Standard, Population & Customised fetal size charts 3 – Population/reference or healthy/standard?

Some of the confusion about fetal charts arises from the language used to describe the various non-customised sorts. It is confusing. “Population” and “reference” are different ways to describe one type of chart. “Healthy” or “standard” different names for the other type.

Population/reference charts

Older fetal growth charts were based on populations of more or less randomly selected pregnant women, including those whose pregnancy had, or was destined to have, problems of one sort or another. The idea was that the charts would “do what it said on the tin”; ten percent of fetuses below the 10^th centile, ten percent above the 90^th and so on.

But populations vary, so doctors often noticed that the charts were “wrong”, and derived a new set for their local population. Many hundreds of different charts were developed. In the UK the most popular was the Chitty chart* (click here), developed from measurements made on 663 consecutive women booking at Kings College Hospital in South London in the early 1990s. Such charts are also called reference charts because they “refer” to a particular population. However, I will use the term “population chart”.

As an aside, local population charts, for example for Pacific islanders, South Indian women, or for that matter “women booking in Kings College Hospital in South London in the early 1990s”, are a type of customised chart. So long as they are used for the population on which they were developed, they are “customised” for that particular population. But true customisation in the modern sense, which we will discuss tomorrow, is more sophisticated.

Healthy/standard charts

These are derived from healthy well-nourished populations excluding, as far as possible, problem pregnancies. The idea is that they provide a “standard” measure of growth in the absence of disease or nutritional constraints. I will use the term “standard” in what follows.

One of the controversies in creating standard charts is to decide what constitutes a healthy population. We can all agree that smokers, and women with raised blood pressure or anaemia should be excluded, but what about short women? Perhaps they are short because of malnutrition early in life. What about underweight, or overweight women? The details matter but the principle is simple. We want to measure fetal growth against other healthy pregnancies.

In theory any difference between standard charts and real world populations reflects the amount of disease and abnormal nutrition in the real world. The fact that in the UK for example more than ten percent of the population lies below the 10^th centile reflects the UK’s many smokers, whose babies are abnormally small. The fact that more than ten percent lie over the 90^th centile reflects our many overweight or diabetic women. The discordance is a “feature not a bug!”  Intergrowth-21 (click here) and WHO (click here) are standard charts.

Tomorrow customisation (click here).

Jim Thornton

* The eagle eyed will notice that Altman and Chitty were attempting to create a “standard” chart, and indeed they excluded some unhealthy pregnancies such as those with hypertension and diabetes.