VBAC Probability Estimator

VBAC success depends on prior obstetric history, current pregnancy characteristics, and planned labor management. This tool provides a quick risk-benefit snapshot to inform shared decision-making.

Maternal age (years)

BMI

Number of prior cesareans

Cervical dilation on admission (cm)

Prior vaginal delivery (before or after cesarean)Prior successful VBAC

Indication for previous cesarean

Labor onset for current pregnancy

Estimated success probability

70%

Risk category

High chance of VBAC success

Proceed with trial of labor after cesarean (TOLAC) if no contraindications; ensure facility can perform emergency cesarean.

Counseling points

No prior vaginal delivery lowers success; consider readiness of cervix and fetal size.
High probability of success. VBAC reduces maternal morbidity and future placenta accreta risk.

Estimates approximate published VBAC prediction models (e.g., Grobman). Use validated local calculators when available and ensure immediate surgical capability for all TOLACs.

How to Use This Calculator

Gather obstetric history

Note prior vaginal births, cesarean indications, and number of cesareans.

Assess current labor circumstances

Labor onset (spontaneous vs induction) and cervical dilation influence success rates.

Use results for shared decision-making

Discuss individual risks, facility resources, and patient preferences when planning TOLAC vs repeat cesarean.

Formula

Estimated VBAC success (%) = 60% + modifiers:

Prior vaginal birth (+15%), prior VBAC (+10%)
Maternal age 35–39 (−7%), ≥40 (−12%)
BMI 30–34.9 (−7%), ≥35 (−12%), <20 (+3%)
Nonrecurrent indication (+5%), malpresentation (+7%), fetal distress (+3%), arrest disorder (−10%)
Spontaneous labor (+5%), induction (−7%), no labor (−12%)
Two prior cesareans (−5%), cervical dilation ≥4 cm (+3%), <2 cm (−4%)

Bounded between 15% and 90%. Reference: patterns from Grobman et al., MFMU VBAC studies.

Full Description

VBAC offers benefits—reduced hemorrhage, shorter recovery, and fewer placenta accreta disorders in future pregnancies—but carries a uterine rupture risk (~0.5–1%). Success is most likely with prior vaginal delivery, spontaneous labor, nonrecurrent cesarean indications, and favorable cervical exams. Advanced maternal age, elevated BMI, induction of labor, and recurrent indications (e.g., arrest disorder) lower success rates. Use this tool as a counseling aid; always confirm candidacy according to ACOG/SMFM guidelines and ensure surgical readiness for emergent cesarean.

Frequently Asked Questions

Is VBAC safe after two cesareans?

Yes for carefully selected patients in experienced centers. Uterine rupture risk increases slightly; ensure continuous monitoring and immediate OR access.

Can I be induced and still attempt VBAC?

Induction is possible but lowers success and raises rupture risk. Mechanical ripening and low-dose oxytocin are preferred; avoid prostaglandin E1 (misoprostol).

Do epidurals affect VBAC success?

No. Epidural analgesia does not reduce VBAC success and may facilitate rapid anesthesia if cesarean becomes necessary.

When should I choose repeat cesarean?

Elective repeat cesarean is reasonable with low predicted VBAC success, patient preference, contraindications (e.g., classical scar), or resource limitations.