VBAC Risk Score (Flamm Model)

Why use it

VBAC offers significant benefits over elective repeat cesarean — shorter recovery, avoidance of major abdominal surgery, reduced risk of placenta previa/accreta in future pregnancies, and lower maternal morbidity in successful cases. However, trial of labor carries a 0.5–0.9% risk of uterine rupture, which can be catastrophic. The Flamm scoring system provides an individualized, evidence-based probability estimate that enables clinicians and patients to make better-informed decisions about TOLAC versus ERCD, replacing anecdotal or population-level statistics with patient-specific predictions. It is endorsed by ACOG Practice Bulletin No. 205 as a useful clinical tool for VBAC counseling.

About

The Flamm VBAC Admission Scoring System was developed by Flamm and Geiger and published in Obstetrics and Gynecology in 1997. It was derived from a prospective multicenter study of 5,022 women undergoing trial of labor after cesarean (TOLAC) at 19 hospitals in the Kaiser Permanente health system, making it one of the largest VBAC prediction datasets at the time of publication. The score assigns points across six clinical variables assessed at hospital admission: maternal age, ethnic/racial group, history of prior vaginal delivery (before or after the index cesarean), indication for the prior cesarean, cervical effacement, and cervical dilation. These six variables generate a score from 0 to 10, with higher scores corresponding to progressively higher predicted probabilities of successful vaginal delivery. The Flamm score was designed to be applied at the time of hospital admission for TOLAC — integrating both antepartum factors (age, ethnicity, prior delivery history, prior cesarean indication) and intrapartum cervical findings (effacement and dilation). It provides a clinically actionable probability estimate that can guide shared decision-making about trial of labor versus elective repeat cesarean delivery (ERCD). Predicted VBAC success rates range from approximately 49% at the lowest scores (0–2) to 89% at the highest scores (≥10). External validation studies, including analyses from the MFMU Cesarean Registry (Grobman et al. 2007), have confirmed the predictive value of the Flamm model, although a separate nomogram-based model was subsequently developed by Grobman et al. as an alternative prediction tool.

When to use

• Women with a history of one prior low transverse uterine incision presenting for delivery planning
• Admission counseling for patients requesting trial of labor after cesarean (TOLAC)
• Antepartum discussions about delivery route at 36–38 weeks gestation
• Shared decision-making when patient is undecided between TOLAC and elective repeat cesarean delivery (ERCD)
• NOT validated for women with prior classical (vertical) uterine incision — TOLAC is generally contraindicated in these cases regardless of score
• NOT validated for women with more than one prior cesarean (limited data; ACOG recommends individualized counseling)
• NOT a substitute for clinical assessment of uterine scar integrity, obstetric history, or facility capacity for emergency cesarean

Formula

Pearls & pitfalls

• The Flamm score is an admission scoring system — cervical effacement and dilation should be assessed at the time of hospital admission, not antepartum, for maximum predictive accuracy.
• Prior vaginal delivery AFTER a cesarean (i.e., a prior successful VBAC) is the single strongest positive predictor in the model (+4 points), reflecting that prior uterine scar integrity has already been proven under labor conditions.
• The indication for the prior cesarean is crucial — a prior cesarean for dystocia or failure to progress (FTP/CPD) carries a higher recurrence risk than non-recurrent indications (malpresentation, placenta previa, fetal distress). Women with prior cesarean for FTP score lower on this variable.
• Racial and ethnic group contributes to the score based on population-level differences in VBAC success rates observed in the Kaiser derivation cohort — African-American and Hispanic women had statistically lower VBAC success rates than other groups in this dataset.
• The Flamm score does NOT account for: interdelivery interval, estimated fetal weight, BMI, or oxytocin use — variables that were later incorporated in the Grobman 2007 nomogram (MFMU model). Consider using the Grobman model when these additional factors are clinically relevant.
• A score does not account for uterine rupture risk directly — the predicted probability is of VBAC success (vaginal delivery), not of safe labor. All TOLAC patients require continuous electronic fetal monitoring and immediate availability of emergency cesarean regardless of score.
• The Flamm score was derived from a Kaiser Permanente population in California in the early 1990s — some populations may have different baseline VBAC success rates. The predicted probabilities are approximate estimates.
• ACOG Practice Bulletin No. 205 (2019) states that women with one prior low transverse cesarean should be offered TOLAC if there are no contraindications, and that prediction models can support but not replace individualized counseling.
• A low Flamm score is not a contraindication to TOLAC — it is a counseling tool. Patient preference, facility readiness, and clinical judgment remain paramount in the final decision.
• The Grobman MFMU nomogram (2007) is an alternative validated tool that includes BMI, labor characteristics, and recurring vs. non-recurring indication — consider this for a more granular risk estimate when those variables are known.

Next steps

Score 0–2 (predicted VBAC success ~49%): VBAC is possible but less likely. Provide balanced counseling about both TOLAC and ERCD. Many clinicians consider ERCD reasonable at this score, particularly if additional risk factors are present. Score 3–6 (predicted success ~60–79%): Moderate probability. Individualized discussion of risks and benefits. Most women are reasonable candidates for TOLAC at well-resourced facilities. Score 7–10 (predicted success ~82–89%): High probability of VBAC success. TOLAC is strongly supported, provided uterine scar characteristics and facility resources are appropriate. For all scores: counsel on absolute risk of uterine rupture (~0.5–0.9%), institutional ability to perform emergency cesarean within 30 minutes, fetal monitoring requirements, and the patient's values and preferences. Document the decision and score in the medical record.

Evidence

Flamm BL, Geiger AM. Vaginal birth after cesarean delivery: an admission scoring system. Obstet Gynecol. 1997;90(6):907-910. PMID:9397098. Derivation cohort: n=5,022 TOLAC patients, 19 Kaiser Permanente hospitals. AUROC 0.71. Predicted VBAC success rates validated against observed outcomes. Grobman WA et al. Prediction of uterine rupture associated with attempted vaginal birth after cesarean delivery. Am J Obstet Gynecol. 2007. MFMU network external validation. Grobman WA et al. Development of a nomogram for prediction of vaginal birth after cesarean delivery. Obstet Gynecol. 2007;109(4):806-812. PMID:17400840. ACOG Practice Bulletin No. 205: Vaginal Birth After Cesarean Delivery. Obstet Gynecol. 2019;133(2):e110-e127.

References

1. Flamm BL, Geiger AM. Vaginal birth after cesarean delivery: an admission scoring system. Obstet Gynecol. 1997;90(6):907-910. — link
2. Grobman WA, Lai Y, Landon MB et al. Development of a nomogram for prediction of vaginal birth after cesarean delivery. Obstet Gynecol. 2007;109(4):806-812. — link
3. ACOG Practice Bulletin No. 205: Vaginal Birth After Cesarean Delivery. Obstet Gynecol. 2019;133(2):e110-e127. — link
4. Landon MB, Hauth JC, Leveno KJ et al. Maternal and perinatal outcomes associated with a trial of labor after prior cesarean delivery. N Engl J Med. 2004;351(25):2581-2589. — link
5. Guise JM et al. Vaginal birth after cesarean: new insights on maternal and neonatal outcomes. Obstet Gynecol. 2010;115(6):1267-1278. — link