A paper published in 2019 helped me to find the language to capture something I was aware of in relation to CTG monitoring but had found difficult to put into words. Rogers, Entwistle and Carter’s thought-provoking paper was about one of the ethical issues that arises from the use of screening tests – that of overdiagnosis. What exactly is overdiagnosis? They explain:
Overdiagnosis is a counter‐intuitive phenomenon as detecting instances of disease is usually considered to be beneficial (given assumptions about the availability of effective treatments and the value of knowledge about prognoses). Overdiagnosis upends these assumptions as there are more harms than benefits from the diagnosis, often because the detected condition would not have progressed to advanced disease, thus there is no benefit to detection. Overdiagnosis is also counterfactual in that the diagnosis and any associated interventions cause harm on balance because people’s overall health states and experiences are worse than they would otherwise have been without the diagnostic intervention.
Rogers, et al., 2019, p. 237.
One way of thinking about what CTGs do is to consider them as a diagnostic tool, looking for a condition called “fetal distress”. Let me give you some examples to make this clearer.
- Alex opts to have intermittent auscultation during her labour. She has a vaginal birth. The baby is born in good condition at 11 am.
In an alternate universe, Alex made different choices…
- Alex opts for CTG monitoring during her labour. Alex’s CTG recording is interpreted as abnormal, and the obstetrician tells Alex her fetus is “distressed”. She accepts their recommendation for a caesarean section. The baby is born in good condition at 10 am. Alex is grateful to her obstetrician and believes CTG monitoring saved her baby’s life.
There was no way for Alex number two, her family, or even her obstetrician and midwife to know it, but she has been overdiagnosed. Her caesarean section was not strictly necessary as, in her case, her fetus was never at risk for harm.
The thing with overdiagnosis is you don’t know when it happens to you…
As we can see from Alex’s alternate timelines, she and her family have no way of knowing that Alex number two would have been no worse off if she didn’t say yes to CTG monitoring. Unlike Dr Who, we can’t drop into our alternative future timelines to see what the consequences of our choices are.
It is well known that the sensitivity of CTG monitoring is poor. That is, when the CTG is considered as abnormal, the possibility that the fetus is actually at risk of harm and will therefore benefit from being born sooner is really low (astonishingly gobsmackingly low actually). Calculating the sensitivity of CTG monitoring is quite tricky as it depends on what you count as an abnormal CTG, and which outcome you are looking at. Nelson and co-authors (1996) calculated that for every 1000 fetuses exhibiting multiple late decelerations, or decreased beat-to-beat variability of the fetal heart rate, or both, then 1.9 would go on to develop cerebral palsy if corrective action were not taken. In other words, the sensitivity of CTG monitoring is 0.19%, giving a false positive rate of 99.8%. Stillbirth during labour is one fifth as common as cerebral palsy so the sensitivity for detecting heart rate patterns that predict the risk for this is lower still.
For every 1000 women who are told something must be done because the CTG is very abnormal, there are potentially two whose baby might avoid cerebral palsy. The other 998 were overdiagnosed, and this group will include women and babies who were no worse off, but also some who experienced complications which could have been avoided if the CTG had not been used. But I would bet almost all of those 998 women, and their care providers, will probably consider CTG monitoring as a life saving and risk reducing technology. Some of them will buy chocolates for the staff to show their gratitude, and one of them might even become health minister one day and fund the purchase of more CTG machines and CTG education programs.
Why CTG monitoring seems to work
Overdiagnosis is therefore one of the reasons why the myth that CTG monitoring is effective is so difficult to shift from our collective consciousness. The irony is that if CTGs didn’t overdiagnose so often, fewer people would believe their lives were made better because of CTG monitoring.
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References
Nelson, K., Dambrosia, J., Ting, T., Grether, J. (1996). Uncertain value of electronic fetal monitoring in predicting cerebral palsy. New England Journal of Medicine, 334(10), 613 – 619. https://dx.doi.org/10.1056/nejm199603073341001
Rogers, W. A., Entwistle, V. A., & Carter, S. M. (2019). Risk, overdiagnosis and ethical justifications. Health Care Analysis, 27(4), 231-248. doi:10.1007/s10728-019-00369-7
Categories: CTG, EFM, Reflections
Tags: caesarean section, Cerebral palsy, Dr Who, False positive, Overdiagnosis, sensitivity
Birth Small Talk 
Many thanks for this review. As a Midwifery educator it is so hard to change the deeply ingrained practice of ‘routine CTGs’
Many thanks
Meredith
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Thanks Meredith – education is a major part of the solution.
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Hi Kirsten, overdiagnosis researcher here – great and easy read, just wanted to let you know that I am referring to this blogpost in an upcomming chapter on overdiagnosis in birthing – thanks for your insights into the CTG iatrogenic harms (yes, I’m also using the cochrane review etc.) Cheers and keep up the good work!
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Thank you! I’m glad you find it helpful.
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