Understanding VBAC and TOLAC
VBAC (vaginal birth after cesarean) refers to successful vaginal delivery in a pregnancy following a previous cesarean section. TOLAC (trial of labour after cesarean) is the broader term encompassing all patients who attempt vaginal delivery after C-section—not all TOLAC attempts result in VBAC. This distinction matters: approximately 60–80% of women attempting TOLAC achieve successful vaginal delivery, though success rates vary considerably based on individual factors.
Cesarean delivery has become increasingly common; over 30% of births in developed nations now occur via surgical delivery. For many women with a previous C-section, TOLAC presents an opportunity to experience vaginal birth, potentially with shorter recovery and lower surgical risks compared to repeat cesarean delivery. However, this pathway carries its own considerations that warrant careful discussion with your obstetrician.
VBAC Success Probability Model
The Grobman model, derived from a large prospective cohort study, predicts the likelihood of successful vaginal delivery after prior cesarean section. The calculator uses your personal and clinical characteristics to generate a probability score, with results ranging from under 40% to over 80% depending on your profile.
BMI = Weight (kg) ÷ Height (m)²
Success probability is determined by integrating coefficients for:
• Maternal age (years)
• Ethnicity/race category
• Pre-pregnancy BMI
• History of prior vaginal delivery
• Recurrent indication for prior cesarean
Age— Your age at the time of pregnancy, in years. Younger age is associated with higher VBAC success.BMI— Body mass index calculated from pre-pregnancy weight and height. Lower BMI within normal range correlates with better outcomes.Ethnicity— Racial or ethnic background, as this variable influences success rates in the Grobman cohort.Prior vaginal delivery— Whether you have delivered vaginally before any cesarean section.Recurrent indication— Whether the original reason for cesarean (e.g., cephalopelvic disproportion, arrest disorder) is likely to recur in this pregnancy.
Clinical Factors That Influence Success
Maternal age: Younger women typically have higher success rates. Success begins to decline gradually after age 35, and falls more markedly after 40, though many women in older age groups still achieve VBAC.
BMI and weight: A BMI below 30 (overweight threshold) is associated with improved outcomes. Excessive weight gain during pregnancy can further reduce success likelihood. Tracking pre-pregnancy weight and healthy gestational weight gain are practical steps to optimise chances.
Prior vaginal delivery: Women who have delivered vaginally—especially before a cesarean—have considerably higher VBAC success rates (often 70–90%). This history is one of the strongest predictive factors.
Reason for prior cesarean: Non-recurring indications (such as transient issues like infection or malpresentation) carry better prognosis than persistent anatomical or medical concerns like cephalopelvic disproportion.
Ethnicity: The model shows variation by ethnicity, with Hispanic and Black women tending to have slightly lower predicted success rates in the original Grobman cohort. These population-level differences do not predict individual outcome and should not override individualised clinical counselling.
Risks and Rare But Serious Complications
The most serious but uncommon complication of TOLAC is uterine rupture, occurring in 0.5–1% of attempted vaginal deliveries after prior cesarean. Rupture can lead to rapid intra-abdominal bleeding, emergency hysterectomy, maternal haemorrhage, and potentially fetal loss or injury. Immediate surgical intervention is required.
Other TOLAC-related risks include:
- Failed TOLAC: Requiring emergency cesarean delivery during labour, with associated surgical risks and recovery time.
- Infection: Postpartum endometritis occurs slightly more often after failed TOLAC than elective repeat cesarean.
- Blood transfusion: Required in approximately 1–2% of TOLAC cases, versus 0.5–1% for planned repeat cesarean.
Conversely, planned repeat cesarean carries its own morbidity: increased rates of placental abnormalities, bowel/bladder injury, and higher maternal recovery burden. Comprehensive risk–benefit discussion with your obstetrician is essential.
Practical Considerations for VBAC Planning
Use these evidence-based pointers to optimise your TOLAC experience and decision-making.
- Pre-pregnancy weight management matters — If you are considering TOLAC for a future pregnancy, achieving a BMI under 30 beforehand measurably improves success odds. Even modest weight loss (5–10% of body weight) can shift your predicted probability upward. Discuss realistic weight targets with your healthcare provider.
- Discuss the recurring indication with your doctor — If your prior cesarean was for a potentially recurring reason (like cephalopelvic disproportion), clarify with your obstetrician whether that condition is likely to persist. This conversation directly affects your personalised success estimate and delivery plan.
- Plan labour support and hospital setting carefully — TOLAC is safest when conducted in a hospital with immediate access to emergency surgery and blood products. Continuous fetal monitoring is standard. Supportive labour companions and early epidural access reduce labour stress and may improve outcomes.
- Do not rely solely on this calculator for decision-making — This tool applies population-level statistics; your individual anatomy, medical history, and obstetric circumstances are unique. Use the probability estimate as a starting point for detailed conversations with your obstetrician, who can factor in elements the calculator cannot capture.