VBAC Calculator

Q: What is a good VBAC success score?

A score of 70% or higher counts as a good chance. Scores between 40% and 69% are moderate. Under 40% is lower. A score above 60% to 70% usually means a trial of labor is likely to work. A lower score does not mean you cannot try. It just means the odds of a repeat C-section are higher.

Q: How many C-sections can you have and still try for a VBAC?

Most doctors let you try after one or two low transverse C-sections. After two, the risk of the scar tearing goes up a little, so not every hospital or doctor will agree. After three or more C-sections, most doctors will plan a repeat C-section instead.

Q: How long should you wait between a C-section and a VBAC?

Most doctors want at least 18 months between the C-section birth and the next birth. That gives the scar on your womb time to heal and get strong. Waiting less than 18 months raises the chance the scar opens during labor.

Q: Is VBAC safer than a repeat C-section?

It depends on your situation. A successful VBAC is the safest option. You skip surgery, lose less blood, and heal faster. A failed trial of labor that ends in an emergency C-section carries more risk than a planned repeat C-section. That is why your success odds matter so much when you pick.

Q: What kind of C-section scar allows a VBAC?

A low transverse scar. That is a side-to-side cut low on the womb. Most C-sections today use this type, and it holds up well in labor. A classical (up-and-down) cut, a T-shaped cut, or a past uterine rupture usually rules out VBAC. The skin scar you see does not always match the cut on your womb, so ask for your surgery records.

Introduction

If you have had a C-section before, you may want to try a vaginal birth this time. Doctors call this VBAC, which stands for vaginal birth after cesarean. This VBAC calculator gives you an estimate of your chance of having a successful vaginal birth.

The tool uses the MFMU (Maternal-Fetal Medicine Units Network) prediction model. You enter six things: your age, your weight before pregnancy, your height, whether your past C-section was due to an arrest disorder (labor that stopped moving forward), your past vaginal birth history, and whether you take medicine for long-term high blood pressure. The calculator then shows your chance of VBAC success as a percentage.

You also get charts, your pre-pregnancy BMI, and a step-by-step look at the math. This helps you see how each answer changes the result.

This is an estimate, not a promise. Use it to help you talk with your doctor or midwife about the safest birth plan for you and your baby.

How to use our VBAC Calculator

Enter six details about your age, body size, and birth history. The calculator gives your chance of having a successful vaginal birth after cesarean (VBAC), plus your BMI, your odds of success, and the chance a trial of labor ends in a repeat C-section.

Maternal Age (years): Type your age in years on your due date. You can enter any age from 14 to 60.

Pre-Pregnancy Weight: Type what you weighed before you got pregnant. Pick pounds (lb), kilograms (kg), or stone (st) from the drop-down box.

Height: Type how tall you are. Pick inches (in), centimeters (cm), meters (m), or feet (ft). Your weight and height are used to find your pre-pregnancy BMI.

Arrest Disorder: Choose "Yes" if your past C-section happened because labor stopped moving forward, such as your cervix stopped opening or the baby stopped coming down. If not, choose "No."

Obstetric Vaginal Delivery History: Pick one: you have never had a vaginal birth, you had a vaginal birth before your C-section, or you had a vaginal birth after a C-section (a prior VBAC).

Treated Chronic Hypertension: Choose "Yes" if you had high blood pressure before pregnancy and took medicine for it. If not, choose "No."

Click Calculate to see your VBAC success score, charts, and a step-by-step math breakdown. Click Reset to start over.

What Is VBAC?

VBAC stands for vaginal birth after cesarean. It means giving birth through the vagina after having a baby by C-section before. To try for a VBAC, you go through labor. Doctors call this a TOLAC, or trial of labor after cesarean. If labor goes well and the baby is born vaginally, that is a VBAC. If problems come up, the doctor does a repeat C-section instead.

Who Can Try for a VBAC?

Most people with one past C-section and a low, side-to-side (low transverse) cut in the womb can try. VBAC is usually a good option if:

You had one or two past C-sections with a low transverse scar
Your pelvis looks big enough for the baby
The baby is head-down and full term
The hospital can do an emergency C-section fast if needed

VBAC is usually not safe if you had a vertical (up-and-down) scar on the womb, a past uterine rupture, or certain other womb surgeries.

What Affects Your Chance of Success

About 6 or 7 out of 10 people who try a VBAC give birth vaginally. Some things raise or lower those odds:

Age: Younger moms tend to have a better chance.
Body weight and height (BMI): A lower pre-pregnancy BMI is linked to higher success.
Past vaginal births: If you have given birth vaginally before, especially a past VBAC, your odds go up a lot.
Reason for the first C-section: If labor stalled last time (an arrest disorder), success is a bit lower.
Treated high blood pressure: Chronic hypertension that needs medicine lowers the odds some.

Benefits and Risks

A successful VBAC means no surgery, less blood loss, less infection risk, a shorter hospital stay, and a faster recovery. It also lowers risks in future pregnancies, since each C-section adds scar tissue and placenta problems.

The main risk of a TOLAC is uterine rupture, when the old scar on the womb opens during labor. This is rare. It happens in fewer than 1 out of 100 births with a low transverse scar, but it is an emergency for both mom and baby. That is why VBAC should happen in a hospital that can do surgery right away.

How the Prediction Works

This tool uses the MFMU (Maternal-Fetal Medicine Units) VBAC model built by Grobman and colleagues. It takes your age, pre-pregnancy BMI, birth history, past C-section reason, and blood pressure status, and turns them into a single percent chance of a vaginal birth. Newer versions of this model leave out race and ethnicity, and this tool does too.

Important Note

This number is an estimate, not a promise. It does not replace advice from your doctor or midwife. Talk with your care team about your scar type, your health, your hospital, and what matters most to you before you choose between a planned VBAC and a planned repeat C-section.

Formulas used

Unit conversion to SI (weight and height)

W_{kg} = W_{unit} \times k_{mass}, \qquad H_{m} = H_{unit} \times k_{length}

Pre-pregnancy BMI

\text{BMI} = \frac{W_{kg}}{H_{m}^2}

Log-odds (MFMU VBAC logistic model)

z = 3.766 - 0.039 \cdot \text{Age} - 0.060 \cdot \text{BMI} - 0.671 \cdot \text{Arrest} + 0.888 \cdot \text{AnyVaginal} + 1.003 \cdot \text{PriorVBAC} - 0.464 \cdot \text{CHTN}

VBAC success probability

P = \frac{1}{1 + e^{-z}}

Success probability as a percentage

P_{\%} = P \times 100

Odds of success

\text{Odds} = \frac{P}{1 - P}

Chance trial of labor ends in repeat cesarean

P_{\text{repeat C-section}} = 100\% - P_{\%}

Frequently asked questions

What is a good VBAC success score?

A score of 70% or higher counts as a good chance. Scores between 40% and 69% are moderate. Under 40% is lower.

A score above 60% to 70% usually means a trial of labor is likely to work. A lower score does not mean you cannot try. It just means the odds of a repeat C-section are higher.

How many C-sections can you have and still try for a VBAC?

Most doctors let you try after one or two low transverse C-sections.

After two, the risk of the scar tearing goes up a little, so not every hospital or doctor will agree. After three or more C-sections, most doctors will plan a repeat C-section instead.

How long should you wait between a C-section and a VBAC?

Most doctors want at least 18 months between the C-section birth and the next birth.

That gives the scar on your womb time to heal and get strong. Waiting less than 18 months raises the chance the scar opens during labor.

What are the signs of uterine rupture during labor?

Warning signs include:

A sudden drop in the baby's heart rate (the most common sign)
Sharp pain at the scar that does not go away between contractions
Contractions that suddenly slow or stop
Vaginal bleeding
Fast heart rate or low blood pressure in the mom
Losing the feeling of the baby in the birth canal

This is why VBAC labor needs steady baby heart monitoring in a hospital.

Can you be induced for a VBAC?

Yes, but with care. Pitocin (oxytocin) and a balloon catheter can be used to start labor.

Prostaglandin drugs like misoprostol should not be used. They make the womb squeeze too hard and raise the risk of the scar tearing. Induced labor carries a slightly higher rupture risk than labor that starts on its own.

Does an epidural affect VBAC success?

No. An epidural is safe during a VBAC and does not lower your chance of success.

A common worry is that it will hide the pain of a uterine rupture. It does not. The main warning sign is a change in the baby's heart rate, not pain. Good pain control may even help you stay with labor longer.

What is an arrest disorder in labor?

An arrest disorder means labor stopped moving forward.

There are two kinds:

Arrest of dilation: the cervix stopped opening, even with strong contractions
Arrest of descent: the baby stopped moving down the birth canal while you pushed

If this caused your first C-section, your VBAC odds drop a bit, because the same thing may happen again.

Does BMI affect VBAC success?

Yes. A higher pre-pregnancy BMI lowers the chance of a vaginal birth.

Each point of BMI cuts the odds a little. Someone with a BMI of 40 has a clearly lower chance than someone with a BMI of 22, with everything else the same. Higher BMI is linked to longer labor and more labor problems.

Is VBAC safer than a repeat C-section?

It depends on your situation. A successful VBAC is the safest option. You skip surgery, lose less blood, and heal faster.

A failed trial of labor that ends in an emergency C-section carries more risk than a planned repeat C-section. That is why your success odds matter so much when you pick.

What kind of C-section scar allows a VBAC?

A low transverse scar. That is a side-to-side cut low on the womb. Most C-sections today use this type, and it holds up well in labor.

A classical (up-and-down) cut, a T-shaped cut, or a past uterine rupture usually rules out VBAC. The skin scar you see does not always match the cut on your womb, so ask for your surgery records.

Does having a prior vaginal birth improve VBAC odds?

Yes, a lot. It is the strongest good factor in the model.

A vaginal birth before your C-section raises your odds. A past VBAC raises them even more, often to 85% to 90%. Your body has already shown the baby can fit and the scar held during labor.

How much does a baby's size affect VBAC success?

Bigger babies lower the odds. Babies over 8 pounds 13 ounces (4,000 grams) have a lower VBAC success rate.

But size guesses before birth are often wrong by a pound or more. Doctors do not usually cancel a VBAC plan just because a scan says the baby looks big.

What happens if a VBAC fails?

You have a C-section. Labor stops and the baby is born by surgery, often the same day.

This happens for about 3 or 4 out of every 10 people who try. Common reasons are labor stalling or the baby's heart rate dropping. Most mothers and babies do fine, but recovery takes longer than a planned C-section.

Can you have a VBAC with twins?

Yes, in many cases. If the first twin is head-down and you have one low transverse scar, most doctors will let you try.

Success rates are close to those for single babies. You will need a hospital with a team ready for a fast C-section if needed.

Does treated high blood pressure lower VBAC chances?

Yes, by a small amount. Chronic high blood pressure that needs medicine cuts the odds a bit.

It often comes with other things that make labor harder, like induction before the due date or problems with the placenta. It does not rule out a VBAC on its own.

How common is VBAC in the United States?

About 1 in 3 people who have had a C-section try for a vaginal birth. Of those who try, roughly 70% succeed.

The overall VBAC rate is near 14% of all births to people with a past C-section. Many hospitals do not offer VBAC because they cannot staff an operating room around the clock.

Rate this calculator