How this calculator works
This is the Grobman/MFMU 2021 first-prenatal-visit model without race or ethnicity. It was developed for people attempting labour with a term singleton cephalic pregnancy after one prior low-transverse caesarean. The vaginal-history categories are mutually exclusive; prior VBAC does not also receive the before-caesarean coefficient. The estimate concerns vaginal birth among those undertaking a trial of labour, not uterine rupture or the safety of labour. No score threshold should be used to deny access to an otherwise appropriate option. The result is an estimate for information only and is not medical advice.
Formula
w = -5.952 - 0.023 age - 0.024 weight kg + 0.056 height cm - 0.597 arrest indication + 0.868 vaginal birth only before caesarean + 1.869 prior VBAC - 0.966 treated chronic hypertension. Probability = 1/(1 + exp(-w)).
Worked example
At age 30, weight 70 kg, height 165 cm and no other factors, w = 0.918 and estimated probability is about 71.46%. A prior VBAC changes w to 2.787 and the estimate to about 94.20%.
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Frequently asked questions
Is this a uterine rupture score?
No. It estimates the chance of vaginal birth during a trial of labour, which is a different outcome.
Why is there no ethnicity input?
The 2021 model was specifically derived without race or ethnicity. It is not the earlier 2007 model with those terms removed.
Does a low estimate rule out a trial of labour?
No. A probability estimate supports discussion and must not be used as an eligibility cut-off.