Prevalence-induced concept change is an interesting psychological phenomenon (Levari et al., 2018). What exactly is it? And what on earth does it have to do with CTGs?
What is it?
Prevalence-induced concept change is a form of bracket creep. The meaning of a concept gradually changes over time. The driver for this change in meaning is when the events captured by the original concept have become less frequent over time. Levari and colleagues explored this in a series of studies. One of the studies asked participants to look at 1,000 dots and indicate which dots were blue. The dots ranged on a colour gradient from very blue to very purple. After showing them a series of images to establish a baseline blue dot count, participants were then shown either a series of images with the same proportion of very blue dots, or a series where the proportion of very blue dots decreased over time.
Participants shown the same proportion of very blue dots remained consistent over time, each time they were shown a new image of dots. But participants who where shown the images with fewer and fewer blue dots did not, as you might expect, find fewer blue dots each time. Instead, what happened was the participants labelled about the same number of dots as blue while selecting increasingly purplish dots from the image. The concept of what blue was (in terms of the activity they were taking part in) shifted as blue appeared less often. This is prevalence-induced concept change.
You might think that, yeah well, that’s interesting and all but has nothing to do with the real world. The same research team explored this further by showing participants a collection of research proposals. Some were seriously ethically dubious, ranging through to others that were ethically fabulous. As the prevalence of ethically dubious trails was reduced over time, participants started to reject ethically not-too-bad research proposals, keeping the balance between rejections and acceptances at about the same number as they originally started with. You can see the potential impact this could have on a real life ethics committee as the quality of the research proposals they are asked to consider gets better over time.
What does this have to do with CTG use?
Right from the start, CTG use was favoured for women considered to be high risk for problems relating to low oxygen levels in labour. For example, in 1971 Beard et al. wrote about their experience of using CTG monitoring with “high risk cases”. During the one year period of their study, the perinatal mortality rate for the overall population was 21 per 1,000 births, and the proportion of women considered to be high risk was 19.7%.
Let’s travel forward to 1985, and to the Dublin trial (MacDonald et al.). The perinatal mortality rate for the overall population was similar, at 22 per 1,000 births, with 22.5% of the women considered to be high risk. So we can see there’s a degree of stability in the concept of high risk, relating to stability in the prevalence of perinatal mortality.
Perinatal mortality has reduced in the past 40 years (for reasons that almost certainly have nothing to do with fetal monitoring practices). In Australia in 2022 (the most recent reported year), the Australian Institute of Health and Welfare noted a perinatal mortality rate of 10 per ,1000 births. The risk of experiencing the death of a fetus or baby is half what it was in the two research papers referred to above. This is the equivalent of presenting people with 50% fewer really blue dots.
So what impact has this reduction in prevalence had on the proportion of women who are told they are high risk? Is it still about 20% of the population of birthing women? Clearly, that would be no. Finding this number in an official document is challenging and I’m yet to nail one down (let me know if you know of a source). When I was collecting data for my PhD, 90% of women in birth suite in the high risk category. When I talk to others in different parts of the world, that seems to be their experience too.
So what I’m seeing is a form of prevalence-induce concept change around what “high risk” means in relation to the use of CTG monitoring. This is the equivalent of more and more purple-ish dots being told they are actually blue dots, and being managed with interventions aimed at helping blue dots (but never proven to be useful for purple-ish dots). There is no current agreed standard about how high risk someone needs to be to be classed as high risk (how blue is blue?). What gets chosen to go on the list of risk factors and what doesn’t is prone to cultural bias, and this makes it easily susceptible to prevalence-induced concept change.
I’d love to know how the numbers work out in your local hospital. Drop a message in the comments box and let me know what percent of women are considered high-risk and what the perinatal mortality rate is. It would make a really interesting bit of research to map perinatal mortality rates against the proportion of women labelled high risk – hit me up if you are interested in making this your research project!
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References
Beard, R. W., Brudenell, J. M., Feroze, R. M., & Clayton, S. G. (1971). Intensive Care of the High Risk Fetus in Labour. BJOG: An International Journal of Obstetrics and Gynaecology, 78(10), 882-893. https://doi.org/10.1111/j.1471-0528.1971.tb00199.x
Levari, D., Gilbert, D., Wilson, T, Sievers, B., Amodio, D., & Wheatley T. (2018). Prevalence-induced concept change in human judgement. Science, 360, 1465-1467. https://www.science.org/doi/10.1126/science.aap8731
MacDonald, D., Grant, A., Sheridan-Pereira, M., Boylan, P. C., & Chalmers, I. (1985, Jul 01). The Dublin randomized controlled trial of intrapartum fetal heart rate monitoring. American Journal of Obstetrics & Gynecology, 152(5), 524-539. https://www.ncbi.nlm.nih.gov/pubmed/3893132
Categories: CTG, EFM, Philosophy, Reflections
Tags: Blue dots, Bracket creep, high risk, Prevalence-induced concept change, Psychology
Birth Small Talk 
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Thanks for sharing Beth
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