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Figure 1 The British 1990 boys infant weight chart, covering 22 weeks of gestation to 12 months. The nine centiles range from 0.4th to the 99.6th, and infants whose weights fall outside the extreme centiles are referred. (© Child Growth Foundation.)

Figure 1 The British 1990 boys infant weight chart, covering 22 weeks of gestation to 12 months. The nine centiles range from 0.4th to the 99.6th, and infants whose weights fall outside the extreme centiles are referred. (© Child Growth Foundation.)

the score is 0. The 2nd centile is approximately 2SDs below the median at all ages, whereas the 75th centile is 1.3 SDs above the median. Therefore, the 2nd centile corresponds to an SD score of —2 and the 75th centile to +1.3.

Height is normally distributed, but much anthropometry including weight is not—it has a skew distribution. The link between centiles and SD scores can be extended to measurements that are not normally distributed using a technique called the LMS method, which is the basis for many growth reference charts. The telltale sign of a skewness-adjusted chart is that the centile curves are asymmetrically spaced at each age; for example, the gap between the 25th and 50th centile curves is less than the 50th-75th centile gap.

The British growth reference centiles (Figure 1) are defined by their corresponding SD scores. The centiles are spaced two-thirds of an SD score apart, so the 0.4th centile corresponds to — 2.67 SD scores. Similarly, the WHO chart spaces its centiles 1 SD score apart, from —3 to +3, which allows it to cover the wide range of anthropometry seen internationally.

Charts for use in the developing world tend to be simpler in design, often with only two or three centile curves. They highlight a region on the chart where children's individual growth curves should lie, and mothers are taught that children with curves within this region are healthy children. One common chart is the Road to Health chart (Figure 2). The advantage of the chart is that it can also include public health information about the timing of breast feeding, immunizations, etc.

The most difficult part of the measurement process is plotting the data accurately. For use in the developing world, the Direct Reading scale is a clever device that links the weighing scale to the chart so that as the child is weighed a pen records the scale deflection on the chart. This and the Road to Health chart can be obtained via Teaching Aids at Low Cost (TALC).

Another direct-reading chart is the Nabarro height-for-weight wall chart (also available through TALC). The child is first weighed, and then he or she stands against the chart at the point corresponding to his or her weight. The chart consists of

Figure 2 The TALC Road to Health chart. (Reproduced by kind permission of Teaching-Aids at Low Cost.)

vertical bars, and the height of the relevant bar is compared to the child's height. If the child is thin, he or she will be taller than other children of the same weight, and the upper section of the bar is color coded in red to flag excessive thinness, also known as wasting.

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