Standard, Population & Customised fetal size charts 16 – summary

That’s it folks. Let’s summarise.

Standard growth charts tell you how big a healthy fetus from a healthy pregnancy and healthy mother should be. Population charts plot the average for your local population, including unhealthy pregnancies (click here). Customised charts adjust the size on the basis of individual parental features (click here).

Customisation improves detection of pathology if we can be sure that the feature customised on, is not only objective and reproducible, but also not associated with pathology (click here).  Customisation on factors associated with pathology, such as smoking, condemn the small sick fetus damaged by smoking, to being classified as “normal for smoking”. Fortunately no-one customises on maternal smoking.

But some people do customise on maternal ethnicity, weight, height and parity. Ethnicity is not objective (click here) and is associated with pathology (click here). Maternal weight is objective, but at both extremes is associated with pathology (click here). Maternal height is objective but also associated with pathology, albeit only at the lower end (click here). Parity is objective but also associated with pathology (click here).

Fetal sex is objective and negatively associated with pathology, with larger male fetuses having higher mortality (click here). This should make it an ideal factor on which to customise, and is the reason why, after birth, paediatricians routinely use separate charts for boys and girls. But it’s not suitable for routine prenatal use at present.

The main customised charts available in the UK are produced by The Perinatal Institute (click here). They provide customised charts for fundal height and for estimated fetal weight. Their training programmes for measuring fundal height are excellent, but the fetal weight estimates on which those charts are based use Hadlock’s outdated formulae. Their customisation formulae for both fundal height and fetal weight are secret. Their charts customise on maternal ethnicity, weight, height and parity, and are therefore likely to condemn the fetuses of first pregnancies and of mothers whose ethnicity, weight and height are markers of past and present deprivation, to having pathology missed.

Fetal growth charts should be created on large populations, using careful techniques to avoid bias (click here). Only the two standard charts created by Intergrowth-21 (click here) and by WHO (click here) have done this.  A well-publicised academic dispute (click here) between the Intergrowth-21 and WHO authors, involved accusations of plagiarism, but had nothing to with the science. Both charts are scientifically rigorous. However, the Intergrowth-21 sample was twice the size of WHO’s, and that group used slightly more advanced techniques for avoiding bias, so their chart is to be preferred.

Claims that the introduction of customised charts caused the recent fall in UK stillbirth rates do not bear close scrutiny (click here).

Suggestions that standard charts are flawed because they detect different rates of growth restriction or macrosomia in different populations make no sense; that is a feature of growth standards.

Empirical comparisons (click here) between customised and population charts are unhelpful because they compare two type of customised chart. All empirical studies suggesting that customised charts detect more pathology than standard charts have also reported higher false positive rates with customisation. No empirical study has ever shown greater detection rates of pathology for a fixed false positive rate. The best quality empirical study (the POP study from Cambridge), where results were also concealed from clinicians, showed no difference with customisation.

Population and customised charts, as currently available, cannot be recommended.

The Intergrowth-21 or WHO fetal growth standard charts should be used.

Jim Thornton

For an addendum on customised twin charts (click here), and for thoughts on the DESIGN trial (click here).