Predictors of Success during Induction Trial of Labor After Cesarean: A Calculator

This calculator estimates the probability of successful vaginal birth after cesarean (VBAC) during induction trial of labor after cesarean (TOLAC) using the beta coefficients reported by Rogerson and colleagues. Use the antepartum model for counseling in the prenatal period and the time-of-delivery model when planned-delivery variables are available.

Antepartum model

Use for counseling before the delivery admission. Includes chronic hypertension, tobacco category, prior cesarean indication, prior NSVD, prior VBAC, and interval since last delivery.

Chronic hypertension Does the patient have chronic hypertension?
Equation coding: cHTN = 1 for yes; cHTN = 0 for no.
Tobacco use Current, former, or never tobacco use?
Equation coding uses two indicator variables: current tobacco = 1 only if current use; former tobacco = 1 only if former use. Never tobacco is the reference category, so both are 0.
Prior cesarean indication Was the indication for the primary cesarean labor dystocia?
Equation coding: labor dystocia indication = 1 for yes; 0 for no.
Enter prior non-VBAC normal spontaneous vaginal deliveries. Do not count the same delivery in both NSVD and VBAC fields.
Enter prior vaginal births after cesarean.
Enter interval in years. For 18 months, enter 1.5.
Antepartum model result Area under the receiver operating characteristic
(ROC) curve: 0.73 (95% CI 0.69-0.78)
Incomplete

Enter all antepartum fields to calculate automatically.

The result will display as an estimated probability of successful VBAC during induction TOLAC.

Time-of-delivery model

Use when delivery-admission variables are known. The paper calls this the time-of-delivery model and includes BMI, simplified Bishop score, and indicated induction.

Tobacco use Current, former, or never tobacco use?
Equation coding uses two indicator variables: current tobacco = 1 only if current use; former tobacco = 1 only if former use. Never tobacco is the reference category, so both are 0.
Prior cesarean indication Was the indication for the primary cesarean labor dystocia?
Equation coding: labor dystocia indication = 1 for yes; 0 for no.
Induction indication Is the induction medically indicated?
Equation coding: indicated induction = 1 when medically indicated; 0 when elective, date-based, or not medically indicated.
Enter prior non-VBAC normal spontaneous vaginal deliveries. Do not count the same delivery in both NSVD and VBAC fields.
Enter prior vaginal births after cesarean.
Dilation, station, and effacement sub-scores only. Range 0-9.
Time-of-delivery model result Area under the receiver operating characteristic
(ROC) curve: 0.75 (95% CI 0.71-0.79)
Incomplete

Enter all time-of-delivery fields to calculate automatically.

This model adds cervical favorability, BMI, and whether induction is medically indicated.

Clinical summary

How to interpret the result

The displayed percentage is the model-estimated probability of successful VBAC during an induction TOLAC. For example, 70% means an estimated probability of 0.70, not a guarantee of success and not a direct estimate of uterine rupture or neonatal outcome.

  • Antepartum model: best for prenatal counseling when delivery-admission variables are not yet known.
  • Time-of-delivery model: generally preferred when simplified Bishop score, most recent BMI, and induction indication are known.
  • Model performance: reported AUROC was 0.73 for the antepartum model and 0.75 for the time-of-delivery model.
  • Validation limitation: Future work includes validation with another cohort, so results should be interpreted as derivation-cohort estimates.
Equations used and variable coding

Logistic transformation:

Probability = 1 / (1 + e^(-LP))

LP is the linear predictor. Binary indicators are coded 1 when present/yes and 0 when absent/no. Tobacco is entered as two separate indicator variables, with never tobacco use as the reference category.

Antepartum linear predictor:

LP = -0.61(cHTN) + 0.68(current tobacco) - 0.69(former tobacco) - 0.41(labor dystocia prior cesarean) + 0.36(NSVD) + 1.33(VBAC) - 0.08(delivery interval years) + 0.82

Time-of-delivery linear predictor:

LP = 0.77(current tobacco) - 0.65(former tobacco) - 0.42(labor dystocia prior cesarean) + 0.39(NSVD) + 1.27(VBAC) - 0.07(delivery interval years) + 0.17(simplified Bishop score) - 0.04(BMI) - 0.41(indicated induction) + 2.08
Equation variable What to enter Coding used in the equation
cHTNChronic hypertension1 = yes; 0 = no.
current tobaccoCurrent tobacco use1 = current use; 0 = not current use.
former tobaccoHistory of tobacco use but not current use1 = former use; 0 = not former use. If never tobacco use, both current tobacco and former tobacco are 0.
labor dystocia prior cesareanPrimary cesarean indication was labor dystocia, arrest disorder, failed progress, or failed induction1 = yes; 0 = no.
NSVDNumber of prior non-VBAC normal spontaneous vaginal deliveriesEnter the count as a whole number. Do not double-count prior VBACs here.
VBACNumber of prior vaginal births after cesareanEnter the count as a whole number.
delivery interval yearsInterval since most recent deliveryEnter years as a decimal. Example: 18 months = 1.5 years.
simplified Bishop scoreDilation, station, and effacement sub-scores onlyEnter 0-9.
BMIMost recent body mass indexEnter kg/m².
indicated inductionMedically indicated induction1 = yes; 0 = no, elective, or date-based induction.
Definitions and input notes
  • TOLAC: trial of labor after cesarean.
  • VBAC: vaginal birth after cesarean.
  • NSVD: normal spontaneous vaginal delivery. Enter prior non-VBAC NSVDs separately from prior VBACs to avoid double counting.
  • Delivery interval: years since the most recent delivery; months divided by 12 may be used.
  • Simplified Bishop score: dilation, station, and effacement only; range 0-9.
  • Labor dystocia: indication for the primary cesarean was labor dystocia, arrest disorder, failed labor progress, or failed induction.
  • Medically indicated induction: code as 1 when the induction is medically indicated in the model sense; code as 0 when not medically indicated, elective, or date-based.
  • Model output: the percentage is an estimated probability of successful VBAC during induction TOLAC, not an estimate of uterine rupture risk and not a substitute for individualized eligibility assessment.
References
  1. Rogerson D, Diaz Luevano C, Christofferson A, Agarwal R, Ottum P, Cowles K, Canfield D, Jacobs M, Gyamfi-Bannerman C. Predictors of Success during Induction Trial of Labor After Cesarean: A Calculator.
  2. Perinatology.com Bishop Score Calculator.

Derivation population

Live singleton pregnancies undergoing induction with a recorded induction agent and 1-2 prior low transverse or unknown-scar cesarean deliveries at a single center; multiple gestation and more than two prior cesarean deliveries were excluded.

Development note

This calculator was developed at The University of California San Diego to predict the likelihood of successful VBAC during induction of labor TOLAC. The antepartum model is designed for counseling in the prenatal period, while the time-of-delivery model is designed for counseling at the time of planned delivery. The calculator, including the antepartum and time-of-delivery models, is available for further clinical validation; contact .

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Disclaimer: All calculations must be confirmed before use. The suggested results are not a substitute for clinical judgment. Neither Perinatology.com nor any other party involved in the preparation or publication of this site shall be liable for any special, consequential, or exemplary damages resulting in whole or part from any user's use of or reliance upon this material.