Understanding Your VBAC Probability: What the Grobman Model Measures
The MFMU VBAC prediction model — often called the Grobman calculator — was developed from a multicenter cohort of over 11,800 women who underwent a trial of labor after cesarean. It estimates the probability of successful vaginal birth using eight pre-labor factors: maternal age, BMI, prior vaginal delivery, prior VBAC, reason for initial cesarean, ethnicity, labor onset type, and cervical dilation on admission. The model produces a percentage score representing individualized VBAC success probability, not a population average.
ACOG Practice Bulletin No. 205 recommends that TOLAC (trial of labor after cesarean) be offered to most women with a single prior low-transverse uterine incision. Absolute contraindications are limited to prior uterine rupture, prior classical or T-shaped incision, and inability to perform an emergency cesarean. The VBAC probability score supports shared decision-making — it is not a gate to care.
Key Factors That Shape Your VBAC Score
These four clinical factors have the strongest influence on your personalized probability estimate:
- SPONTANEOUS LABOR ONSET
Spontaneous labor is associated with the highest VBAC success rates. When labor begins on its own, the cervix is typically more favorable, oxytocin receptor density is optimal, and uterine contractility is more physiological — all factors that reduce the risk of arrest disorders and emergency repeat cesarean.
- PRIOR VAGINAL DELIVERY
A documented prior vaginal delivery is the single strongest modifiable predictor of VBAC success in the Grobman model. It confirms that the bony pelvis is adequate for fetal descent and that the maternal pushing reflex and soft tissue compliance are favorable for vaginal birth.
- BMI & LABOR DYSTOCIA RISK
Higher pre-pregnancy BMI is associated with longer labor, higher rates of labor dystocia, and greater likelihood of repeat cesarean during TOLAC. This is likely mediated by soft tissue factors, altered oxytocin sensitivity, and higher rates of macrosomia. Weight optimization before conception can meaningfully improve VBAC candidacy.
- CERVICAL DILATION ON ADMISSION
Greater cervical dilation on hospital admission is a strong real-time predictor of VBAC success. Patients admitted at ≥4 cm have significantly higher success rates than those admitted earlier. This reflects both cervical ripeness and the biologic favorability of the labor process already underway.
When Should You Discuss VBAC with Your Provider?
Ideally, VBAC counseling begins in the second trimester so you have time to identify a supportive birth facility, review your operative report for uterine incision type, and develop a birth plan. Use our Pregnancy Weight Gain Calculator to track BMI trajectory — maintaining a healthy weight gain pattern is one of the modifiable factors that can improve your VBAC candidacy before delivery.