MFMU VBAC Calculator

Updated Sep 11, 2026 By Jehan Wadia

Introduction

If you had a C-section before, you may want to try a vaginal birth this time. Doctors call this a VBAC, short for vaginal birth after cesarean. The first step is a trial of labor. This MFMU VBAC Calculator estimates how likely that labor is to end in a vaginal birth.

The tool uses the MFMU Network model built by Grobman and colleagues. It asks for six things you already know:

Your age in years
Your weight before this pregnancy
Your height
Whether your last labor stopped moving forward before your C-section
Whether you have given birth vaginally before, and if it was a VBAC
Whether you take medicine for long-term high blood pressure

The calculator turns your weight and height into a BMI, puts all six answers into the model, and gives you a percent chance of VBAC success. You also see your chance of a failed trial of labor, the math done step by step, and charts that show how the answer would change with a different history or BMI.

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

How to use our MFMU VBAC Calculator

Enter six details about your age, body size, and past births. The calculator shows your predicted chance of a successful VBAC (vaginal birth after cesarean) as a percent, plus your BMI and a step-by-step math breakdown.

Maternal Age (years): Type your age in full years at the time of this birth. For example, type 28.

Pre-Pregnancy Weight: Type your weight from before this pregnancy. Tap lbs or kg to pick your unit. The number switches for you when you change units.

Height: Type your height without shoes. Use the ft / in boxes for feet and inches, or tap cm to type it in centimeters. Your weight and height give your pre-pregnancy BMI.

Arrest Disorder During Prior Cesarean: Pick Yes if your past labor stopped moving forward (your cervix stopped opening or the baby stopped coming down) and that led to the C-section. Pick No if it did not.

Prior Obstetric History: Pick the one that fits best. Choose No Prior Vaginal Delivery, Vaginal Delivery Before Cesarean, or Prior Successful VBAC. If you have had a VBAC, pick that one.

Treated Chronic Hypertension: Pick Yes if you had high blood pressure before this pregnancy and take medicine for it. Otherwise pick No.

Press Calculate to see your result, or Clear / Reset to start over.

What Is a VBAC?

VBAC stands for vaginal birth after cesarean. It means a person who had a C-section in the past gives birth vaginally the next time. To try for a VBAC, you go through labor. Doctors call this a trial of labor after cesarean (TOLAC). Most people who try do give birth vaginally. Some end up needing another C-section.

About the MFMU VBAC Model

The MFMU Network (Maternal-Fetal Medicine Units Network) studied thousands of births to find out what makes a VBAC more or less likely. Researchers led by Dr. William Grobman turned those findings into a math formula. The formula uses a few simple facts about you and gives one number: your chance of a successful vaginal birth.

What Affects Your Chances

Age: Chances drop a little as age goes up.
Pre-pregnancy BMI: BMI comes from your height and weight before pregnancy. A higher BMI lowers the odds.
Arrest disorder: This means your past labor stopped moving forward, so you had a C-section. It lowers the odds.
Past vaginal birth: If you have given birth vaginally before, your odds go up a lot. A past VBAC helps the most.
Treated chronic high blood pressure: High blood pressure that started before pregnancy and needs medicine lowers the odds.

Why It Matters

A successful VBAC often means less blood loss, no surgery cuts, a shorter hospital stay, and a faster recovery. It also lowers risks in future pregnancies. But if labor does not work out, a repeat C-section during labor can carry more risk than a planned one. There is also a small chance of uterine rupture, where the old scar opens. This is rare, but it is serious.

How to Use the Result

The score is an estimate, not a promise. A higher number means a VBAC is more likely to work. A lower number does not mean you cannot try. Many other things matter too, such as your scar type, how many C-sections you have had, your baby's size and position, how far along you are, and what your hospital can handle in an emergency.

Important: Older versions of this model included race and ethnicity. Newer versions removed them, because race is not a medical cause. This tool uses only the health and history factors listed above. Always talk with your doctor or midwife before you choose how to give birth.

Formulas used

Unit conversions (weight and height)

W_{kg} = W_{lb} \times 0.45359237, \qquad H_{m} = \left(12 f + i\right) \times 0.0254 = \frac{H_{cm}}{100}

Pre-pregnancy BMI

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

MFMU VBAC logistic model logit

w = 3.766 - 0.039\,(\text{Age}) - 0.060\,(\text{BMI}) - 0.632\,I_{\text{arrest}} + 0.888\,I_{\text{vaginal}} + 1.003\,I_{\text{VBAC}} - 0.395\,I_{\text{HTN}}

Predicted probability of successful VBAC

p = \frac{e^{w}}{1 + e^{w}}

Success percentage

P(\text{successful VBAC}) = p \times 100\%

Estimated failed trial-of-labor rate

P(\text{failed TOLAC}) = 100\% - P(\text{successful VBAC})

Frequently asked questions

What percentage of VBAC attempts are successful?

About 60% to 80% of people who try labor after a C-section give birth vaginally. That is roughly 3 out of 4. Your own odds depend on your age, BMI, blood pressure, and past births.

Who is a good candidate for a VBAC?

You may be a good candidate if you have:

One or two past C-sections with a low transverse (side-to-side, low on the uterus) cut
No past uterine rupture
No other major uterine surgery
A healthy pregnancy with the baby head down
A hospital that can do an emergency C-section fast

A past vaginal birth makes you an even stronger candidate.

What disqualifies you from having a VBAC?

Doctors usually say no to a VBAC if you have:

A classical or T-shaped uterine cut (a vertical cut high on the uterus)
A past uterine rupture
Past surgery that went through the full wall of the uterus, like fibroid removal
Placenta previa or another reason vaginal birth is not safe

Three or more past C-sections may also rule it out at many hospitals.

What is the risk of uterine rupture during a VBAC?

The risk is low. For one past low transverse C-section, rupture happens in about 0.5% to 0.9% of labors, or roughly 1 in 150. The risk is much higher with a vertical (classical) scar, which is why VBAC is not offered then.

What are the signs of uterine rupture in labor?

The most common first sign is a sudden change in the baby's heart rate on the monitor. Other signs include:

Sharp pain that does not go away between contractions
Contractions that slow or stop
Bleeding from the vagina
The baby's head moving back up
Fast heart rate or low blood pressure in the mother

This is why VBAC labor is watched closely in a hospital.

How long should you wait after a C-section before trying a VBAC?

Most doctors suggest waiting at least 18 to 24 months from your C-section to your next birth. Gaps shorter than 18 months raise the chance of the scar opening, because the uterus needs time to heal fully.

Can you have a VBAC after two C-sections?

Often yes. Many hospitals offer a trial of labor after two low transverse C-sections, and success rates are close to those after one. The rupture risk is a bit higher, so not every hospital or doctor will agree. Ask yours early in pregnancy.

Can labor be induced for a VBAC?

Yes, but with care. Pitocin (oxytocin) and a Foley balloon can be used. Misoprostol and other prostaglandin drugs are not used, because they raise the chance of the scar opening. Induction also lowers the chance the labor ends in a vaginal birth.

What is an arrest disorder in labor?

An arrest disorder means labor stopped moving forward. Either the cervix stopped opening for several hours with good contractions, or the baby stopped moving down the birth canal. If that caused your first C-section, your chance of a VBAC is lower, but still possible.

Why does a past vaginal birth raise VBAC success so much?

Your body has already done it. The pelvis and cervix have proven they can let a baby through, so labor tends to move faster. A past VBAC helps most of all, because it shows the scarred uterus handled labor fine.

Is a VBAC safer than a repeat C-section?

It depends on how labor goes. A successful VBAC has the fewest problems: less blood loss, shorter stay, faster recovery. A failed trial that ends in an emergency C-section has more risk than a planned repeat one. Each extra C-section also raises the risk of placenta problems in future pregnancies.

Can you get an epidural during a VBAC?

Yes. An epidural is safe during a trial of labor and does not hide the signs of uterine rupture. Pain relief may even help you keep going with labor instead of asking for surgery.

Does the baby's size change VBAC odds?

Yes. Babies over about 8 pounds 13 ounces (4,000 g) lower the chance of a vaginal birth, especially if you have never given birth vaginally. But ultrasound size guesses can be off by a pound or more, so size alone should not stop you from trying.

What VBAC success score is considered low?

Scores under about 60% are usually called low. Once the predicted chance drops below roughly 60% to 70%, the risks of a failed labor start to outweigh the gains. A low score does not block you from trying, but it is a good reason for a careful talk with your doctor.

Can you have a VBAC at home or in a birth center?

Doctors advise against it. A trial of labor after a C-section should happen where an emergency C-section and blood transfusion can start right away. Minutes matter if the scar opens.

Does going past your due date lower VBAC chances?

A little. After 40 weeks, babies get bigger and induction becomes more likely, and both lower the odds of a vaginal birth. Many people still have a successful VBAC past their due date.

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