Standard, Population & Customised fetal size charts 2 – technical scan stuff

Yesterday, in my first fetal size chart post (click here), I teased those doctors who customise children’s growth charts on the basis of US citizenship!  I will be more serious as I consider fetal growth charts, but first let’s get some technical issues out of the way. We can’t weigh, or measure babies directly in-utero. We have to scan them. And we have to do the scan right.

1. How to do the scans

As image quality improved, techniques changed. For example measuring the bi-parietal diameter (width) of the head from the proximal surface of the near skull bone to the proximal surface of the far one, used to be the standard technique. It always underestimated the actual measure, but was necessary because the distal surface of the far bone was indistinct. As image quality improved we can now easily see the outer surface of the distal skull bone, so we now measure “outer to outer”. But some old charts are based on the outdated method.

As outlining technology improved, calculating circumferences from two diameters measured at right angles was superseded by direct outline measures. But some old charts used the outdated method.

The need to create charts from correctly aligned scans showing the landmarks clearly, causes problems for population charts (more on these tomorrow) which unavoidably include overweight women whose scan views are often sub-optimal, but whose exclusion would distort the result. This is not a problem for the creators of standard charts because overweight women are excluded by definition.

2. How to make the actual measurements

If the ultrasonographer placing the measuring calipers can see the value he’s coming up with, he may unconsciously adjust the position to get a normal value, or report to the nearest whole or half millimetre.  The way to avoid such bias and digit preference is to get someone who doesn’t know the woman to place the calipers, ovoids or other boundary markers on a stored image, and only reveal the measurement after placement is judged correct. It’s akin to using a random zero sphygmomanometer to measure blood pressure. Easy in principle, but expensive in practice. Few, if any, of the older charts even attempted it.

3. How many pregnancies to study.

Smoothed growth charts can be produced with small samples. They look good, but by definition include few measurements at the upper and lower centiles, making these outer lines imprecise. Charts based on large samples typically used routinely collected scan data and suffer from the measurement biases described above.

In practice only two modern charts have taken measures using the modern techniques, avoided bias, and had a sufficiently large sample size to estimate the outer centiles with any degree of precision. These are Intergrowth-21 (click here) and WHO (click here). We will return to them. Tomorrow some more technical stuff, the difference between reference or population charts and healthy or standard charts (click here).

Jim Thornton