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.
* 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
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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 a hospital, served as an expert witness in a legal proceeding, and has traveled the country educating hundreds of professionals and highly motivated parents. Even more have accessed her trainings online. 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 the Secretary for the California Association of Midwives and the California Association of Licensed Midwives. Her favorite flavor of ice cream is peanut butter chocolate. And mint chip. And coffee.