VBAC Risk Score Calculator - MFMU TOLAC Success Probability

A VBAC risk score calculator is a clinical decision tool that turns 11 published MFMU inputs into a single percentage chance of ending a trial of labor after cesarean in a vaginal birth, also called a successful TOLAC. This VBAC risk score calculator uses the MFMU Network multivariable logistic regression to combine age, BMI, prior delivery history, hypertensive status, gestational age, labor type, and the admission cervical exam into one probability that can be quoted in a counseling conversation.

• • Antenatal counselling: A pregnant person with one prior low-transverse cesarean can preview the chance of a successful TOLAC before labor begins.
• • Admission triage: When a patient arrives in labor, refresh the probability using the cervical exam and the planned labor type.
• • Informed consent: The percent output gives a concrete number to discuss alongside the small risk of uterine rupture.

The MFMU model was developed on more than 11,000 women with one prior low-transverse cesarean at 19 U.S. academic centers and is the calculator ACOG references for vaginal birth after cesarean.

A VBAC risk score calculator output is an estimate, not a prediction. It does not diagnose uterine rupture, predict neonatal outcome, or replace a clinical exam.

Estimate the gestational age that feeds the MFMU model with the pregnancy due date calculator before locking in the VBAC risk score inputs.

The VBAC risk score calculator takes 11 clinical inputs from the published MFMU intrapartum model, multiplies each by a published coefficient, sums them into a linear log-odds index L, and converts L into a probability with the standard logistic function. Only the patient-specific inputs change the output.

• Maternal age (years): Linear; older age lowers the probability.
• Body mass index (kg/m^2): Pre-pregnancy or first-trimester; higher BMI lowers the probability.
• Any prior vaginal delivery: Strong positive contributor; the pelvis has previously accommodated a vaginal birth.
• Prior successful TOLAC (prior VBAC): The largest positive contributor in the published model.
• Recurring prior cesarean indication: Arrest of dilation or descent, or cephalopelvic disproportion, lower the probability more than a non-recurring indication.
• Gestational age at delivery (weeks): Modelled linearly; later gestational age slightly lowers the probability.
• Hypertensive disorder of pregnancy: Chronic hypertension, gestational hypertension, or preeclampsia lower the probability.
• Labor induction: Spontaneous and augmented labor are the reference; induction carries the negative coefficient.
• Cervical dilation (cm) at admission: Higher dilation raises the probability.
• Cervical effacement (percent) at admission: Higher effacement raises the probability.
• Fetal station at admission: Higher (less negative) station raises the probability.

The MFMU coefficients follow the published multivariable logistic regression for one prior low-transverse cesarean. The c-statistic is about 0.75, so the inputs rank a successful TOLAC above a failed one 75% of the time in the original U.S. cohort. The bands (low through very high) are heuristic.

According to Grobman et al., AJOG 2009, the probability of a successful TOLAC is calculated from a multivariable logistic regression that includes maternal age, body mass index, any prior vaginal delivery, prior successful TOLAC, recurring prior cesarean indication, hypertensive disorders of pregnancy, gestational age at delivery, labor induction, and the admission cervical exam (dilation, effacement, and station).

Confirm the early-pregnancy BMI with the BMI in pregnancy calculator so the MFMU BMI coefficient is using the same units.

Four ideas come up in almost every discussion of the model.

The two strongest positive coefficients are prior successful TOLAC and any prior vaginal delivery. The strongest negative for a typical candidate is no prior vaginal combined with induced labor and an unfavorable cervical exam. The published 2009 model does not include race, ethnicity, or estimated fetal weight; later reviews have reinforced that choice because race-based terms are observational, not biological.

Match the gestational age input to the dating method used by the gestational age calculator so the TOLAC probability does not drift between visits.

Work through the inputs in the order below. Most are known before labor; a few (dilation, effacement, station) come from the admission cervical exam.

1. 1 Enter maternal age and BMI: Use age in years and pre-pregnancy or first-trimester BMI.
2. 2 Mark the prior delivery history: Any prior vaginal delivery, prior successful TOLAC, and whether the prior cesarean was for a recurring indication (arrest or CPD) or a non-recurring one.
3. 3 Add the current pregnancy context: Enter gestational age at delivery in weeks and mark whether the pregnancy is complicated by a hypertensive disorder.
4. 4 Pick the labor type and cervical exam: Choose spontaneous, augmented, or induced. Then enter dilation, effacement, and station from the admission exam.
5. 5 Read the predicted probability and band: The result panel shows the predicted probability, a band label, and the top three contributors.

Example workflow: A 32-year-old at 39 weeks with BMI 26, a prior cesarean for breech, no prior vaginal, no prior VBAC, no hypertensive disorder, and a favorable admission exam (4 cm, 70% effaced, station -1) in spontaneous labor scores in the very high band. Switching her to induced labor with a 1 cm, 30% effacement, station -3 admission drops the band to low, with every other input unchanged.

For an overview of the same pregnancy workflow where TOLAC candidacy is usually discussed, the pregnancy calculator is the next page to open.

A VBAC risk score calculator is not a substitute for clinical judgment, but a structured score helps.

• • Translates 11 published inputs into one number: Combines age, BMI, prior delivery history, indication, gestational age, hypertensive status, labor type, and the cervical exam into a single percent.
• • Tied to the named MFMU model: The coefficient set matches the Grobman 2009 intrapartum model, so the result is reproducible across visits.
• • Shows which inputs move the score: The top-contributors panel lists the three inputs that contributed the most to the linear predictor.
• • Useful for pre-labor and admission scenarios: The default cervical exam and labor type can be edited for a pre-labor estimate or the actual admission.
• • Bands the percent into a clinical label: A 70% and a 30% can feel similar in a paragraph; the band label gives a quick read at the bedside.

The calculator also helps document the shared decision-making conversation. The percent can be quoted in the chart note.

For a full peripartum risk picture, the VTE risk in pregnancy calculator covers venous thromboembolism risk alongside the TOLAC probability.

The MFMU model has well-known strong and weak inputs. Understanding them avoids over-reading a small change in the percent.

• • The model is observational and was developed in 19 U.S. academic centers in 2009. Calibration is best in similar cohorts, so the VBAC risk score may be too high or too low in different populations.
• • The model does not predict uterine rupture, neonatal outcome, postpartum hemorrhage, or length of labor. It only estimates the chance of ending a TOLAC in a vaginal birth.
• • The model was developed and validated for one prior low-transverse cesarean. ACOG notes that two prior low-transverse cesareans may also be considered candidates for TOLAC with appropriate counseling, an experienced clinician, and the ability to perform an emergency cesarean.

Race and ethnicity were not used in the published 2009 intrapartum equation. The result should always be read alongside the full clinical picture, not as a stand-alone number.

According to ACOG Practice Bulletin No. 205 (2019), a candidate with one prior low-transverse cesarean, no contraindication, and a clinician able to perform an emergency cesarean is generally appropriate for TOLAC, with reported success rates of 60 to 80 percent in selected candidates and higher when a prior vaginal delivery or prior successful TOLAC is present. The same bulletin notes that two prior low-transverse cesareans may also be considered with counseling.

According to AHRQ Evidence Report on Vaginal Birth After Cesarean (2010), a validated prediction tool such as the MFMU calculator can help frame the TOLAC discussion by putting the chance of success and the small risk of uterine rupture into perspective for the individual patient.

If TOLAC is attempted, the APGAR score calculator is the next score to document once the baby is delivered.