Charts to Measure Growth

Growth charts are usually constructed using cross-sectional data—each reference child contributes a single measurement to the data set. This allows children in each age group to be ranked by size to identify the required centiles, which can then be plotted against age. James Tanner coined the term ''distance to indicate size attained,'' meaning the distance the child has travelled on the journey from conceptus to adult.

Growth is the rate of change of size, or velocity in Tanner's notation. To measure growth in an individual child requires at least two measurements separated in time. However, the growth chart is based on cross-sectional data, which provide no information about growth. This is an irony of the conventional growth chart: It is designed to measure size, not growth. It flags poor growth when the child's growth curve rises more slowly than the centile curve, but it does not distinguish between mild and severe faltering. The chart effectively has only three growth categories: normal (i.e., tracking along cen-tiles as recommended by the Road to Health chart), above average growth (i.e., crossing centile curves upwards), and below average growth or crossing centiles downwards. Within the latter two categories, the chart does not grade the rate of centile crossing.

Tanner introduced notation to distinguish between distance or size charts, on the one hand, and velocity or growth charts, on the other hand. Velocity charts display centiles of growth velocity by age, and probably the most useful such chart is for height velocity measured over 1 year. In theory, velocity is better than distance for detecting short-term growth faltering, but in practice it is more difficult to measure because it involves two measurement errors, not one. It is also more complicated to monitor because the process of charting velocity involves taking the two measurements, calculating the velocity in units of cm/year or kg/year, and then plotting it at the mid-age point on the velocity chart.

Other forms of charts have been described that provide more information about growth velocity. Thrive lines are extra lines superimposed on the cen-tile chart to quantify the rate of centile crossing of weight in infancy. Infants with monthly weight measurements who are growing on the 5th velocity centile track along the thrive lines. If they track in this way for 1 month it is a sign of moderate weight faltering, but if it continues then the faltering is progressively more severe. The thrive lines take into account the child's age and sex and adjust for regression to the mean. Figure 3 illustrates the thrive lines superimposed on the weight chart of Figure 1. An infant measured twice 4 weeks apart whose weight curve tracks along the thrive lines (i.e., crosses centiles downwards) is growing on the 5th velocity centile, indicating moderate weight faltering.

Wright designed a weight monitoring chart in which the centile curves are spaced according to the infant's chance of crossing them in a given period of time. Large infants tend to cross centiles downwards more rapidly than small infants—this is a consequence of regression to the mean. Therefore, the centiles are relatively widely spaced at the top end and become progressively closer together at lower centiles. The chart is designed to simplify the assessment of infants recovering from failure to thrive.

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