VBAC Calculator
Use our free Vbaccalculator Calculator to get personalized health results. Based on validated medical formulas and clinical guidelines.
Formula
P(success) = 1 / (1 + e^(-logit)); logit = 3.766 - 0.039(age) - 0.060(BMI-24) + factors
Where the logit is computed from the Grobman MFMU model with coefficients for maternal age, BMI, prior vaginal delivery (+1.003), non-recurring indication (+0.482), cervical dilation (+0.103 per cm), and ethnicity adjustments. The probability is then calculated using the logistic function. This validated model was derived from a large multicenter observational study.
Worked Examples
Example 1: Favorable VBAC Candidate
Problem: A 28-year-old woman with BMI 24, one prior cesarean for breech presentation (non-recurring), previous vaginal delivery, admitted at 2 cm dilation at 39 weeks. Calculate VBAC success probability.
Solution: Base logit: 3.766\nAge adjustment: -0.039 x 28 = -1.092\nBMI adjustment: -0.060 x (24-24) = 0\nPrevious vaginal delivery: +1.003\nNon-recurring indication: +0.482\nCervical dilation: +0.103 x 2 = +0.206\nLogit = 3.766 - 1.092 + 1.003 + 0.482 + 0.206 = 4.365\nProbability = 1/(1 + e^(-4.365)) = 0.987 = 98.7%\nUterine rupture risk: 0.5% (spontaneous labor)
Result: VBAC Success Probability: 98.7% - Excellent candidate for trial of labor
Example 2: Less Favorable VBAC Candidate
Problem: A 38-year-old woman with BMI 35, one prior cesarean for failure to progress (recurring), no prior vaginal delivery, admitted at 1 cm dilation at 40 weeks. Calculate VBAC success probability.
Solution: Base logit: 3.766\nAge adjustment: -0.039 x 38 = -1.482\nBMI adjustment: -0.060 x (35-24) = -0.660\nNo previous vaginal delivery: 0\nRecurring indication: 0 (no bonus)\nCervical dilation: +0.103 x 1 = +0.103\nLogit = 3.766 - 1.482 - 0.660 + 0.103 = 1.727\nProbability = 1/(1 + e^(-1.727)) = 0.849 = 84.9%\nUterine rupture risk: 0.5%
Result: VBAC Success Probability: 84.9% - Moderate candidate, discuss risks and benefits thoroughly
Frequently Asked Questions
What is VBAC and who is eligible to attempt a trial of labor after cesarean?
VBAC (Vaginal Birth After Cesarean) refers to a vaginal delivery in a woman who has previously had one or more cesarean deliveries. A trial of labor after cesarean (TOLAC) is the attempt to have a vaginal birth. Most women with one prior low transverse cesarean incision are candidates for TOLAC, with overall success rates of 60-80%. Eligibility requires a prior low transverse uterine incision, no other uterine scars or rupture history, a physician available throughout labor capable of performing emergency cesarean, and the availability of anesthesia and operating room for emergency surgery. Contraindications include prior classical (vertical) uterine incision, prior uterine rupture, certain types of prior uterine surgery, and placenta previa.
What factors most strongly predict successful VBAC?
The strongest predictors of VBAC success identified by the Maternal-Fetal Medicine Units Network (MFMU) include a history of previous vaginal delivery (the single strongest predictor, increasing success probability by approximately 20-30%), a non-recurring indication for the prior cesarean (such as breech presentation rather than failure to progress), younger maternal age, lower BMI, greater cervical dilation at admission, and spontaneous onset of labor rather than induction. The MFMU prediction model combines these factors into a probability score. Women with a predicted success rate above 70% generally have outcomes comparable to or better than elective repeat cesarean when considering both maternal and neonatal outcomes across current and future pregnancies.
What is uterine rupture and how common is it during VBAC attempt?
Uterine rupture is the most serious complication of TOLAC, occurring when the previous cesarean scar separates during labor. Complete rupture involves tearing through all layers of the uterine wall and is a surgical emergency requiring immediate cesarean delivery. The risk of complete uterine rupture is approximately 0.5-0.7% for women with one prior low transverse cesarean section in spontaneous labor. This risk increases to approximately 1.0% with labor induction using oxytocin and 2-3% with prostaglandin agents (which are generally contraindicated for VBAC). With two prior cesareans, the rupture risk is approximately 0.9-1.8%. Signs of rupture include sudden severe abdominal pain, fetal heart rate abnormalities, vaginal bleeding, and loss of fetal station.
How should the VBAC success score be used in clinical decision-making?
The VBAC success score should be used as one component of shared decision-making between the patient and her healthcare provider, not as the sole determinant of delivery method. A predicted success rate above 70% is generally considered favorable, while rates below 50% suggest that repeat cesarean may be the safer option, though the final decision depends on patient values and institutional factors. It is important to discuss both the benefits of successful VBAC (shorter recovery, lower infection risk, better outcomes in future pregnancies) and the risks of failed TOLAC (emergency cesarean carries higher complication rates than planned repeat cesarean). The calculator cannot account for factors such as fetal size estimate, uterine scar thickness on ultrasound, and the specific clinical circumstances of the current pregnancy, all of which influence the ultimate recommendation.
How do I interpret the result?
Results are displayed with a label and unit to help you understand the output. Many calculators include a short explanation or classification below the result (for example, a BMI category or risk level). Refer to the worked examples section on this page for real-world context.
Can I share or bookmark my calculation?
You can bookmark the calculator page in your browser. Many calculators also display a shareable result summary you can copy. The page URL stays the same so returning to it will bring you back to the same tool.