Update 1/20/12 – Someone who believed this birth myth to be true, told me that the source of this information was an OB from St. Louis who presented at the 2011 ICAN conference. I contacted ICAN and they said that the person must be referring to Dr. George Macones. Yet, no one on the ICAN Board, who were seated at the front table during his presentation, remembers him saying that induced, unscarred women have the same risk of uterine rupture as a VBAC mom. Additionally, I would think that if he gave a statistic like that, everyone would have remembered because it is quite a remarkable statement as you will see shortly. While many women repeat, believe, and defend this statement, no one has supplied one study to me to support it.

Update 1/21/12 – Ruth S Beattie Dicken, the Speaker Chair of the 2011 ICAN conference contacted me via Facebook and said, ” Dr. Macones did not say that. Nor did any other OB. I sat in on every session with OB speakers.”

Update 1/21/12: The difference in uterine rupture (UR) rates between unscarred, induced uteri and scarred uteri is significant: 2.2 per 10,000 in an unscarred, induced uterus and 64 in 10,000 in a scarred uterus. But it’s not that the risk of UR is so large in a scarred mom, it’s that it’s so very, very small in an unscarred mom, even when she is induced.

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OBs are often vilified (rightfully so) for giving women inflated rates of uterine rupture and I’ve documented several examples over the years: Another VBAC Consult Misinforms, Scare tactics vs. informed consent, Hospital VBAC turned CS due to constant scare tactics, and A father says, Why invite the risk of VBAC?. But the midwife (or OB, but it’s generally a midwife) who gives false information that minimizes the risk of rupture is just as harmful to the VBAC mom. Since I wrote Lightning strikes, shark bites, and uterine rupture, I’ve been making mental notes of other birth myths that seem to be forwarded from woman to woman, without anyone asking, “That’s a great statistic! What’s the source?”

There is one that I hear quite often:

A woman without a prior cesarean whose labor is induced is just as likely as a VBAC mom to experience an uterine rupture.

Recently, I heard it again and I really wanted to know if there was some study that demonstrated this. It’s a logical conclusion that inducing an unscarred woman would increase her risk of rupture as uterine rupture is listed as a risk for Pitocin and prostaglandins (such as Cytotec and Cervidil) but how much does induction increase the risk of uterine rupture in an unscarred uterus? And does the rate of rupture increase so much that it is the same as the risk of rupture in a VBAC mom? I had unsuccessfully looked for that information in the past, so I went to my Facebook page and asked if anyone had a source.

Several women responded who had heard this information, two of which from their midwives which is really frightening. Unfortunately, no one who responded could cite where they heard this information. So I started looking and found Uterine rupture in the Netherlands: a nationwide population-based cohort study (Zwart, 2009).

This study included 358,874 total deliveries, making it “the largest prospective report of uterine rupture in women without a previous cesarean in a Western country.” It also differentiates between uterine rupture and dehiscence which is really important because we want to measure the rate of complete rupture. You can read the study in its entirety here.

The role of induction in scarred and unscarred uterine rupture

Zwart utilized multiple methods of induction: cervical prostaglandins (sulproston, dinoproston, and misoprostol aka Cytotec), oxytocin (Pitocin) and mechanical dilatation. Prostaglandin “dosages ranged from 0.5 to 2.0 mg with a minimal interval of 4 h in between,” but they do not provide the dosages of the women who ruptured.

Of the 208 scarred and unscarred uterine ruptures, 130 (62.5%) occurred during spontaneous labor reflecting 72% of scarred ruptures and 56% of unscarred ruptures. 28 (13.5%) ruptures occurred during cervical prostaglandin induction. 22 (10.6%) ruptures occurred during oxytocin (Pitocin) induction.

It seems that there were women who were induced with prostaglandins and Pitocin as measured in Table 5. But there is no measure for women who ruptured and were induced with both prostaglandins and Pitocin in any of the uterine rupture tables.

There is no mention of Bishop’s score, but they did provide the “reasons for induction with prostaglandins [in scarred women which] included (nearly) post-term pregnancy (n = 10), intra uterine fetal death/ multiple congenital abnormalities (n = 5), elective (n = 3), pregnancy induced hypertension (n = 2), intra uterine growth restriction (n = 1) and prelabour rupture of membranes (n = 1).”

Interestingly, this Netherlands-based study found “there was a trend towards more liberal use of prostaglandins for induction of labour in low-volume hospitals as compared to middle- and high-volume hospitals (24.4% versus 13.0% of cases, P = 0.29).”

It’s also interesting that there were no maternal deaths even though “18 [unscarred] women (72%), rupture occurred outside office hours.”

The rate of uterine rupture in an induced labor without a prior cesarean

The study found, ” In 11 women [without a prior cesarean who experienced a uterine rupture], labour was induced, in all but one with prostaglandins.” Said in another way, 40% of the unscarred women who ruptured were induced with prostaglandins versus only 12.1% of scarred moms who ruptured.

So Zwart found that it’s not the Pitocin that causes the ruptures in unscarred moms, it’s the prostaglandins. This is logical because prostaglandins are harder to control. If the uterus is hyper-stimulating due to prostaglandins, they continue to work on the uterus even after they have been removed from the cervix. Pitocin, on the other hand, has a short half-life so the body responds quicker to the drip being turned off in the event of uterine hyper-stimulation.

While we know that there are 332,885 unscarred women included in this study, we don’t know the number or percentage of unscarred women who were induced. We need this information in order to calculate the rate of uterine rupture in induced, unscarred women.

So I did a little looking and I found Verhoeven (2009) which states, “In The Netherlands induction rates have remained stable over the last decades at approximately 15%.” Since the induction rate has been stable, and this study included 97% of births in The Netherlands between August 1, 2004 and August 1, 2006, I feel comfortable using this 15% rate of induction to calculate the rate of uterine rupture in induced, unscarred women.

So when we take 15% of the 332,885 unscarred women in the study, we get 49,933 induced, unscarred women.

Dividing the 11 ruptures that occurred in induced, unscarred women by 49,933 total induced, unscarred women, we get the following uterine rupture rate in induced, unscarred women: 0.022% or 2.2 per 10,000 deliveries.

Now let’s look at the rate of uterine rupture in women with a prior cesarean: “25,989 trials of labor were attempted in the Netherlands during the study [resulting in 183 ruptures.] The risk of uterine rupture would then be 0.64%” or 64 in 10,000 deliveries. This rate includes ruptures in induced and spontaneous labors, but we do know that 72% of those ruptures occurred during spontaneous labors.

In other words, a woman with a prior cesarean section has a uterine rupture risk 29 times greater than the risk of uterine rupture due to induction in a woman without a prior cesarean, 0.64% vs. 0.022%.

Another way to look at the data is: you would need to induce 4,546 women without a prior cesarean in order to get one uterine rupture due to induction.

While I hadn’t seen the numbers until now, I was always very skeptical when I heard this rumor. I’m glad to finally have hard numbers to share.

Risk of uterine rupture in a spontaneous labor without a history of uterine surgery

Next, since I had all the data available, I wanted to calculate how induction affects the rate of uterine rupture in an unscarred woman. Remember that 10 of the 11 ruptures in induced, unscarred women occurred during the use of prostaglandins and we don’t have information on the dosage in those labors.

We already established that the rate of rupture in an induced, unscarred labor was 0.022% or 2.2 per 10,000 deliveries.

The remaining 14 ruptures of the 25 total ruptures occurred during spontaneous labor.

14 spontaneous ruptures among 282,952 spontaneous labors in unscarred women, gives us a 0.0049% uterine rupture rate or .49 per 10,000 deliveries.

As I suspected, an unscarred woman induced with prostaglandins has a greater risk of uterine rupture than an unscarred woman in a spontaneous labor, but now we have exact figures: 0.022% vs. 0.0049%. Prostaglandin induction in an unscarred woman increases her risk of uterine rupture almost 5 times, but the overall risk is still extremely low.

Moving forward

It was interesting to note that among women with a prior cesarean, 72% of ruptures occurred during spontaneous labor. The scar itself, that prior cesarean surgery, is what increases the risk of uterine rupture the most. With this in mind, the researchers state:

With 29% of all previous caesareans being performed for breech presentation, we clearly show the negative side effects and long-term adverse consequences of routinely performing elective caesarean for breech delivery . . . the only means of reducing the incidence of uterine rupture is to minimize the number of inductions of labor and to closely monitor women with a uterine scar. . . Ultimately, the best prevention [of uterine rupture] is primary preventions, i.e. reducing the primary cesarean delivery rate. The obstetrician who decides to perform a caesarean has a joint responsibility for the late consequences of that decision, including uterine rupture.

This is why more hospitals offering breech vaginal birth and VBAC, such as Portland, OR based Oregon Health & Science University (OHSU), is so important. Read more about OHSU’s mission to reduce the cesarean rate.

As I say in Myth: Risk of uterine rupture doesn’t change much after a cesarean:

While the risk of rupture in a spontaneous labor after one prior low transverse cesarean is comparable to other obstetrical emergencies, it is important for women weighting their post-cesarean birth options to know that their risk increased substantially due to their prior cesarean. It is important for them to understand the risks and benefits of VBAC vs. repeat cesarean. It is important for them to have access to accurate information and be able to differentiate between a midwife’s/blogger’s/doula’s/birth advocate’s/person on Facebook’s hopeful opinion vs. documented statistics.

I implore those who interact with, and have impact on, women weighing their birth options: do not pass along information, no matter how great it sounds, if you don’t have a well-designed scientific study supporting it. If you hear a statistic you would love to use and share, just ask the person who gave you this information, ”What is the source?” and use the citation anytime you quote the statistic. But if the person doesn’t have a well-designed scientific study, be wary and don’t use the stat. This way, we can reduce the rumor and increase the amount of good information on the Internet. I know, a lofty goal.

I use the data in this same study to debunk the myth: the risk of uterine rupture is roughly double, or not much different, from an unscarred uterus. . . more dangerous information from what should be trusted sources.

Read more birth myths debunked including Lightning strikes, shark bites, and uterine rupture and Myth: Risk of uterine rupture doesn’t change much after a cesarean.

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Notes: This study found that there were 183 ruptures after a prior cesarean and states in the abstract that this reflects a rate of 0.051% or 5.1 per 10,000 deliveries. But the problem is, they divided the number of uterine ruptures after a cesarean by the total number of women (with a prior cesarean and without.) It’s only towards the end of the study do they state the risk of uterine rupture in a woman after a prior cesarean is 0.64%. So, this is a little confusing and is another example of why reading the entire study, rather than just the abstract, is so important.