How Often Does the Scar Give Way in VBAC Labors?

by | Apr 10, 2023 | VBAC (Vaginal Birth after Cesarean)

If you’re considering whether to plan a VBAC, without doubt the scariest part of that decision is the worry that the scar might give way with potentially dire consequences for your baby. This post will drag that monster out of the closet and provide you a detailed look. My hope is that giving it a shape and form will enable you to base your choice to plan VBAC or repeat cesarean on solid information rather than pure emotion.

To that end, this post will examine what factors reduce or increase the probability of scar rupture, by which I mean events where the scar gives way completely, causing symptoms such as heavy bleeding or fetal distress. I’ll report averages and ranges as best I can so that you have quantitative data on which to base a decision. After that, I’ll compare severe adverse outcome rates with planned VBAC versus planned repeat cesarean because your decision is about the consequences of this choice, not a specific event attached to it. Finally, I’ll wrap up with your take-away, including actions you can take to maximize the probability of an uneventful vaginal birth should you choose to plan VBAC.

What Are the Odds of the Scar Giving Way?

Let’s start by answering how often in general symptomatic scar separation occurs in a VBAC labor. A systematic review, a study that pools data from multiple studies on a particular topic, reported a rate of 5 per 1000 labors in 8 studies totaling 57,419 births.13 A large Canadian study (58,704 births) of VBAC labor in the next pregnancy following a cesarean delivery in the first pregnancy reported a rate of 3 per 1000.41 Put another way, the baseline odds of not having a problem with the uterine scar in a VBAC labor are 995 to 997 out of 1000.

While good to know, those populations were made up of a mixture of people whose odds varied according to multiple factors. So let’s drill down and see what those factors might be.

Before we do that, though, we first need to consider two overarching factors that pull scar rupture rates in opposite directions. On the one hand we have inducing or augmenting labor (giving I.V. oxytocin to strengthen contractions), which increases the odds of scar rupture, and on the other we have having a prior VBAC or VBACs, which decreases them.

Overarching Effects on the Probability of Scar Rupture

The Effect of Induction and Augmentation

Because of differences among studies in who was included and how labors were managed, study results vary widely. What you can see, though, is a consistent pattern: stimulating contractions increases risk of scar rupture, and the greater the stimulation, the greater the increase.3, 11, 39, 43 The lowest scar rupture rates will be seen when labor begins and progresses on its own. Rates increase markedly when labor is induced or a labor that began spontaneously is augmented and especially when labor induction includes treatment for an unfavorable cervix. In further support of the relationship between intensity of uterine stimulation and scar rupture, studies also show that scar rupture rates rise in parallel with maximum oxytocin dose and duration at maximum oxytocin dose.6, 7

Scar rupture rate with:

  • No use of oxytocin
    • average rate: 5 per 1000
    • range: 2-8 per 1000
  • Spontaneous onset of labor (labor could be augmented)
    • average: 6 per 1000
    • range: 1-12 per 1000
  • Augmentation of labor with IV oxytocin (Pitocin, Syntocinon)
    • average: 14 per 1000
    • range: 5-19 per 1000
  • Labor induction with oxytocin
    • average: 12 per 1000
    • range: 3-25 per 1000
  • Labor induction with prostaglandin E2, a cervical ripening agent (Cervidil, Dinoprostone, Minprostin)*
    • average: 28 per 1000
    • range: 11-43 per 1000

*Because of its strong association with scar rupture, misoprostol (Cytotec), another prostaglandin used for ripening an unfavorable cervix, is contraindicated for use in VBAC labors.1

The Effect of Prior Vaginal Birth

Vaginal birth in relation to cesarean delivery can mean vaginal birth (or births) before the cesarean and vaginal birth (or births) after the cesarean, i.e. VBAC. Again, it’s complicated because each study differs in who it includes and how they are managed, but the overall pattern with prior VBAC is clear: Having a VBAC under your belt, so to speak, provides protection against scar rupture.5, 11, 20 Also, one of the studies found that having an increasing number of VBACs had no effect on scar rupture,26 which resolves the concern that multiple VBACs might overstress the scar. In contrast, however, vaginal birth before the cesarean doesn’t seem to make much of a difference.

Scar rupture rate with:

  • One or more VBACs (may or may not also have vaginal births prior to the cesarean)
    • average: 4 per 1000
    • range: 1-7 per 1000
  • Only vaginal births prior to the cesarean
    • average: 8 per 1000
    • range: 5-13 per 1000
  • No prior VBAC (mixture of women with no prior vaginal births and vaginal births before the cesarean)
    • average: 13 per 1000
    • range: 9-16 per 1000
  • No vaginal births
    • average: 12 per 1000
    • range: 6-16 per 1000

One of the studies gives us a sense of the degree to which prior VBAC protects against scar rupture in induced labors.26 Scar rupture rates were:

  •   4 per 1000 with induced labor and one prior VBAC*
  •   6 per 1000 with induced labor and two or more prior VBACs*
  • 14 per 1000 with induced labor and no prior VBACs

*The slight difference between these two rates is probably due to chance.

What Individual Characteristics Influence the Probability of Scar Rupture?

This brings us to a list of individual characteristics to consider against the background of these overarching factors.

Number of Previous Cesareans

VBAC labor after two cesareans increases the probability of scar rupture compared with after one;29, 35 however, the absolute numbers can vary considerably. Compare scar rupture rates in this systematic review:35

  • 16 per 1000 with planned VBAC after two cesareans
  •  7 per 1000 with planned VBAC after one cesarean

with the rates found in this study at a single, large hospital:29

  • 6 per 1000 with planned VBAC after two cesareans
  • 3 per 1000 with planned VBAC after one cesarean

What might explain the difference? The review authors observe that one of the studies included in their review found that most cases of scar rupture occurred in labors that were induced or augmented.29, 35 The results in the study at the large hospital support this explanation.29 The hospital’s policy was that women planning VBAC after two cesareans could only be induced by rupturing membranes and could not have labor augmented. The take-away here is that the risk of scar rupture—especially in labors after two cesareans—is reduced when labor both begins and progresses on its own.

You will want to take this into account as well: while there may be heightened risks with labor with more than one uterine scar, adverse outcomes with repeat cesareans rise too. The review reported that rates of hysterectomy, transfusion, newborn intensive care admission, and perinatal death or neurologic injury were similar between women planning VBAC after two cesareans and women planning a third cesarean.35

High BMI

High body mass index (BMI greater than or equal to 30) does not increase risk of scar rupture compared with lower BMI:15

  • 7 per 1000 in women with BMI greater than or equal to 30
  • 8 per 1000 in women with BMI 18.5 to 24.9

A second study had no comparison group, but scar rupture rate was very low:40

  • 2 per 1000 in women with BMI greater than or equal to 30

Big Baby (Macrosomia)

The threshold for macrosomia, which means “big body,” is commonly set at 4000 g, or 8 lb 13 oz. Studies report increased rates of scar rupture with babies weighing 4000 g or more compared with babies weighing less;3, 11 however, induction and augmentation are more common with bigger babies.18 which would inflate the probability of scar rupture in that weight category.

Scar rupture rate:

  • Birthweight > 4000 g
    • average: 16 per 1000
    • range: 4-28 per 1000
  • Birthweight < 4000 g
    • average: 9 per 1000
    • range: 2-12 per 1000

A major problem for deciding whether to plan VBAC or repeat cesarean, though, is that the data in these studies come from actual birthweights, but your decision would be based on predicted birthweight, and the prediction that the baby will weigh 4000 g or more has little better than a 50-50 chance of being correct.2 That means the decision to plan a repeat cesarean because of predicted macrosomia is likely to be based on a prediction that turns out not to be true.

Longer Pregnancy Duration

Scar rupture was more likely in pregnancies of longer duration;3, 11 however, most occurred in induced labors.14, 42 As with labors with macrosomic babies, results are skewed by this hidden factor.

Scar rupture rate:

  • Pregnancy duration greater than 41 weeks vs. 37 weeks to 40 weeks 6 days
    • average: 15 per 1000 vs. 7 per 1000
    • range: 3-27 per 1000 vs. 2-12 per 1000
  • Pregnancy duration greater than 40 weeks vs. 37 weeks to 39 weeks 6 days
    • average: 12 per 1000 vs. 9 per 1000
    • range: 11-13 per 1000 vs. 8-10 per 1000

Shorter Interval Between Pregnancies

I can’t give you averages or ranges because studies chose different intervals to compare and while five of my six sources chose intervals between deliveries, the sixth chose interval between pregnancies,3, 11 all of which made it impossible for me to merge their data. What can be gleaned is that a short interval between pregnancies increases risk of scar rupture and that waiting at least 9 months before trying to conceive (interval between deliveries of 18 months) substantially decreases risk of scar rupture. Also, a long interval between pregnancies (60 months or more) didn’t increase scar rupture rate.33

Uterine Scar Thickness

Three systematic reviews all concluded that ultrasound measurements of uterine scar thickness did not establish a cut-off with good enough predictive value of the scar giving way to be useful in deciding whether to plan VBAC. One review also noted that studies mostly evaluated defects in the scar at planned repeat cesarean, not complete separation, and we don’t know how well finding a scar window incidentally during surgery correlates with a scar giving way during labor.13, 17, 19

Older Maternal Age

Older maternal age increases the probability of scar rupture,3, 11 although it seems likely that age would be a marker for other factors, not the cause itself. Scar rupture rates were:

  • Age 35 or more
    • average: 11 per 1000
    • range: 3-18 per 1000
  • Age less than 35
    • average: 6 per 1000
    • range: 2-9 per 1000

Preterm Labor in Current Pregnancy

Preterm labor doesn’t increase the probability of scar rupture compared with labor at term, and rates are very low.3, 10, 34 Scar rupture rates were:

  • Labor at less than 37 weeks gestation
    • average: 2 per 1000
    • range: 1-3 per 1000
  • Labor at 37 weeks or more
    • average: 5 per 1000
    • range: 2-7 per 1000

Of note, one of the studies reports on the effects of induction and augmentation.10 Among the eight scar ruptures occurring in preterm VBAC labors, seven were in labors that were induced or augmented.

Uterine Incision Type

There are several uterine incision types:

  • The low-transverse uterine incision is standard. It is a horizontal incision made low on the uterus where the uterus is mostly connective tissue rather than muscle, which makes for a stronger scar.
  • The low-vertical incision may be done at preterm deliveries when the lower portion of the uterus hasn’t developed sufficiently to permit a transverse incision.
  • J or inverted-T incisions may occasionally be performed if it becomes necessary to make more room to deliver the baby.22
  • The classical vertical incision is so-named because it was the type in use decades ago, now replaced by the low-transverse type. It may still be used today in special situations such as emergent cesareans or if the placenta is covering the area where doctors would normally do the transverse incision.

The American College of Obstetricians & Gynecologists VBAC guidelines state that because of the heightened risk of scar rupture, classical vertical incisions and J and inverted-T incisions contraindicate labor.1 They further state that because studies disagree on whether low-vertical incisions pose heightened risk, VBAC labor is not contraindicated.

Previous Preterm Cesarean

I have only one study comparing scar rupture rates in VBAC labor following a cesarean or cesareans before 37 weeks gestation with rates with only a prior cesarean or cesareans at 37 weeks or later.31 Scar rupture rates were:

  • 10 per 1000 with previous preterm cesarean
  •  7 per 1000 with only previous term cesarean

External Cephalic Version for Breech

A study collected data from multiple studies of external cephalic version, a hands to belly technique for turning breech babies head down, in women with previous cesareans.16 Totaling 549 cases in all, not one reported that the procedure caused a problem with the scar.

Severe Adverse Outcomes with Planned VBAC Versus Planned Repeat Cesarean

As I said earlier, the issue isn’t scar rupture per se but the broader consequences of choosing to plan a VBAC versus a repeat cesarean. Here’s data from a Canadian study of 58,704 women planning VBAC after having their first baby via cesarean (scar rupture rate 3 per 1000) compared with 138,836 women planning repeat cesarean (scar rupture rate 0.5 per 1000).41 I’m only going to report on outcomes in that study that would—or could—have permanent consequences because however serious, those are the ones crucial to decision making. These would be hysterectomy, neonatal death, and neonatal seizures, a symptom of neurologic injury from which some, and probably most babies, will make a complete recovery.9 Also, because these outcomes are so rare, I’m reporting them per 10,000 deliveries instead of per 1000 as I’ve been doing above. (For more information, including comparison data on outcomes other than those reported on in this study, see my blog post “Should You Plan a VBAC or a Repeat Cesarean?”.)

  • hysterectomy
    • 6 per 10,000 with VBAC labor
    • 6 per 10,000 with planned repeat cesarean
  • neonatal death
    • 2 per 10,000 with VBAC labor
    • 1 per 10,000 with planned repeat cesarean
  • neonatal seizures
    • 14 per 10,000 with VBAC labor
    •  6 per 10,000 with planned repeat cesarean

There are other considerations here as well. First, most adverse outcomes of all kinds occur in labors that end in repeat cesareans whether that be because of the scar giving way or other problems.21, 41 Therefore, the higher the VBAC rate, the lower the rates of severe adverse outcomes. The VBAC rate in this study was only 50 percent. By contrast, among 12 studies of women planning hospital VBAC with no prior vaginal births, VBAC rates ranged from 61 to 79 percent.11 In addition, as I wrote earlier, severe adverse outcome rates rise with accumulating repeat cesareans, and having a VBAC gets you off that track.

Your Take-Away

So, what’s the overall take-away?

  • The best outcomes will be achieved in labors uncomplicated by problems with the scar and that end in vaginal birth. That means that having care providers who are supportive of VBAC and skilled at managing VBAC labors will maximize your chances of an uneventful, spontaneous vaginal birth.
  • Having a VBAC protects against scar problems in subsequent labors. That means that if there is a possibility of your having more children after this pregnancy—and if you aren’t doing something permanent about your fertility, that possibility exists—planning a VBAC protects your and any future baby’s health and wellbeing.
  • Scar rupture rates vary widely within populations sharing the same characteristic, and one reason for that is whether labor was induced or augmented with I.V. oxytocin. That means that avoiding those interventions whenever possible will reduce your risk.

When then, would it be possible to avoid those interventions?

To begin with, inductions are not just done because medical complications have arisen that necessitate delivery. As we saw above, they are also done for suspected macrosomia and for reaching pregnancy durations short of 42 weeks, the demarcation point defining postterm pregnancy. The intent of these precautionary inductions is to avert adverse outcomes arising from difficult delivery of a large baby or that may occur even in uncomplicated ongoing pregnancies. However, as we have seen, induction increases, not reduces, the risk of severe adverse outcomes, so precautionary inductions don’t accomplish their intended goal. You may wish to consider this if induction is proposed to you for these reasons.

The other rationale for precautionary inductions and for augmenting labor is to reduce cesareans. Let’s see if they deliver on that.

Induction does not. Studies report that induction with a favorable cervix results in similar repeat cesarean rates compared with labor starting on its own, and induction with an unfavorable cervix increases the odds of cesarean delivery.11, 36

Research also shows that repeat cesareans for progress delay have largely to do with clinician judgment and could have been avoided. In proof of this, a study found that over half of the repeat cesareans for progress delay or failed induction were done before 6 cm dilation, the kickover point from early to active labor.44 Before 6 cm, slow progress should rarely be an indication for cesarean because progress is often slow in this phase of labor.4 Another study found that women whose prior cesareans were for progress delay were given 2 to 2 ½ hours less time in the VBAC labor than women whose prior cesareans were for reasons unrelated to progress, but they are the very women likely to benefit by being given more time to progress.32 Furthermore, studies of non VBAC labors consistently find that women are much more likely to have cesareans when doctors incorrectly believe the baby to be big than when the baby actually weighs 4000 g or more, but their doctors didn’t suspect it.8, 12, 25, 27, 28, 30, 37, 38

Finally, we have evidence that high VBAC rates can be achieved without augmentation. A study of freestanding birth centers (birth centers independent of hospitals), where augmentation would not be available, reported an 81 percent VBAC rate in women with no prior vaginal births compared with a range of 61-79 percent reported in hospital studies.11, 23

In summary, we have solid evidence that precautionary induction doesn’t deliver on reducing cesareans either, and the need to augment labor can be reduced by labor management that promotes progress and by having patience.

As for situations where induction or augmentation is indicated, should that become the case, studies document that they can be managed in ways that minimize the risk of overly stressing the scar.24, 29

Basically, it comes down to my recommendation in the first bullet: if you decide to plan a VBAC, find care providers who are supportive of VBAC and who are skilled in managing them, and you will have maximized your odds of having an uncomplicated labor that ends in a spontaneous vaginal birth.

For additional information on VBAC issues, see my blog post “New ACOG VBAC Guidelines: Same Old, Same Old or a Step Forward?”.


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