Formula Used
This calculator uses logistic regression.
The probability equals one divided by one plus e raised to the negative score.
The score is built from selected clinical factors.
Antepartum Model
Score = cHTN(-0.61) + current tobacco(0.68) + former tobacco(-0.69)
+ labor dystocia(-0.41) + prior vaginal births(0.36)
+ prior VBACs(1.33) + delivery interval(-0.08) + 0.82.
Time of Delivery Model
Score = current tobacco(0.77) + former tobacco(-0.65)
+ labor dystocia(-0.42) + prior vaginal births(0.39)
+ prior VBACs(1.27) + delivery interval(-0.07)
+ Bishop score(0.17) + BMI(-0.04)
+ indicated induction(-0.41) + 2.08.
Probability = 1 / (1 + e^-score).
The result is multiplied by 100 for a percentage.
About Vaginal Birth After C Section Planning
Vaginal birth after c section is often called VBAC.
It means birth through the vagina after a previous cesarean.
A planned labor attempt is often called TOLAC.
Both terms appear in counseling and chart notes.
The decision is personal.
It is also clinical.
A calculator can organize known factors.
It cannot replace medical judgment.
Why Estimates Matter
A clear estimate helps frame the discussion.
It can show how past births may affect success.
Prior vaginal birth often improves the estimate.
Prior successful VBAC usually adds more confidence.
A prior cesarean for labor dystocia may lower the estimate.
Body mass index and Bishop score can also matter.
Timing since the last delivery is included.
These values should come from reliable records.
Clinical Review Is Essential
Scar type is very important.
A known low transverse incision is commonly reviewed.
Unknown scars need careful evaluation.
Prior uterine rupture needs specialist review.
Placenta previa or serious bleeding changes the plan.
Fetal position, gestational age, and facility resources also matter.
The labor unit should be ready for urgent care.
These details are outside a simple percentage.
Using the Result
Use the percentage as a conversation aid.
Do not treat it as a promise.
A higher value does not remove risk.
A lower value does not automatically forbid TOLAC.
Ask about benefits, risks, and alternatives.
Discuss repeat cesarean as well.
Bring operative notes when possible.
Review induction plans before admission.
Save the CSV or PDF result.
Share it during prenatal visits.
Final decisions should match patient goals.
They should also match clinical safety.
FAQs
What does this calculator estimate?
It estimates the chance of a successful vaginal birth after a previous cesarean using selected obstetric factors. It is only a counseling aid.
Is this result medical advice?
No. The result is educational. A qualified clinician must review your full history, operative report, current pregnancy, and hospital resources.
Which model should I choose?
Use the antepartum model during pregnancy planning. Use the time of delivery model when Bishop score, BMI, and induction status are available.
What is a Bishop score?
A Bishop score summarizes cervical readiness for labor. This calculator uses the simplified score range from zero to nine.
Why does prior VBAC matter?
A previous successful VBAC is a strong favorable factor in the model. It suggests the body has delivered vaginally after cesarean before.
Why ask about labor dystocia?
Labor dystocia means slow or difficult labor progress. A prior cesarean for this reason may reduce the estimated chance of VBAC success.
Can I download my result?
Yes. Submit the form with CSV or PDF download. The file includes inputs, estimate, model name, and a safety note.
What should I discuss with my clinician?
Discuss scar type, prior records, placenta location, fetal position, induction plan, emergency access, personal goals, and repeat cesarean options.