VBAC Calculator (Vaginal Birth After Cesarean Section)

The calculation evaluates a combination of weighted clinical factors to determine an aggregate prediction score. First, maternal body mass index (BMI) is calculated using weight and height, expressed as BMI = weight / (height * height). This BMI value, along with maternal age, race, prior history of vaginal deliveries (both pre- and post-cesarean), and the primary indication for the previous cesarean, is mapped to a validated scoring algorithm. The total score (Score = Base Clinical Score + BMI Score) is then converted into a percentage probability representing the chance of a successful vaginal delivery.

Consider a 32-year-old expectant mother weighing 70 kg with a height of 1.65 meters, presenting for a trial of labor after one prior low-transverse cesarean delivery performed because of breech presentation (no prior vaginal births). First, her BMI is computed: 70 / (1.65 * 1.65) = 25.7 kg/m². Next, the clinical scoring algorithm assigns points based on her age, favorable indication for the previous cesarean (breech rather than arrest of descent), and lack of prior vaginal deliveries. Combining her base score with her BMI score yields an aggregate score of 6. Using the logistic conversion function, this composite score corresponds to an estimated vaginal birth success probability of approximately 71%, offering a helpful metric for her clinical consultation.

Always input accurate measurements for height and pre-pregnancy or early-pregnancy weight to ensure your BMI score is correctly categorized. Remember that this calculator provides a statistical estimate and should serve only as a starting point for a detailed discussion with your obstetrician or midwife. Every pregnancy is unique, and clinical factors not captured by a numerical score can significantly influence your birth outcomes.