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The use (and abuse) of reference centiles with an application to weight gain in pregnancy.

Centile charts are commonly used in many areas of health research and practice, e.g. growth charts for children, Doppler ultrasonography in pregnancy and assessment of cholesterol levels at different ages. Yet there are a number of aspects of both their construction and application that are problematic and it is some of these issues that will be raised in this paper. The objective of the paper is to outline, in a non-technical way, some of the issues that need to be considered by the practitioner in estimating and using reference centile charts, but which frequently are either not known or ignored. These include: (i) the choice of reference population; (ii) how to estimate centiles; (iii) formally incorporating previous measurements on an individual, e.g. the interpretation of a child's weight that is on the 50th percentile for its age will be different if it has been moving along the 90th percentile at previous ages than if it has consistently been on the 50th percentile; and (iv) evaluation of centile charts used as a screen for problems. The concepts are introduced using an aspect of a study conducted at Tygerberg Hospital where centile charts for maternal weight gain in pregnancy were developed and assessed for their usefulness in detecting light-for-gestational-age (LiGA) births. The reference centile charts for maternal weight show poor discriminating ability between LiGA and normal births. These results support arguments in favour of abandoning the routine weighing of pregnant women.