A growth chart reflects the size and growth of its reference population. When the target population is materially different in size, as happens with ethnic minorities in the industrialized world (e.g., Hispanics in the United States or Asians in the United Kingdom), the chart's centiles can be misleading. More than 3% of such children may be found below the 3rd height centile, and the relevant cutoff for referral should take ethnicity into account. One simple way to do this is to estimate an offset, measured in SD score units, to apply to the chart for a given ethnic minority group. For example, Southeast Asian children in the United Kingdom are approximately 0.4 SDs shorter than ethnic Caucasians, corresponding to approximately half a channel width on the chart, the distance between adjacent centiles.
The differences in height between ethnic groups can be explained partly by genetics (i.e., differences in parental height) and partly by differences in the environment. However, the parental height differences reflect the environment of previous generations, so it is not feasible to ascribe the differences purely to genes or the environment—the two are inextricably linked. If the child's height is appropriate for the heights of his or her parents, this should provide reassurance.
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