VBAC Calculator
Use our free Vbaccalculator Calculator to get personalized health results. Based on validated medical formulas and clinical guidelines.
Calculator
Adjust values & calculateUterine Rupture Risk by Labor Type
Benefits of Successful VBAC
Risks of TOLAC
Formula
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.
Last reviewed: January 2026
Worked Examples
Example 1: Favorable VBAC Candidate
Example 2: Less Favorable VBAC Candidate
Background & Theory
The VBAC Calculator applies the following established principles and formulas. Health and medicine calculators are grounded in validated physiological measurement methods established through decades of clinical research. Body Mass Index, or BMI, is calculated by dividing weight in kilograms by height in meters squared (kg/m²), a formula originating from Adolphe Quetelet's 19th-century statistical work and later codified by the WHO into standard classifications: underweight below 18.5, normal weight 18.5 to 24.9, overweight 25 to 29.9, and obese at 30 and above. Basal Metabolic Rate quantifies the minimum energy required to sustain life at rest. The Mifflin-St Jeor equation, published in 1990 and widely regarded as the most accurate for most adults, calculates BMR as (10 × weight in kg) + (6.25 × height in cm) − (5 × age) ± sex adjustment. The older Harris-Benedict equations, revised in 1984 by Roza and Shizgal, remain in common use. Total Daily Energy Expenditure is derived by multiplying BMR by a physical activity factor ranging from 1.2 for sedentary individuals to 1.9 for extremely active ones, following the methodology validated by doubly labeled water studies. Body fat percentage can be estimated without laboratory equipment using the U.S. Navy circumference method, which uses neck, waist, and hip measurements, or via BMI-derived equations adjusted for age and sex. The Jackson-Pollock skinfold method offers higher precision with calipers. Blood pressure classification, according to the American College of Cardiology and the 2017 ACC/AHA guidelines, defines normal as below 120/80 mmHg, elevated as 120 to 129 systolic, and hypertension stage 1 as 130 to 139 systolic or 80 to 89 diastolic. Target heart rate zones for aerobic exercise are derived from maximum heart rate estimates, most commonly using the formula 220 minus age in years, with moderate-intensity training typically defined as 50 to 70 percent of maximum heart rate and vigorous intensity at 70 to 85 percent, consistent with CDC and American Heart Association guidelines. These thresholds guide safe and effective cardiovascular conditioning.
History
The history behind the VBAC Calculator traces back through the following developments. The history of health measurement stretches back to ancient Greece, where Hippocrates around 400 BCE laid the foundation for observational medicine by systematically recording patient symptoms, diet, and environment. His humoral theory, though scientifically superseded, established the principle that the body operates as an interconnected system subject to measurable imbalance. The transformation toward modern medicine accelerated in the 19th century. Louis Pasteur and Robert Koch developed germ theory in the 1860s and 1870s, identifying microorganisms as disease agents and enabling targeted interventions. Florence Nightingale, working during the Crimean War in the 1850s, introduced statistical analysis to nursing practice, demonstrating through data visualization that sanitation reduced mortality. Her work is foundational to evidence-based health measurement. The discovery of vitamins in the early 20th century, beginning with Casimir Funk's coinage of the term in 1912 and culminating in the isolation of vitamins A through K, created the field of nutritional science and gave rise to dietary reference intake frameworks. The World Health Organization, founded in 1948, subsequently established global standards for health metrics, disease classification through the International Classification of Diseases, and recommended daily allowances. The BMI as a clinical screening tool gained traction in the 1970s through Ancel Keys' large-scale epidemiological work, which validated Quetelet's index as a population-level obesity indicator. Through the 1980s and 1990s, the Framingham Heart Study produced landmark data linking cholesterol, blood pressure, and lifestyle factors to cardiovascular disease risk, directly shaping the numeric thresholds still used in health calculators. The evidence-based medicine movement, formalized by Gordon Guyatt and colleagues at McMaster University in the early 1990s, demanded that all health recommendations derive from systematically graded clinical evidence. The digital health era beginning in the 2000s brought these formulas to consumer devices, wearable sensors, and smartphone applications, expanding access to health self-monitoring on a global scale and enabling population-level data collection that continues to refine clinical reference ranges.
Frequently Asked Questions
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.
How do I verify VBAC Calculator's result independently?
The Formula section on this page shows the equation used. You can reproduce the calculation manually or in a spreadsheet using those steps. Compare your answer against the worked examples in the Examples section, which use known reference values so you can confirm the calculator is behaving as expected.
References
Reviewed by Rahul Singh, Health & Wellness Specialist · Editorial policy