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Standard, Population & Customised fetal size charts 4 – principles

September 6, 2019

Previous posts (click here and here) set the scene; how to get accurate scan measures, and the difference between reference and standard charts. Now the main event.

Customisation

This means creating a special chart based on one or more “custom” features of the individual being measured. Vets do it all the time – they couldn’t use the same chart for a Shire horse and a Shetland pony! Customisation makes sense for equines because even healthy foals differ in size.

But what about human smokers and non-smokers? Their fetuses  differ in size too, but customised charts for smokers would be bonkers. What’s the difference?

The principle is as follows. Charts are used to predict pathology, death, brain damage etc.

For factors unrelated to the pathology sought, customisation is appropriate. 

A Shetland pony is not a malnourished Shire horse, so it is good practice to have different charts to detect both malnourished Shire Horses and malnourished Shetland ponies.

But for factors lying on the pathway which leads to pathology (e.g. smoking is on the pathway to stillbirth) customisation is inappropriate. 

Note the choice of words, “lying on the pathway” rather than “causing”.  For this purpose, correlation is equivalent to causation; customisation on a measure which is simply correlated with pathology is also inappropriate. Consider social class. Babies of mothers in lower social classes tend to be smaller, but it is unlikely that social class affects size directly. More likely the effect is mediated via differences in diet, smoking or some other behaviour. But it would still be inappropriate to produce separate customised charts for different social classes.

Note two. Strictly the principle relates to the relative strength of the size/factor relationship to the outcome we wish to predict.  If the relationship between smoking and stillbirth is stronger than that between size and stillbirth, customisation on smoking would have the net effect of reducing the identification of babies who were destined to die.  If smoking was less strongly related there might theoretically be a way to customise charts by smoking status which improved detection of stillbirth. The latter might apply to maternal height (see later post).

In the next few posts we’ll look at some real factors for which doctors have suggested customisation, and see if they make sense. First race and ethnicity (click here).

Jim Thornton

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