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. In a large U.S. study, the scar tore in 0.9% of people with more than one past C-section who tried labor, compared with 0.7% after one, a difference that was not significant. 4 Not every hospital or doctor offers a trial of labor after two. After three or more C-sections, most doctors will plan a repeat C-section instead.

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

It depends on your situation. A successful VBAC is linked to fewer problems for the mother and fewer complications in future pregnancies. 6 You skip abdominal surgery, lose less blood, and recover faster. 2 A failed trial of labor carries more risk for mother and baby than either a successful VBAC or a planned repeat C-section. 6 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 across the lower part of the womb. It is the most common type and carries the least chance of tearing in a later labor. 2 A high vertical (classical) cut has the highest risk of rupture. 2 A T-shaped cut or a past uterine rupture usually rules out VBAC too. You cannot tell what kind of cut was made on your womb from the scar on your skin, so ask for your surgery records. 2

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.¹ 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 both go into the model, and the results also show 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 it is not successful, you will need another C-section.²

Who Can Try for a VBAC?

A low, side-to-side (low transverse) cut is the most common kind of cut on the womb and has the lowest chance of tearing in a later labor.² VBAC is usually a good option if:

You had one or two past C-sections with a low transverse scar⁴
Your pelvis looks big enough for the baby
The baby is head-down and full term
The hospital can handle emergencies that threaten you or your baby²

VBAC should not be tried if you are at high risk of the scar tearing.² A high vertical (classical) cut on the womb has the highest risk.² A past uterine rupture or certain other womb surgeries can also rule it out.

What Affects Your Chance of Success

In a large U.S. study of people with one past C-section who tried labor, 74% gave birth vaginally.¹ Some things raise or lower those odds:

Age: Younger moms tend to have a better chance.¹
Body weight and height: A lower pre-pregnancy weight and a taller height are both linked to higher success.¹
Past vaginal births: If you have given birth vaginally before, especially a past VBAC, your odds go up a lot.¹
Reason for the first C-section: If labor stalled last time (an arrest disorder), success is lower.¹
Treated high blood pressure: Chronic hypertension that needs medicine lowers the odds.¹

Benefits and Risks

A successful VBAC means no abdominal surgery, a shorter recovery, a lower risk of infection and less blood loss.² If you plan to have more children, it may also help you avoid problems linked to several C-sections, such as problems with the placenta in future pregnancies.²

The main risk of a TOLAC is uterine rupture, when the old scar on the womb opens during labor. This is rare: in a large U.S. study it happened in 0.7% of people with a low transverse scar who tried labor.³ It is very serious and may harm both you and your baby.² That is why VBAC should happen in a hospital that can manage emergencies.²

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 weight and height, birth history, past C-section reason, and blood pressure status, and turns them into a single percent chance of a vaginal birth.¹ The newest version of this model leaves out race and ethnicity, and this tool uses it.¹

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) ¹

z = -5.952 - 0.023 \cdot \text{Age} - 0.024 \cdot W_{kg} + 0.056 \cdot H_{cm} - 0.597 \cdot \text{Arrest} + 0.868 \cdot \text{VagBefore} + 1.869 \cdot \text{PriorVBAC} - 0.966 \cdot \text{CHTN}

VBAC success probability ¹

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.

In a large U.S. study, the scar tore in 0.9% of people with more than one past C-section who tried labor, compared with 0.7% after one, a difference that was not significant.⁴ Not every hospital or doctor offers a trial of labor after two. 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?

A longer gap is safer for the scar on your womb.

In one study, people who gave birth 18 months or less after their C-section were three times as likely to have the scar open during labor as people with a longer gap.⁵

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. In a large U.S. study, the scar tore in 1.0% of induced labors, compared with 0.4% of labors that started on their own.³ Induction can also lower the chance of a vaginal birth.²

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 are lower.¹

Does BMI affect VBAC success?

Yes. In the model, each extra kilogram of pre-pregnancy weight lowers the odds a little (an odds ratio of 0.98 per kg), and each extra centimeter of height raises them (1.06 per cm).¹

So for two people of the same height, the one with the higher BMI gets a lower estimate.

Is VBAC safer than a repeat C-section?

It depends on your situation. A successful VBAC is linked to fewer problems for the mother and fewer complications in future pregnancies.⁶ You skip abdominal surgery, lose less blood, and recover faster.²

A failed trial of labor carries more risk for mother and baby than either a successful VBAC or a planned repeat C-section.⁶ 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 across the lower part of the womb. It is the most common type and carries the least chance of tearing in a later labor.²

A high vertical (classical) cut has the highest risk of rupture.² A T-shaped cut or a past uterine rupture usually rules out VBAC too. You cannot tell what kind of cut was made on your womb from the scar on your skin, so ask for your surgery records.²

Does having a prior vaginal birth improve VBAC odds?

Yes, a lot. It is the strongest good factor in the model.¹

A vaginal birth before your C-section raises your odds (an odds ratio of 2.38), and a past VBAC raises them even more (6.48).¹ 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.

In a large U.S. study, 73.4% of people who tried labor after a C-section gave birth vaginally, so the rest, about a quarter, had a C-section.³ 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. In the model, treated chronic high blood pressure lowers the odds of a VBAC (an odds ratio of 0.38).¹ For a 30-year-old weighing 71 kg at 156 cm tall, the estimate falls from 59.4% to 35.8% when treated high blood pressure is added.¹

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?

Fewer people try than in the past. At the U.S. academic centers in one large study, the share who tried labor fell from 48.3% in 1999 to 30.7% in 2002.³ Of those who tried, 73.4% gave birth vaginally.³

Some hospitals do not offer VBAC because their staff do not feel they can provide the emergency care it may need.²

Sources

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