​If I were to ask you, “What is the risk of uterine rupture during a planned VBAC after a classical cesarean section?” how would you respond? Most would say 4-9% citing ACOG, the American College of Obstetricians and Gynecologists. But where did those numbers come from? Asking that question led me down a rabbit hole of studies 100+ years old, manipulated data, “expert opinion,” and the perpetual ripple effect of one statistic last referenced in ACOG’s 1999 VBAC guidelines.

What is a classical cesarean section?

The overwhelming majority of cesareans in the United States are low transverse meaning that they are a horizontal incision on the lower uterine segment. For a variety of reasons, some people may receive a classical or T-incision.

A classical uterine incision is a vertical incision into the upper part of the uterus called the fundus. A T-incision, sometimes called an inverted-T, is a low transverse incision with a low vertical extension that can go into the fundus.

Why would someone need a classical uterine incision?

Doctors perform classical uterine incisions for a number of reasons but the most common is during a preterm, breech delivery. (Landon, 2011) This is because before 30 weeks, the lower uterine segment may not be fully developed.

Another indication for a classical uterine incision is when the fetus is transverse, meaning the fetus is laying across the abdomen rather than head down (vertex) or foot/butt down (breech).  It is also used when the physical development of the fetus is atypical, such as spina bifida, where a larger incision is needed.

What does ACOG say about classical cesarean section and uterine rupture?

When you try to track down the often cited 4-9% uterine rupture rate associated with labor after a prior classical cesarean section, you are led to ACOG’s 1999 VBAC guidelines. That is the last time ACOG cites that specific statistic. However, here’s the problem. That number doesn’t just include uterine ruptures. It also includes something called uterine dehiscence which is not a uterine rupture.

What is a uterine dehiscence?

Before we go any further, let’s take a step back and discuss the difference between a uterine rupture and a uterine dehiscence. The pregnant uterus has two layers: the uterine muscle, called the myometrium, and the outer layer, called the serosa. A uterine rupture is when both layers separate. Sometimes they are called “true” or “complete” uterine ruptures. It is a split of the full thickness of the uterus, creating an opening.

The pregnant uterus has two layers: the uterine muscle, called the myometrium, and the outer layer, called the serosa. A uterine rupture is when both layers separate.

Uterine ruptures often present with fetal heart rate abnormalities, hemorrhage, and/or abdominal or referred pain, particularly in the shoulder. Uterine ruptures can harm the fetus by diminishing, or eliminating, oxygen delivery which can result in brain damage or death. The primary risk to birthing women or people is excessive blood loss. As a result, uterine ruptures are medical emergencies even though not all uterine ruptures will result in an adverse outcome.

Whereas a uterine dehiscence is when the myometrium separates, but the serosa stays intact. They can also be called a uterine window or an “incomplete” rupture. Dehiscence is often asymptomatic, does not require surgical repair, and is often only identified during a cesarean delivery or via ultrasound. Dehiscences typically do not pose any risk to the fetus or birthing parent. They are not medical emergencies. 

Do studies categorize uterine dehiscences as uterine ruptures?

So here’s the challenge: Sometimes studies classify uterine dehiscences as uterine ruptures and this is especially true among older studies. This inflates the reported uterine rupture rate and gives the impression that uterine rupture is happening far more often than it is.

When reading medical studies, it’s critical to determine how a specific study defines uterine rupture.

To give you an idea of how often dehiscences occur, the Landon 2004 study reported a 0.7% rate of uterine rupture and a 0.7% rate of uterine dehiscence among the almost 18,000 people who labored after one prior low transverse cesarean. Imagine if Landon reported a 1.4% rate of uterine rupture rather than putting dehiscences in their own category? It would tell a very different story. For clarity, all the studies we cite here at VBAC Facts® distinguish between uterine rupture and dehiscence and if they don’t, we clearly disclose that within our discussion. 

When reading medical studies, it’s critical to determine how a specific study defines uterine rupture. Does it count full thickness openings alone? Or are they including any “uterine defect?” This is a key distinction if we wish to nail down the true risk of uterine rupture. As ACOG warns us in their 2019 guidelines, these two events “are not consistently distinguished from each other in the literature and often are used interchangeably.” They say this because some of the studies they cite combine these two events under the umbrella term “uterine rupture.”

What studies are strong enough to measure uterine rupture?

The other thing to keep in mind when measuring uncommon events like uterine rupture is that you need sufficient people in your study in order for it to be powerful enough to report accurate rates. Studies that don’t include a lot of people can either underreport or overreport rates of uncommon events.

It’s ideal to have at least several hundred people included in any uterine rupture research. Generally speaking, the more people you have in a study, the more reliable the numbers will be. (Yes, we run into coding issues with birth certificate data, where we get some of our largest studies, but, generally speaking, the more people who participate, the more accurate the numbers will be.)

What do experts say about uterine rupture after classical cesarean section?

The citations ACOG provides in their 1999 VBAC guidelines for the 4-9% uterine rupture risk after a classical c-section are a 1997 study by Scott and a 1992 study by Pridjian. These are the same articles that ACOG cites for their 4-9% uterine rupture risk associated with T incisions and 1-7% for low vertical incisions. How were these studies conducted and, more importantly, did they classify uterine dehiscences as uterine ruptures?

Most importantly, ACOG identifies both these articles as level three evidence – which means they are expert opinions rather than medical studies.

Unfortunately, due to the age of these articles, the full text is no longer available, and requests to the authors for a full text version were not answered. Additionally, Scott’s article no longer offers a public abstract and the abstract from Pridjian’s article reads like general guidelines only referencing classical cesareans by stating any incision in the active part of the uterus is “an absolute contraindication” to labor. But the abstract does not indicate that any study took place that found a 4-9% risk of rupture among those with a prior classical cesarean incision.

Most importantly, ACOG identifies both these articles as level three evidence – which means they are expert opinions rather than medical studies. This is critical. Expert opinion means that this is simply what someone thinks. They are not collecting data. They are not conducting a study. 

Per a 2005 historical review on vaginal birth after classical cesarean by de Costa, all recent studies on VBAC exclude those with a previous classical cesarean (except one that we discuss here). So in order to get an idea of where this number came from, we are going to have to get into the way back machine.

​The source of the 4% uterine rupture rate after a classical cesarean

In reading de Costa, the 4% classical cesarean section uterine rupture statistic is based on two studies by Holland from 1921. The first study examined the outcomes and characteristics surrounding 97 uterine ruptures. Though it is unclear if dehiscences are included in that figure, it is very likely they were given how medical studies of that time classified dehiscences. Of those 97 events, 54 occurred with classical scars and 36 occurred before labor.

At the time, most cesareans occurred within the fundus as surgery in the lower uterine segment was still in its early days.

At the time, most cesareans occurred within the fundus as surgery in the lower uterine segment was still in its early days. Most people who ruptured only had one prior cesarean, but others had as many as three. Suture material varied from catgut to silk to even reindeer tendon. 

For Holland’s second study, he collected cesarean section data from obstetricians practicing in the British Isles. He was able to assemble data on “1103 women of whom 78 had a subsequent vaginal birth and 352 a subsequent repeat cesarean.” (de Costa, 2005) So 430 births total. Scar type was not specified but as classical scars were customary at the time, it’s quite likely that most or all cesareans occurred in the fundus.

18 uterine ruptures occurred, many of which were identified as dehiscences, but the exact number was not shared. Further, that number represents ruptures and dehiscences that occurred before labor, during labor, and those discovered during a cesarean. 

It’s from those two studies – including dehiscences – that Holland “concluded that the overall incidence of rupture of cesarean section scars (all types) in a subsequent pregnancy or labor was 4%.”  (de Costa, 2005) Holland made a few observations of what he saw as potential drivers for uterine rupture including imperfect healing and placenta over the scar. He also noted that uterine rupture occurred “almost as often in late pregnancy as in labor.” (de Costa, 2005) 

The source of the 9% uterine rupture rate after a classical cesarean

De Costa also shares the source for the 9% statistic (and this is going to blow your mind!) In 1956, Dewhurst published a study of 68 women who had 103 classical cesareans, so some had more than one classical cesarean, and 16 women who had both classical and lower uterine segment cesareans, some of which also had multiple cesareans. He observed a uterine rupture rate of 6%. 

It’s from this incredibly small sample size of 135 classical cesareans that he calculated his uterine rupture rates. 

The next year, in 1957, he published a review of six studies, including his own which, remember, included the experiences of 84 women, some of whom had multiple classical cesareans. Two other studies didn’t include any classical scars and one study included only 16 classical cesareans. It’s from this incredibly small sample size of 135 classical cesareans that he calculated his uterine rupture rates. 

So among those people who labored after a prior classical uterine incision, 4.7% had a uterine rupture, but Dewhurst also calculated the rate of uterine rupture among those who just delivered vaginally… want to guess the rate? 8.9%. 

By calculating the uterine rupture rate among just those that delivered vaginally, rather than including all of those who labored after a classical cesarean section, he was able to increase the reported uterine rupture rate. Let’s remember that this rate very likely includes dehiscences as well. Nevertheless, this was the number that people glommed onto. 

As De Costa shares, “The impression is given, and has since been widely quoted in the literature, that 8.9% of women with previous classical Caesarean section attempting vaginal delivery had uterine rupture – this is not the case.” 

“From that date on, in both the American and the European literature, there was almost universal caution against TOS [trial of scar].” 

So what was the impact of this study? De Costa continues, “Up until the mid 1950s, TOS [trial of scar] after classical Caesarean section was still reasonably common on both sides of the Atlantic… Dewhurst was firmly opposed to the concept of TOS [trial of scar] for women with prior classical Caesarean section and his paper marks a turning point in obstetricians’ attitudes to the subject. From that date on, in both the American and the European literature, there was almost universal caution against TOS.” 

Dewhurst also emphasized that uterine rupture from a fundal scar “could be more severe” than a uterine rupture occurring in the lower uterine segment as other researchers had noted, but given their small sample sizes, it’s questionable whether they actually found this or if this was their best guess. 

It is astonishing the incredible influence this mid-1950s study has had even though it included only 135 classical cesareans and very likely labeled uterine dehiscences as uterine ruptures.

The source of the 12% uterine rupture rate after a classical cesarean

Landon 2011 cites a vaginal birth after classical cesarean uterine rupture rate of 1 to 12%. I wanted to know where those numbers came from. Turns out, that 12% classical VBAC uterine rupture rate is also based on a very small study, yet this figure continues to be repeated. 

In 1988 Halperin published a retrospective study of birth after preterm cesarean. They included 70 classical cesareans and 71 low transverse cesareans. There were four dehiscences, five ‘thin’ scars, and no uterine ruptures. How do you think this data was interpreted and communicated? 

As the dehiscences occurred at a rate of 12%, but they were called uterine ruptures, thus was born the “hard science statistic” that laboring after a classical cesarean carries a 12% uterine rupture risk.

If you guessed that the dehiscences were called uterine ruptures, you are right. As the dehiscences occurred at a rate of 12%, but they were called uterine ruptures, thus was born the “hard science statistic” that laboring after a classical cesarean carries a 12% uterine rupture risk. A single study. Of 70 classical cesareans. With no uterine ruptures. Is your mind blown yet? In fact, ACOG’s 1994 VBAC guidelines mention a 12% uterine rupture rate associated with classical cesareans. Halperin is who they cite as the source.

When we examine the actual source of the 4-9%, and even 12% uterine rupture risk with classical VBAC, we see that they fall flat. The studies were incredibly small, called dehiscences uterine ruptures, or the numbers were calculated in such a way to inflate the uterine rupture rate like reporting the uterine rupture rate among only those who delivered vaginally versus all who labored after a cesarean.

So given the very limited available data, what do birthing women and people need to know about classical VBAC, including T/J incisions? We cover that here.

Want to learn more about this topic? We dive into all the details in our continuing education training, “Decoding Classical, T/J, and Low Vertical VBAC: Navigating Limited Knowledge and Unknown Risk” available through VBAC Facts® Professional Membership.

Jen

Resources Cited

American College of Obstetricians and Gynecologists. (1994). ACOG Committee Opinion Number 143: Vaginal delivery after a previous cesarean birth. Washington, DC: The American College of Obstetricians and Gynecologists.

American College of Obstetricians and Gynecologists. (1999). ACOG Practice Bulletin No. 5: Vaginal birth after cesarean delivery. Obstetrics & Gynecology.

American College of Obstetricians and Gynecologists. (2019). ACOG Practice Bulletin No. 205. Vaginal birth after cesarean delivery. Obstetrics & Gynecology, 133(2), e110-e127.  https://journals.lww.com/greenjournal/Abstract/2019/02000/ACOG_Practice_Bulletin_No__205__Vaginal_Birth.40.aspx

De Costa, C. (2005). Vaginal birth after classical Caesarean section. Australian and New Zealand Journal of Obstetrics and Gynaecology, 45, 182-186. https://doi.org/10.1111/j.1479-828X.2005.00387.x

Dewhurst, C. (1956). Reports of societies – rupture of the uterus. The Journal of Obstetrics and Gynaecology of the British Empire, 63, 125-129.

Dewhurst, C. (1957). The ruptured Caesarean section scar. The Journal of Obstetrics and Gynaecology of the British Empire, 113-118.

Eames, D. H. (1953). A study of the management of pregnancies subsequent to Caesarean section. American Journal of Obstetrics & Gynecology, 65, 944-953.

Halperin, M. E., Moore, D. C., & Hannah, W. J. (1988). Classical versus low-segment transverse incision for preterm Caesarean section: maternal complications and outcome of subsequent pregnancies. British Journal of Obstetrics & Gynecology, 990-996.

Holland, E. (1921). Rupture of the Caesarean section scar in subsequent pregnancy or labour. The Journal of Obstetrics and Gynaecology of the British Empire, 28, 488-522.

Landon, M. B., Hauth, J. C., Leveno, K. J., Spong, C. Y., Leindecker, M. S., Varner, M. W., . . . Miodovnik, M. (2004). Maternal and perinatal outcomes associated with a trial of labor after prior cesarean delivery. New England Journal of Medicine, 351, 2581-2589. https://doi.org/10.1056/NEJMoa040405

Landon, M. B., & Lynch, C. D. (2011). Optimal timing and mode of delivery after cesarean with previous classical incision or myomectomy: a review of the data. Seminars in Perinatology (Vol. 35, No. 5, pp. 257-261). https://doi.org/10.1053/j.semperi.2011.05.008

Pridjian, G. (1992). Labor after prior cesarean section. Clinical Obstetrics and Gynecology, 35(3), 445-56. https://doi.org/10.1097/00003081-199209000-00004

Scott, J. R. (1997). Avoiding labor problems during vaginal birth after cesarean delivery. Clinical Obstetrics and Gynecology, 40(3), 533-541.