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Standard, Population & Customised fetal size charts 5 – race ethnicity

September 6, 2019

At first sight customisation by maternal (or paternal) race or ethnic group appears sensible. Han Chinese women, for example, tend to be smaller than say Swedish women, and have smaller babies. Surely we should plot their baby’s growth on different charts.

But how can we be sure that the ethnic “differences” we observe are not on, or correlated with, the pathway to the pathology we are trying to identify?

Consider the two ethnic groups above, who appear to differ in size, and have had relatively little historical intermingling, Northern Europeans, and the Han Chinese.  Northern European women and their babies are taller, and larger than Han Chinese.  But perinatal mortality is also higher in the Han Chinese.

Both differences are probably at least partly caused by under nutrition among the Han Chinese ancestors who would have lived through two of the worst famines in recent human history, Mao Tse Tung’s Great Leap Forward, and Cultural Revolution. Both will certainly be affected by present day differences in nutrition, and smoking and drinking habits.  If so, the observation that more than n% of Chinese babies fall below the nth centile of Western European charts, is a sign that such babies are genuinely failing to reach their full growth potential, rather than that the charts are wrong.

It doesn’t matter if race or ethnicity causes the adverse pregnancy outcomes, or is just correlated, the fact that there is a correlation should make us think twice about customising on either. Although even a strong correlation in itself doesn’t prove it wrong. If race was less strongly correlated with adverse outcomes than size, customisation on race, at least in theory, might still work.

Tomorrow (click here) we’ll see some practical reasons why successful customisatiom by race or ethnicity is in reality a hopeless quest.

Jim Thornton

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