Myth: 50% of uterine ruptures occur before labor

Myth: 50% of uterine ruptures occur before labor

Becky recently ask this question:

I read somewhere that the risk of uterine rupture is actually higher during pregnancy than during birth. Does anyone have a source for this?

Becky,

I had heard the same thing many times. However, no one who shared this stat with me could ever cite a study substantiating it. I looked and looked on and off for years and never found it.

Instead, I found “Uterine rupture in the Netherlands: a nationwide population-based cohort study” (Zwart, 2009), “the largest prospective report of uterine rupture in women without a previous cesarean in a Western country.” Zwart differentiated between uterine rupture and dehiscence and included 97% of births in The Netherlands between August 1, 2004 and August 1, 2006. All told, Zwart studied 358,874 total deliveries, 25,989 of which were TOLACs.

I have referenced Zwart before when comparing scarred vs. unscarred rupture rates and scarred vs. induced, unscarred rupture rates. Zwart also included data on pre-labor rupture which I will share with you as well.

Scar rupture before labor

Zwart reported that 9% (1 in 11) of scar ruptures (women with prior cesareans) happened before the onset of labor. When we take 9% of the overall scar rupture rate of 0.64% (1 in 156)*, we get a 0.0576% (1 in 1736) risk of a scar rupture before labor.

Unscarred rupture before labor

Zwart (2009) found 16% (1 in 6.25) of ruptures in women without prior cesareans (unscarred ruptures) occurred before labor and an overall unscarred rupture rate of 0.007% (1 in 14,286)*. When we multiply these two numbers, we get a 0.00112% (1 in 89,286) risk of uterine rupture in an unscarred uteri before labor.

Here is a table comparing the numbers:

Overall UR Rate % of URs that Occur Pre-Labor Pre-Labor UR Rate
Scarred Uteri 0.64% 9% 0.0576%
Unscarred Uteri 0.007% 16% 0.00112%

The war of the studies

Remember, all these stats are based on one study. Other studies might find different rates. However, I think Zwart would have the most accurate rates to date as it is “the largest prospective report of uterine rupture in women without a previous cesarean in a Western country.” This is an important factor because uterine rupture in an unscarred woman is an extremely rare event. We need tens of thousands of women in order to get an accurate number. The fact that Zwart includes over 300,000 unscarred women is huge.

Take home message: The risk of uterine rupture before labor is extremely rare especially for unscarred women.

* This statistic includes non-induced/augmented, induced, and augmented labors.

Jen

Resources Cited

Zwart, J. J., Richters, J. M., Ory, F., de Vries, J., Bloemenkamp, K., & van Roosmalen, J. (2009, July). Uterine rupture in the Netherlands: a nationwide population-based cohort study. BJOG: An International Journal of Obstetrics and Gynaecology, 116(8), pp. 1069-1080. Retrieved January 15, 2012, from http://onlinelibrary.wiley.com/doi/10.1111/j.1471-0528.2009.02136.x/full

What do you think?
Leave a comment.

What do you think? Leave a comment.

Jen Kamel

Jen Kamel is the founder of VBAC Facts, an educational, training and consulting firm. As a nationally recognized VBAC strategist and consumer advocate, she has been invited to present Grand Rounds at hospitals, served as an expert witness in a legal proceeding, and has traveled the country educating hundreds of professionals and highly motivated parents. She speaks at national conferences and has worked as a legislative consultant in various states focusing on midwifery legislation and regulations. She has testified multiple times in front of the California Medical Board and legislative committees on the importance of VBAC access and is a board member for the California Association of Midwives.

Learn more >

Free Report Reveals...

Parents pregnant after a cesarean face so much misinformation about VBAC. As a result, many who are good VBAC candidates are coerced into repeat cesareans. This free report provides quick clarity on 5 uterine rupture myths so you can tell fact from fiction and avoid the bait & switch.

VBAC Facts does not provide any medical advice and the information provided should not be so construed or used. Nothing provided by VBAC Facts is intended to replace the services of a qualified physician or midwife or to be a substitute for medical advice of a qualified physician or midwife. You should not rely on anything provided by VBAC Facts and you should consult a qualified health care professional in all matters relating to your health. Created By: Jen Kamel | Copyright 2017 VBAC Facts | Terms of Use | Privacy Policy

 

Myth: Two numbers less than 1% are similar

Myth: Two numbers less than 1% are similar

I have often heard, “If two numbers are less than 1%, they are similar.”  Typically
this is expressed while comparing the risks of rupture in an unscarred versus scarred uterus.   But is this true?  How different can two numbers less than 1% be?

Two numbers less than 1% are no more similar than two numbers greater than 1%

Just because two numbers are less than 1%, that doesn’t make them any more similar than two numbers greater than 1%.  A 2% risk of something happening is very different than an 89% risk.  While they are both greater than 1%, they represent drastically different levels of risk.

2% = 1 in 50 risk

89% = 1 in 1.12 risk

89% represents a 44 times greater risk than 2%.

What about numbers less than 1%?

It might seem rational that since numbers less than 1% are so small, that there wouldn’t be as much of a difference between them.  But numbers less than 1% work in the same way as those greater than 1%.   Let’s run a few and measure the difference.

1 in 100 represents 1%.

1 in 1,000, is the same as 0.1%, and is 10 times smaller than 1%.

1 in 10,000, is the same as 0.01%, and is 100 times smaller than 1%.

1 in 100,000, is the same as 0.001%, and is 1,000 times smaller than 1%.

1 in 1,000,000, is the same as 0.0001%, and is 10,000 times smaller than 1%.

Comparing small risks

According to Zwart* (2009), the risk of uterine rupture in:

– an unscarred mom is 1 in 14,286 (0.007% or 0.7 in 10,000) and

– a scarred mom is 1 in 156 (0.64% or 64 in 10,000).

(Both statistics include non-induced/augmented, induced, and augmented labors.)  Even though both numbers are less than zero, they represent very different levels of risk.  In fact, the risk of rupture in an unscarred mom is 91 times smaller than a scarred mom.  It’s not that the risk of rupture is excessively high in a scarred mom, but that it is so very, very, very low in an unscarred mom.

Using the language from Kim James’ handout Understanding Obstetrical Risk, the risk of rupture in an unscarred mom would be described as “very rare” whereas the risk of rupture in a scarred mom would be described as “uncommon.”

Take away messages

Just because two numbers are less than 1% does not mean that they are similar.  Numbers below 1% represent just as much of a range as numbers greater than 1%.

While the risk of scar rupture is very different than the risk of unscarred rupture, it is similar to other serious obstetrical emergencies such as placental abruption, cord prolapse, and postpartum hemorrhage.

Resources Cited

* Zwart (2009) differentiated between uterine rupture and dehiscence, featured 358,874 total deliveries, 25,989 of which were trials of labor after a cesarean.  Zwart included 97% of births in The Netherlands between August 1, 2004 and August 1, 2006, making it “the largest prospective report of uterine rupture in women without a previous cesarean in a Western country.”

Zwart, J. J., Richters, J. M., Ory, F., de Vries, J., Bloemenkamp, K., & van Roosmalen, J. (2009, July). Uterine rupture in the Netherlands: a nationwide population-based cohort study. BJOG: An International Journal of Obstetrics and Gynaecology, 116(8), pp. 1069-1080. Retrieved January 15, 2012, from http://onlinelibrary.wiley.com/doi/10.1111/j.1471-0528.2009.02136.x/full

What do you think?
Leave a comment.

What do you think? Leave a comment.

Jen Kamel

Jen Kamel is the founder of VBAC Facts, an educational, training and consulting firm. As a nationally recognized VBAC strategist and consumer advocate, she has been invited to present Grand Rounds at hospitals, served as an expert witness in a legal proceeding, and has traveled the country educating hundreds of professionals and highly motivated parents. She speaks at national conferences and has worked as a legislative consultant in various states focusing on midwifery legislation and regulations. She has testified multiple times in front of the California Medical Board and legislative committees on the importance of VBAC access and is a board member for the California Association of Midwives.

Learn more >

Free Report Reveals...

Parents pregnant after a cesarean face so much misinformation about VBAC. As a result, many who are good VBAC candidates are coerced into repeat cesareans. This free report provides quick clarity on 5 uterine rupture myths so you can tell fact from fiction and avoid the bait & switch.

VBAC Facts does not provide any medical advice and the information provided should not be so construed or used. Nothing provided by VBAC Facts is intended to replace the services of a qualified physician or midwife or to be a substitute for medical advice of a qualified physician or midwife. You should not rely on anything provided by VBAC Facts and you should consult a qualified health care professional in all matters relating to your health. Created By: Jen Kamel | Copyright 2017 VBAC Facts | Terms of Use | Privacy Policy