VBAC Calculator (Vaginal Birth After Cesarean)

Your details

Grobman uses prenatal data only. Flamm adds cervical exam findings at admission and is more accurate closer to delivery.
Age at the time of the planned delivery.
years
Weight before the current pregnancy (used to compute BMI).
kg
Height enters the 2021 model directly, not only through BMI: taller people have a higher predicted chance of VBAC.
cm
The 2021 model treats these as three mutually exclusive categories. A previous VBAC is the strongest positive predictor in the equation.
The 2021 model uses this specific indication (labour that stopped progressing), not the broader "potentially recurrent indication" of the 2007 nomogram.
Chronic high blood pressure treated with medication. This variable is only in the 2021 race-free model.
VBAC success probabilityModerate VBAC chance
73.8%

Predicted probability of successful vaginal delivery (model-based estimate)

Grobman linear predictor (w)1.038
Pre-pregnancy BMI23.9kg/m²
73.8% %
Lower<50%Moderate50%-75%Favorable75%+

Estimated VBAC success probability: 73.8%.

  • Your pre-pregnancy BMI is 23.9 kg/m2.
  • BMI is shown for context only. The 2021 model uses pre-pregnancy weight and height as separate terms rather than BMI.
  • Both models were derived from North American populations; accuracy may differ in other settings.

Next stepA moderate probability means VBAC may still be worth attempting if your clinical situation is otherwise suitable. Ask your care team about continuous fetal monitoring and an emergency cesarean backup plan.

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