VBAC Calculator
Use our free VBAC Calculator to get personalized health results. Based on validated medical formulas and clinical guidelines.
Medical disclaimer: This calculator is provided for educational and informational purposes only and does not constitute medical advice, diagnosis, or treatment. Results are general estimates and may not reflect your individual circumstances. Always consult a qualified healthcare professional before making decisions about your health.
VBAC Calculator
Calculator
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Formula: P(success) = 1 / (1 + e^(-logit)); logit = 3.766 - 0.039(age) - 0.060(BMI-24) + factors
Worked example — VBAC Success Probability: 89.3% - Excellent candidate for a trial of labour after cesarean
Formula
P(success) = 1 / (1 + e^(-logit)); logit = 3.766 - 0.039(age) - 0.060(BMI-24) + factors
This is the antenatal MFMU model published by Grobman and colleagues, in the 2021 revision: w = 3.766 - 0.039 x age - 0.060 x pre-pregnancy BMI + 0.888 if there has been any prior vaginal delivery + 1.003 if one of those was a vaginal birth after a cesarean - 0.632 if the prior cesarean was for a recurring indication such as arrest of labour. The probability is then 1 / (1 + e to the minus w). The 2021 revision deliberately removed the race and ethnicity terms that the original 2007 equation contained, because they lowered predicted success for Black and Hispanic patients and pushed them toward repeat cesarean without a causal justification. There is a separate admission model that adds cervical dilation, effacement, station and induction; the number shown here is the antenatal estimate and does not use the dilation field.
Worked Examples
Example 1: Favorable VBAC Candidate
Problem:A 28-year-old woman with BMI 24, one prior cesarean for breech presentation (non-recurring), and one previous vaginal delivery before that cesarean, at 39 weeks. Calculate her antenatal VBAC success probability.
Solution:Base logit: 3.766 Age adjustment: -0.039 x 28 = -1.092 BMI adjustment: -0.060 x 24 = -1.440 Any prior vaginal delivery: +0.888 Prior VBAC: none, so no further credit Indication was breech, which is non-recurring, so no penalty Logit = 3.766 - 1.092 - 1.440 + 0.888 = 2.122 Probability = 1/(1 + e^(-2.122)) = 0.893 = 89.3% Uterine rupture risk: about 0.5% in spontaneous labour after one prior cesarean
Result:VBAC Success Probability: 89.3% - Excellent candidate for a trial of labour after cesarean
Example 2: Less Favorable VBAC Candidate
Problem:A 38-year-old woman with BMI 35, one prior cesarean for failure to progress (recurring), and no prior vaginal delivery, at 40 weeks. Calculate her antenatal VBAC success probability.
Solution:Base logit: 3.766 Age adjustment: -0.039 x 38 = -1.482 BMI adjustment: -0.060 x 35 = -2.100 No previous vaginal delivery: +0 Prior cesarean was for failure to progress, a recurring indication: -0.632 Logit = 3.766 - 1.482 - 2.100 - 0.632 = -0.448 Probability = 1/(1 + e^0.448) = 1/2.565 = 0.390 = 39.0% Uterine rupture risk: about 0.5% in spontaneous labour, roughly doubled if labour is induced
Result:VBAC Success Probability: 39.0% - Below the 50% mark, so the risks and benefits of a trial of labour versus a planned repeat cesarean need a careful discussion
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 does BMI affect VBAC success rates and what should overweight women consider?
BMI has a significant inverse relationship with VBAC success. For each unit increase in BMI above the normal range, the probability of successful VBAC decreases by approximately 1-2%. Women with BMI over 30 have VBAC success rates approximately 15-20% lower than women with normal BMI. However, obesity does not preclude a TOLAC attempt, and the risk-benefit analysis should consider that repeat cesarean in obese women also carries increased risks including wound complications, blood clots, longer recovery, and higher risk of surgical complications. For obese women, the decision between TOLAC and repeat cesarean should be individualized, considering the overall predicted success probability, the patient's values regarding vaginal delivery, and the specific institutional resources available.
Why is a previous vaginal delivery the strongest predictor of VBAC success?
A history of vaginal delivery is the strongest predictor because it provides direct evidence that the maternal pelvis can accommodate vaginal birth and that the patient can progress through normal labor. Studies show that women with a prior vaginal delivery (either before or after their cesarean) have VBAC success rates of 85-95%, compared to 60-65% for women who have never delivered vaginally. A prior vaginal delivery after cesarean (VBAC) is even more predictive than a vaginal delivery before the cesarean. This factor essentially proves pelvic adequacy and labor capability, reducing the uncertainty that makes VBAC prediction challenging for women without vaginal delivery history. The effect is so strong that it often outweighs other negative predictors.
What is the difference between recurring and non-recurring indications for the prior cesarean?
The indication for the prior cesarean is categorized as recurring or non-recurring based on whether the same situation is likely to recur in subsequent pregnancies. Non-recurring indications include breech presentation, placenta previa, fetal distress unrelated to labor progress, cord prolapse, and elective cesarean without labor. These are associated with higher VBAC success rates (70-85%) because they were circumstantial rather than related to the ability to deliver vaginally. Recurring indications include cephalopelvic disproportion, failure to progress (arrest of dilation or descent), and failed induction. These carry lower VBAC success rates (50-65%) because they may reflect persistent anatomical or physiological factors. However, even with recurring indications, many women achieve successful VBAC because fetal size, position, and labor dynamics vary between pregnancies.
How does the number of previous cesarean deliveries affect VBAC candidacy?
Women with two or more prior cesarean deliveries face different considerations than those with a single prior cesarean. The VBAC success rate with two prior cesareans is approximately 60-75%, somewhat lower than the 70-80% for one prior cesarean. The uterine rupture risk increases from approximately 0.5-0.7% with one prior cesarean to 0.9-1.8% with two prior cesareans. Despite these increased risks, ACOG states that TOLAC can be considered in women with two prior cesareans provided there are no other contraindications and the patient is appropriately counseled. Three or more prior cesareans represent a relative contraindication, though some institutions will offer TOLAC in select cases. The cumulative risks of multiple repeat cesareans (adhesions, placenta accreta spectrum, surgical complications) must be weighed against the risks of TOLAC.
Should labor be induced for VBAC, and does induction change success rates?
Labor induction during TOLAC is not contraindicated but does affect both success rates and uterine rupture risk. Spontaneous onset of labor is associated with the highest VBAC success rates (approximately 75-80%) and the lowest rupture risk. Oxytocin induction or augmentation slightly reduces success rates and approximately doubles the uterine rupture risk to 1.0-1.1%. Mechanical cervical ripening (Foley balloon) appears to be the safest induction method, with rupture rates similar to spontaneous labor. Prostaglandin agents (misoprostol, dinoprostone) are generally contraindicated or used with extreme caution because they significantly increase rupture risk (2-3%). The decision to induce should be made carefully, weighing the indication for induction, Bishop score, predicted VBAC success, and the availability of emergency surgical backup.
What monitoring is required during a trial of labor after cesarean?
Continuous electronic fetal monitoring is recommended throughout TOLAC because changes in fetal heart rate patterns are often the earliest sign of uterine rupture. The classic findings of uterine rupture on fetal monitoring include sudden prolonged deceleration, recurrent late decelerations, or fetal bradycardia. Additionally, intravenous access should be established, blood should be typed and screened, and the surgical team capable of performing an emergency cesarean should be immediately available. ACOG recommends that institutions offering TOLAC should be able to begin emergency cesarean delivery within 30 minutes of the decision, though faster response times are preferable. Nursing staff should be familiar with the signs and symptoms of uterine rupture, including sudden pain, vaginal bleeding, loss of station, and maternal hemodynamic instability.
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.
References
Reviewed for accuracy by Rahul Singh, Health & Wellness Specialist · Editorial policy
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