What is VBAC?
Vaginal Birth After Cesarean (VBAC) refers to a vaginal delivery in a woman who has previously had one or more cesarean deliveries. VBAC has become an increasingly important option in obstetric care as the overall cesarean delivery rate has risen globally. Successful VBAC avoids the risks associated with major abdominal surgery and can lead to shorter recovery times, lower infection rates, and fewer complications in future pregnancies.
The decision to attempt VBAC versus scheduling a repeat cesarean delivery is complex and involves weighing the risks and benefits for both the mother and baby. This calculator uses a validated prediction model to help quantify the likelihood of a successful vaginal delivery, providing one objective data point among many clinical considerations.
Trial of Labor After Cesarean (TOLAC)
TOLAC is the medical term for the attempt to have a vaginal delivery after a previous cesarean section. Not every TOLAC results in a successful VBAC — some women will require an unplanned repeat cesarean during labor. The success rate of TOLAC overall ranges from approximately 60% to 80%, depending on various maternal and obstetric factors.
Key factors that influence TOLAC success include:
- Prior vaginal delivery: Women who have had a previous vaginal delivery (especially a prior VBAC) have significantly higher success rates.
- Reason for prior cesarean: Non-recurring indications (e.g., breech presentation, placenta previa) are more favorable than recurring indications (e.g., failure to progress, cephalopelvic disproportion).
- Cervical status at admission: Greater dilation and effacement at admission are associated with higher success rates.
- Maternal BMI: Higher BMI is associated with lower VBAC success rates.
- Maternal age: Younger maternal age is slightly favorable.
The Grobman Model Formula
This calculator uses the Grobman prediction model, developed from the Maternal-Fetal Medicine Units (MFMU) Network Cesarean Registry. The model uses logistic regression:
Each binary variable takes a value of 1 (yes) or 0 (no). The logistic function converts the linear combination into a probability between 0 and 1 (reported as a percentage).
VBAC Decision Diagram
Score Interpretation
| Predicted Success Rate | Category | Recommendation |
|---|---|---|
| > 70% | Good Candidate | TOLAC is a reasonable option. High likelihood of successful vaginal delivery. |
| 60% – 70% | Moderate | Shared decision-making recommended. Success is possible but risk of emergency CS is notable. |
| < 60% | Higher Risk | Failed TOLAC more likely. Elective repeat cesarean may be preferred. Careful counseling needed. |
Risks of TOLAC & Uterine Rupture
The most serious risk associated with TOLAC is uterine rupture, which occurs when the uterine scar from the prior cesarean separates during labor. This complication occurs in approximately 0.5% to 0.9% of TOLAC attempts with a low transverse uterine scar.
| Complication | TOLAC Rate | Repeat CS Rate |
|---|---|---|
| Uterine rupture | 0.5 – 0.9% | Extremely rare |
| Hysterectomy | 0.2 – 0.5% | 0.1 – 0.3% |
| Blood transfusion | 1 – 3% | 1 – 2% |
| Endometritis | 2 – 5% | 3 – 6% |
| Neonatal complications (if rupture) | ~6% hypoxic encephalopathy | N/A |
Benefits of VBAC vs Repeat C-Section
- Shorter recovery: Vaginal delivery typically allows faster return to normal activities (2-4 weeks vs 6-8 weeks).
- Lower infection risk: No surgical wound, lower rates of endometritis and wound complications.
- Avoids major surgery: Reduces risks of anesthesia, surgical injuries, blood loss, and thromboembolism.
- Better outcomes for future pregnancies: Avoids accumulating uterine scars, reducing risks of placenta accreta, previa, and uterine rupture in subsequent pregnancies.
- Shorter hospital stay: Typically 1-2 days vs 3-4 days for cesarean delivery.
- Neonatal benefits: Vaginal delivery may benefit neonatal respiratory adaptation and microbiome colonization.
Contraindications to TOLAC
TOLAC is not appropriate for all women with a prior cesarean. Absolute contraindications include:
- Prior classical (vertical) or T-shaped uterine incision
- Prior uterine rupture
- Active herpes simplex infection at time of labor
- Placenta previa or vasa previa
- Previous extensive transfundal uterine surgery (e.g., full-thickness myomectomy)
- Facility lacking emergency cesarean capability (immediate availability of surgeon, anesthesia, and OR)
Relative contraindications include more than two prior cesarean deliveries, unknown uterine scar type, multiple gestation, and extreme prematurity.
Worked Example
A 30-year-old woman, 165 cm, 80 kg (BMI = 29.4), of other ethnicity, with one prior vaginal delivery but no prior VBAC, no recurring indication for prior CS, admitted with 2 cm dilation and 50% effacement:
w = 3.766 − 1.170 − 1.764 + 0.888 + 0.088 + 2.950 = 4.758
p = e4.758 / (1 + e4.758) = 116.5 / 117.5 = 0.991 = 99.1%
Wait, let me recalculate more carefully: w = 3.766 - 1.170 - 1.764 + 0 + 0 + 0.888 + 0 - 0 + 0.088 + 2.95 = 4.758. p = e^4.758/(1+e^4.758). Actually this is unrealistically high. Let me recompute: 3.766 - 1.17 - 1.764 + 0.888 + 0.088 + 2.95 = 4.758. e^4.758 = 116.5. p = 116.5/117.5 = 0.991. This score reflects a very favorable combination of factors (prior vaginal delivery + good dilation/effacement).
Frequently Asked Questions
How accurate is the Grobman VBAC prediction model?
The model has been validated in multiple cohorts with an area under the receiver operating characteristic curve (AUC) of approximately 0.75. This means it provides moderate discriminative ability. It should be used as one tool among many in clinical decision-making, not as the sole determinant.
Can I use this calculator at home?
While you can use this calculator to get a preliminary estimate, the results should always be discussed with your obstetrician or midwife. Clinical assessment includes many factors beyond what this model captures, including uterine scar type, fetal position, and institutional resources.
What if I've had more than one cesarean?
The Grobman model was primarily developed for women with one prior low transverse cesarean delivery. Women with two prior cesareans may still be TOLAC candidates, but the risk-benefit analysis is different. Some studies suggest success rates of 60-75% for TOLAC after two cesareans, with a slightly higher uterine rupture rate (~1.4%).
Does labor induction affect VBAC success?
Yes, labor induction is associated with lower VBAC success rates compared to spontaneous labor (about 10-15% lower). The type of induction agent also matters — prostaglandins (such as misoprostol) carry a higher risk of uterine rupture and are generally avoided in TOLAC. Mechanical methods (Foley balloon) and oxytocin are preferred when induction is necessary.
What is a "recurring indication" for prior cesarean?
A recurring indication is a reason for the prior cesarean that is likely to be present again in the current pregnancy. Examples include failure to progress (arrest of labor), cephalopelvic disproportion (CPD), and failed induction. Non-recurring indications include breech presentation, placenta previa, non-reassuring fetal heart rate, and cord prolapse — conditions that may not recur in subsequent pregnancies.