VBAC Calculator

What is VBAC and the Trial of Labor After Cesarean (TOLAC)?

Vaginal Birth After Cesarean (VBAC) refers to giving birth vaginally after having had one or more previous deliveries by cesarean section. Attempting a vaginal birth after a prior cesarean is clinically known as a Trial of Labor After Cesarean (TOLAC). According to the American College of Obstetricians and Gynecologists (ACOG Practice Bulletin #205), attempting TOLAC is a safe, appropriate, and reasonable choice for most women with a prior low transverse uterine incision.

The Grobman MFMU VBAC Prediction Model

The most widely utilized and rigorously validated clinical algorithm for predicting VBAC success was developed by Dr. William Grobman and the Maternal-Fetal Medicine Units (MFMU) Network of the National Institute of Child Health and Human Development (NICHD). Based on prospective data from over 11,000 women undergoing TOLAC, the multivariable logistic regression model estimates individual success probability using key antenatal factors:

$$\text{Log-Odds } (w) = 3.655 - (0.040 \times \text{Age}) - (0.038 \times \text{BMI}) - \beta_{\text{Race}} + 0.893 \times (\text{Prior Vaginal Delivery}) + 0.681 \times (\text{Prior VBAC}) - 0.668 \times (\text{Recurring Indication}) - 0.413 \times (\text{Hypertension})$$

The predicted probability of a successful vaginal delivery (\(P\)) is then calculated as:

$$P(\text{VBAC}) = \frac{e^w}{1 + e^w} \times 100\%$$

Clinical Factors Influencing VBAC Success

Interpreting Your VBAC Probability Score

Overall, 60% to 80% of women who attempt a trial of labor after one prior low-transverse cesarean achieve a successful vaginal delivery. Clinical research shows that when a patient's predicted success probability is 60% to 70% or greater, the maternal and neonatal complication rates of TOLAC are lower than or equal to those associated with an elective repeat cesarean delivery (ERCS).

For calculating other pregnancy timelines and milestones, explore the Pregnancy Due Date Calculator and the Pregnancy Conception Calculator.