed choices aligned with their values and circumstances.
MFMU VBAC Prediction Model (simplified Grobman): Logit = 3.766 - (0.039 × age) - (0.067 × BMI) - 0.671 (if African American) - 0.468 (if Hispanic) + 0.888 (if prior vaginal delivery) + 1.003 (if prior VBAC) + 0.446 (if non-recurring indication) + 0.105 × cervical dilation (cm) + 0.017 × effacement (%) Probability = 1 / (1 + e^(-logit))
Result: Predicted VBAC Success: 78.3%
This 32-year-old patient with BMI 28 has several favorable factors: prior vaginal delivery, non-recurring indication (breech), and favorable cervix (2 cm/50%). Her 78% predicted success rate exceeds the commonly used 60-70% threshold, supporting TOLAC as a reasonable option. She should be counseled about the small risk of uterine rupture (~0.5%) and the availability of emergency cesarean.
ACOG Practice Bulletin #205 (2019) supports TOLAC for most women with one prior low-transverse cesarean, including: women with two prior low-transverse cesareans (if otherwise appropriate), twin gestations, and those requiring labor induction. TOLAC should be attempted in facilities with capability for emergency cesarean delivery. The bulletin emphasizes shared decision-making and documentation of the counseling process.
Failed TOLAC requiring intrapartum cesarean carries higher morbidity than either successful VBAC or planned repeat cesarean: more blood loss, more infections, longer hospital stay, and rarely uterine rupture with neonatal consequences. This is why accurate prediction of VBAC success is so important — it helps identify patients whose risk of failed TOLAC outweighs the benefits of attempted vaginal delivery.
VBAC rates vary enormously worldwide: >45% in Nordic countries, ~25-30% in the UK (after NHS guidelines encouraging VBAC), and ~13% in the US. Cultural attitudes, medico-legal environments, and healthcare system structures significantly influence VBAC rates beyond medical prediction models.
ACOG does not set a specific numeric threshold. Most practitioners and institutions consider TOLAC reasonable when predicted success is ≥60-70%. However, the decision ultimately depends on patient values, access to emergency cesarean capability, and individual clinical circumstances. Some patients may choose TOLAC at lower predicted success rates after fully informed consent.
With one prior low-transverse cesarean, uterine rupture risk during TOLAC is approximately 0.5-0.7%. Risk increases with: prior classical/T-incision (4-9%), multiple prior cesareans (~1.4% for two), short inter-delivery interval (<18 months), and labor induction (especially with prostaglandins). Uterine rupture is a life-threatening emergency requiring immediate cesarean.
Yes. Induction reduces VBAC success compared to spontaneous labor. Mechanical methods (Foley bulb) are generally preferred over prostaglandins for VBAC candidates. Misoprostol is contraindicated for VBAC induction due to increased rupture risk. Oxytocin augmentation is acceptable but should be used judiciously.
The MFMU model includes race/ethnicity because it was a statistically significant predictor of VBAC success in the derivation cohort. African American and Hispanic women had lower success rates even after adjusting for other variables. This likely reflects complex interactions of social determinants, healthcare access, and biological factors. While controversial, including race improves calibration of the model.
TOLAC after two prior cesareans is more controversial. Success rates are modestly lower (~60-70% vs 75%), uterine rupture risk is slightly higher (~1-1.4%), and fewer practitioners/institutions offer it. The MFMU model was primarily validated for one prior cesarean. A separate counseling discussion and institutional capability assessment is needed for TOLAC after ≥2 cesareans.
Yes. The model can be used at any point before or during early labor. Without cervical exam data, enter 0 cm dilation and minimal effacement for a pre-labor estimate. The admission model (with cervical data) is more accurate. Grobman also published an "early pregnancy" model without cervical data for prenatal counseling.