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Confusing math misleads women on uterine rupture and mortality risk

A note about the studies and language I use throughout the website: When I use the term “uterine rupture,” I mean a complete uterine rupture. I do not lump complete uterine ruptures and dehiscences into one group. Additionally, when I give a statistic for “uterine rupture in a spontaneous labor” that means a complete uterine rupture in a non-augmented, non-induced labor. While spontaneous labors are sometimes augmented, I only use studies that break out the uterine rupture rates for spontaneous, augmented, and induced labors. And if you have heard that studies always lump dehiscences and uterine rupture together or that studies never spell out uterine rupture rates in spontaneous labors, you have been receiving incorrect information.

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Image of math on chalkboardI recently came across a blog discussing the study A 10-Year Population-Based Study of Uterine Rupture (Kieser, 2002). Kieser is “a population-based study of uterine rupture in one Canadian province for the 10-year period 1988–1997″ including 114,933 total deliveries of which 11,585 were after a cesarean . (You can read the entire study on the Obstetrics & Gynecology website. The link to the full text PDF is on the right hand side of the page.) The blogger in question shares this VBAC study along with statistics she calculated using confusing math which ultimately misrepresents the risk of uterine rupture and infant mortality during a VBAC attempt.*

The following critique demonstrates why it is important to not only know the source of the data (which this blogger provides), but also know that the blogger’s statistics are meaningful. This blogger’s statistics are confusing, hence the weakness of her article.

First, she provides a copy of the study abstract:

Objective: To review the incidence, associated factors, methods of diagnosis, and maternal and perinatal morbidity and mortality associated with uterine rupture in one Canadian province.

Methods: Using a perinatal database, all cases of uterine rupture in the province of Nova Scotia for the 10‐year period 1988–1997 were identified and the maternal and perinatal mortality and morbidity reviewed in detail.

Results: Over the 10 years, there were 114,933 deliveries with 39 cases of uterine rupture: 18 complete and 21 incomplete (dehiscence). Thirty‐six women had a previous cesarean delivery: 33 low transverse, two classic, one low vertical. Of the 114,933 deliveries, 11,585 (10%) were in women with a previous cesarean delivery. Uterine rupture in those undergoing a trial for vaginal delivery (4516) was complete rupture in 2.4 per 1000 and dehiscence in 2.4 per 1000. There were no maternal deaths, and maternal morbidity was low in patients with dehiscence. In comparison, 44% of those with complete uterine rupture received blood transfusion (odds ratio 7.60, 95% confidence interval 1.14, 82.14, P = .025). Two perinatal deaths were attributable to complete uterine rupture, one after previous cesarean delivery. Compared with dehiscence, infants born after uterine rupture had significantly lower 5‐minute Apgar scores (P < .001) and asphyxia, needing ventilation for more than 1 minute (P < .01).

Conclusion: In 92% of cases, uterine rupture was associated with previous cesarean delivery. Uterine dehiscence was associated with minimal maternal and perinatal morbidity. In contrast, complete uterine rupture was associated with significantly more maternal blood transfusion and neonatal asphyxia.

What follows is the entirety of her analysis which is simultaneously full of confusing math that ultimately misleads the reader while lacking clear explanations of what her percentages actually represent.

Error 1: She simply states, “Out of 114,933 deliveries, there were 39 total ruptures (.o34%), with 36 of them having a previous cesarean.”

It’s difficult to figure out what she intends to show here. This is a study about VBAC and she’s calculated a total rupture rate, so someone reading her comment might think that the 0.034% figure relates to the risk of uterine rupture during a VBAC. But let’s look at the numbers she used.

First, the 39 uterine rupture figure she referred to includes both complete ruptures and dehiscences from both scarred and unscarred women. Second, she divided that number by the total number of scarred and unscarred women in the study.

What does the resulting percentage give us? Not anything very helpful. It melds the uterine rupture statistic for complete ruptures and dehiscences from scarred and unscarred women into one single number: 0.034% (or 0.34 per 1,000 VBACs). Since unscarred women represented 90% of all women in the study, it makes sense that the number she arrived at would be so small.

The correct way to determine the uterine rupture and dehiscence rate (which the blogger refers to as the “total rupture rate”) during a VBAC attempt is to identify the number of uterine ruptures (11) and dehiscences (11) from VBAC attempts and then divide that by the total number of VBAC attempts (4516) and then multiply that number by 100. This gives us the “total rupture rate” during a VBAC as 0.49% or 4.9 per 1,000 VBACs which is 14 times larger than the figure of 0.034% that the blogger arrives at.

So again, it’s hard to tell what the blogger intends to show. What message is she sending her readers? What does she intend her “take-away point” to be? I am concerned that since she is reviewing a VBAC study, many readers who are unfamiliar with the generally accepted rate of uterine rupture, but might know that it’s “less than 1%,” might believe that this 0.034% rate of “total rupture” is for VBAC moms. And since the blogger isn’t clear about her math, and that she’s including unscarred moms in her math, I’m certain that women left her site with that inaccurate statistic in their head.

Error #2: She states, “Complete ruptures counted for 18 of those (.016% chance of complete uterine rupture). No mother deaths.”

Again, her confusing math resulted in a misleading statement. Rather than dividing the number of uterine ruptures during a VBAC attempt (11) by the total number of women attempting a VBAC (4516), she divides the number of complete ruptures in scarred and unscarred women by the total number of scarred and unscarred women in the study.

Once again, she melds the statistics for scarred and unscarred women into one very low percentage, but since this is a VBAC study, it’s confusing.

It’s interesting to note that the risk of uterine rupture during a VBAC attempt is provided in the abstract (0.24% or 2.4 per 1,000). So not only has she done the math wrong, but the number she arrives at is 15 times smaller than the actual risk. The end result is that she is misrepresenting the risk of uterine rupture during a VBAC.

Don’t misunderstand. It’s not that the actual risk of uterine rupture during a VBAC found in this study is so large, it’s just that the level of risk that the blogger miscalculated is so very, very small since she included all the unscarred women in her math.

She correctly states that there were no maternal (“mother”) deaths.

Just for comparisons sake, let’s just look at the complete rupture rate among unscarred women from the study because it is a persistent birth myth that the rate of uterine rupture in an unscarred mom isn’t much different than a scarred mom, even if that unscarred mom is induced.

Of the 103,348 unscarred women in the study, 3 had uterine ruptures giving us a uterine rupture rate in unscarred women of 0.0029% or 0.029 in 1,000 labors. This figure is 82.7 times smaller than the risk of uterine rupture in a VBAC attempt.

But don’t take this to mean that the risk of uterine rupture in a VBAC attempt is so large. It’s just that the risk of uterine rupture in an unscarred woman is so very, very small, which is why it is dangerous to perpetuate the myth that the risk of uterine rupture doesn’t change much (or simply doubles) after a cesarean.

Error #3: She states, “2 infant deaths (2 0f 114,933 deliveries is a 0.0017%). One of the deaths the mother had a previous cesarean, one had no previous cesarean.”

The abstract reports that there were two perinatal deaths (stillbirths and deaths in the first 7 days of life) due to uterine rupture, one during a VBAC attempt and one in an unscarred woman. However, the full text of the study (which it doesn’t seem that the blogger read) reveals that there were three neonatal deaths (all deaths in the first 28 days of life) and gives more information on the one neonatal death attributable to uterine rupture in a woman during a VBAC attempt:

There were three neonatal deaths, all in women with a previous cesarean delivery. Two of these were caused by lethal anomalies known before labor. One was secondary to severe asphyxia in a woman with one previous low segment cesarean delivery in spontaneous labor at term. A nonreassuring fetal heart pattern and vaginal bleeding prompted cesarean delivery, which confirmed complete rupture of the cesarean scar.

The correct way to calculate the infant mortality rate due to uterine rupture during a VBAC attempt would be to divide the number of babies who died due to uterine rupture during a VBAC attempt (1) by the total number of VBAC attempts (4516) which gives us 0.022% or 0.22 per 1,000.

But rather than calculating the risk the correct way, she divides the two perinatal deaths from complete rupture (one during a VBAC attempt and one in an unscarred mom), by the total number of scarred and unscarred women included in the study. Again, she melds together the statistics for infant death for scarred and unscarred women into one number which is not helpful to the reader. The blogger’s incorrectly calculated figure of 0.0017% is 15 times smaller than the actual risk of infant death due to uterine rupture during a VBAC attempt because she included all the unscarred women in her math.

Again, please don’t misunderstand. It’s not that the risk of infant death due to uterine rupture during a VBAC attempt is so large. It’s that the blogger’s confusing math resulted in an extremely small number.

She closes her blog post with:

Uterine scar dehiscence is different than complete uterine rupture. Dehiscence is the separation of a preexisting scar that doesn’t disrupt uterine serosa and that does not significantly bleed from its edges. In addition, the fetus, placenta, and umbilical cord must be contained within the uterine cavity, without a need for cesarean delivery due to fetal distress.

That reflects the end of the blogger’s post, but I wanted to offer a few more statistics from this study.

On the scar type of the women who had uterine ruptures or dehiscences:

Of the 36 ruptures in women with a previous cesarean delivery, 33 had a low transverse incision (12 complete, 21 dehiscence), and three, all complete ruptures, had a vertical incision (two classic, one low vertical).

22% of the uterine ruptures occurred before the onset of labor. On the scar type of those ruptures:

Ten [scarred and unscarred] women (four complete, six dehiscence) had no labor: four had two previous low transverse cesareans, one low transverse, two classic, one low vertical, and one no scar.

Please note that 38.9% of complete ruptures in the study (this includes scarred and unscarred women) occurred during spontaneous (not induced, not augmented) labor. More on the ruptures in induced and augmented labors:

Of the 14 [scarred and unscarred] women with complete rupture who had labor, six of 14 (42.9%) received an oxytocic drug for induction (oxytocin or vaginal prostaglandin E2) or augmentation of labor (oxytocin), and six of 15 (40%) of the dehiscence group who labored received an oxytocic drug for induction of labor. . . Our data do not allow the precise degree of risk with the individual types of induction, but the fact that oxytocic agents for induction or augmentation of labor were involved in 43% of complete and 40% of dehiscence highlights the potential risk.

I hope you can see how important it is to understand how bloggers arrive at their statistics, as well as to read the entire study as opposed to just the abstract. There really is so much great data that is in the body of a study and many studies published in Obstetrics & Gynecology are available for free, without registration.

Like Angela Horn’s latest blog post, I believe that we (bloggers, doctors, midwives, doulas, childbirth educators/advocates, and moms who write about birth) are obligated to ensure that the information we provide is correct and accurate to the best of our ability.

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* I submitted a comment offering the correct math, but my comment was never published. I emailed the blogger and was told that she never received the comment. Later I made a couple more comments on other blog posts on her site, answering questions that people asked (specifically about uterine rupture rates in induced, unscarred moms relative to VBAC moms), none of which were published. For whatever reason, she didn’t want to approve my comments. She didn’t want her readers to have access to the information I share on this website. Since then I have learned that this blogger has a history of deleting comments and blocking individuals from reading her website.

So, I figured I would redo the numbers contained in my never approved comment- representing over an hour of number crunching, reading, and writing – and share it here because her readers are likely leaving that article accepting her misleading numbers as reality.

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Kieser, K. E., & Baskett, T. F. (2002, October). A 10-Year Population-Based Study of Uterine. Obstetrics & Gynecology, 100(4), pp. 749-753. Retrieved from http://journals.lww.com/greenjournal/Fulltext/2002/10000/Delivery_After_Previous_Cesarean__A_Risk.22.aspx

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2 comments to Confusing math misleads women on uterine rupture and mortality risk

  • Jana S

    thankyou so much for posting this. I believe I know the blog you are referring to and found this post because I was trying to research why her rates of rupture were so much lower compared to other studies. (The rates she used when interpreting the study anyway) It was very confusing to me so this was very helpful thankyou!

    • Jen Kamel

      Jana,

      It’s my pleasure! I have found that she did this on another VBAC study – combined the stats for scarred and unscarred moms – and people caught it so she put a “disclaimer” up at the top of the post. On this post that I wrote about though, it doesn’t seem from any of the comments that women understand her stats include unscarred moms.

      Warmly,

      Jen

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