VBAC Risk Score
Estimate your probability of successful vaginal birth after cesarean (VBAC) using the validated Grobman nomogram model. Enter age, BMI, ethnicity, and obstetric history for personalized prediction with confidence intervals.
About This Calculator
The VBAC (Vaginal Birth After Cesarean) Risk Score Calculator uses the validated Grobman nomogram model published in Obstetrics & Gynecology (2007) to estimate your personalized probability of a successful vaginal birth after a previous cesarean section. This evidence-based tool is designed for pregnant women with one prior low-transverse cesarean delivery who are considering a trial of labor after cesarean (TOLAC).
The prediction model incorporates six clinical factors: maternal age, pre-pregnancy body mass index (BMI), ethnicity, history of prior vaginal delivery, history of prior successful VBAC, and whether the indication for the previous cesarean was a recurring condition such as cephalopelvic disproportion or failure to progress. The model was developed by the NICHD Maternal-Fetal Medicine Units Network using data from 7,660 women across 19 academic centers and has been externally validated in multiple populations worldwide.
The Grobman score is calculated using logistic regression: Score = 4.188 − 0.0579 × Age − 0.0911 × BMI + Ethnicity Coefficient + 0.5917 × Prior Vaginal Delivery + 1.1757 × Prior VBAC − 0.5618 × Recurring Indication. The probability of successful VBAC is then derived as eScore / (1 + eScore). The calculator also provides a 95% confidence interval to reflect the uncertainty of the estimate.
This tool also provides a 95% confidence interval for the probability estimate, giving you a range within which your true VBAC success probability likely falls. The confidence interval accounts for the sample size and variability in the original study population.
Regional Notes
India: VBAC success rates in Indian hospitals range from 65-85%. The Grobman model has been validated in Indian populations with similar discriminative performance. Discuss with your obstetrician as practice patterns vary between private and public hospitals.
United States: ACOG recommends offering TOLAC to most women with one prior low-transverse cesarean. The VBAC success rate in the US is 60-80%. The Grobman model was developed on a US population and is widely used in American obstetrics.
United Kingdom: NICE guidelines support VBAC counseling using prediction models. UK success rates are 72-76%. The Royal College of Obstetricians and Gynaecologists emphasizes shared decision-making and individualized risk assessment.
Disclaimer: This calculator is for educational purposes only and should not replace professional medical advice. Always consult your healthcare provider for delivery planning.
Frequently Asked Questions
What is a VBAC Risk Score?
The VBAC Risk Score (Grobman model) estimates the probability of successful vaginal birth after a previous cesarean section. It uses six factors: maternal age, BMI, ethnicity, prior vaginal delivery, prior VBAC, and recurring indication for cesarean. The model was developed by the NICHD Maternal-Fetal Medicine Units Network and published in Obstetrics & Gynecology in 2007.
How accurate is the Grobman VBAC prediction model?
The Grobman model was validated on 7,660 women across 19 academic centers and has good discriminative ability with an area under the ROC curve of approximately 0.75. It correctly identifies women with high and low probability of VBAC success, but individual results may vary and should always be discussed with a healthcare provider.
What is a good VBAC success probability?
A VBAC success probability of 70% or higher is generally considered favorable for attempting a trial of labor after cesarean (TOLAC). Probabilities between 50-69% are moderate, while below 50% suggests lower chances. The ACOG recommends shared decision-making between patient and provider regardless of the predicted probability.
What are the risks of VBAC?
The primary risk of VBAC is uterine rupture, occurring in approximately 0.5-1% of trials of labor after cesarean. Other risks include blood transfusion, hysterectomy, and operative injury. These risks must be weighed against the benefits of avoiding major abdominal surgery and the faster recovery associated with vaginal delivery.
Who is not a candidate for VBAC?
VBAC is contraindicated in women with a classic (vertical) uterine incision, more than two prior cesarean sections, prior uterine rupture, or certain medical conditions that preclude vaginal delivery. Women with a prior low-transverse cesarean are generally candidates. Always consult your obstetrician for individual assessment.
Can I use this VBAC calculator if I am pregnant with twins?
The Grobman model was developed for singleton pregnancies at term. Its accuracy for twin pregnancies, preterm gestations, or women with multiple prior cesarean sections has not been validated. Discuss your specific situation with a maternal-fetal medicine specialist.
Is there a difference in VBAC success between IN, US, and UK?
VBAC success rates vary by region: in the US, approximately 60-80% of TOLAC attempts result in successful VBAC; in the UK, rates are similar at 72-76%; in India, reported success rates range from 65-85% depending on the hospital setting. The Grobman model was developed on a US population but is used internationally as a risk stratification tool.
How is the VBAC probability calculated?
The VBAC probability is calculated using logistic regression: the Grobman score is computed from age, BMI, ethnicity, and obstetric history variables. The probability is then derived as e^score / (1 + e^score). The calculation follows the formula published by Grobman et al. in Obstetrics & Gynecology (2007) based on the NICHD MFMU network study of 7,660 women.