In infancy, the most common routine measurement is weight. It is simple to do, the required equipment is reasonably cheap, and it provides a convenient global summary of the infant's size. Birth weight in particular is a useful proxy for fetal growth. An advantage of weight is that it relates closely to the mother's own perception of her child's size.

Infant length is more difficult to measure for several reasons. The optimal equipment is a length board with a sliding footboard, which is expensive and needs regular calibration. Simpler equipment such as a tape measure increases the measurement error dramatically. Most important, proper length measurement requires two trained observers—one to hold the infant's head against the headboard and the other to position the footboard and take the measurement. For these reasons, infant length is often measured either poorly or not at all.

Arm circumference (or mid-upper arm circumference (MUAC)) is a popular alternative to weight in the developing world but less so in the industrialized world. This is because arm circumference measurement can detect malnutrition using a simple cutoff. The equipment (a specially marked inextensible tape) is cheaper and easier to use than weighing scales, and arm circumference is highly correlated with weight.

In the industrialized world, head circumference measured in infancy can detect some rare conditions such as hydrocephalus, which is indicated by a rapid increase in head circumference at approximately the time of birth.

Once past infancy, priorities change. Height becomes much more important, particularly in the industrialized world, where the emphasis is on detecting primary growth disorders such as growth hormone deficiency or Turner's syndrome. Children can be measured standing at approximately 2 years of age using a freestanding stadiometer with counterbalanced headboard. The child's head is positioned in the Frankfort plane (looking straight ahead with the line between the ear hole and eye horizontal), with the child's shoulders, buttocks, and heels touching the back plate, and the observer brings the headboard down gently and reads the height. Alternatively, a second observer can take the measurement while the first checks the child's position. Measurement technique is crucial for height, particularly when it is measured repeatedly. Height velocity is relatively low after infancy, so the height increment over a period of 6 months, for example, may be only 2 or 3 cm depending on age, which can result in excessive measurement error. A competent observer should be able to achieve a measurement error of less than 0.3 cm.

Weight and height are highly correlated, so their assessments are often similar: On average, a tall child is heavy and a short child is light. Once past infancy, weight can be more informative when expressed as an index of weight adjusted for height. There are many weight-for-height indices, but among the most common in the industrialized world is the body mass index, calculated as weight (measured in kilograms) divided by the square of height (in meters). It has been used in adults for decades (the index was originally proposed by Que-telet in the nineteenth century) but in children only relatively recently, and mainly in the area of child obesity. Note that it requires adjustment for age, in the same way as for weight and height. Other weight-for-height indices, used mainly in the developing world, adjust weight for height ignoring the child's age, which is an advantage when the age is not known. However, this leads to biases at certain ages, notably infancy and puberty, when a child's expected weight depends on their age as well as their height.

With the recent steep increase in the prevalence of child obesity, waist circumference has become useful as a measure of central body fatness. Body mass index is less useful because it does not distinguish between fat mass and muscle mass. A child may become fatter over time without becoming heavier simply by losing muscle mass (through inactivity)

Table 1 The suitability of anthropometry for growth monitoring at different agesa

Anthropometry Infancy Preschool Childhood and adolescence

Weight Length/height Body mass index Arm circumference Head circumference Waist circumference aMore asterisks indicate better suitability.

and gaining an equal mass of fat, as occurred in US adolescents during the 1980s. It is easier to measure waist circumference than skinfold thickness (e.g., triceps or subscapular skinfold), and the required equipment is also simpler—an inextensible tape as opposed to a skinfold caliper (the appearance of which often frightens parents and young children).

Table 1 summarizes the value of anthropometry at various stages of childhood, as described previously. The process of anthropometry requires attention to detail: suitable equipment that is regularly maintained and calibrated; observers who are trained in correct measurement technique; and regular quality control sessions in which observers are checked, against both themselves and each other, for measurement precision and accuracy. Only in this way can accuracy and precision be maintained.

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