Procedures for Accurate Measurement

Accurate measurement is a critical component of growth assessment. Weight, in pounds or kilograms, must be determined using an accurate scale. For infants and toddlers, weight, length, and head circumference are obtained. These measures should be performed with the infant naked, and ideally, repeated measures should be performed on the same equipment. Head circumference is determined using a flexible tape measure run from the supraorbital ridge to the occiput in the path that leads to the largest possible measurement. Length is most accurately measured by two examiners (one to position the child), with the child supine on a measuring board. For older children, the measure is stature or height, taken without shoes, using a stadiometer. Measurements obtained in alternative manners, such as marking examination paper at the foot and head of a supine infant, or using a simple wall growth chart with a book or ruler on the head can lead to inaccuracy that may render the measurement useless. It is essential to compare measurements with previous growth trends, repeat any that are inconsistent, and plot results longitudinally.

Derivation and Interpretation of Growth Charts

In 2000, the Centers for Disease Control and Prevention (CDC) published new growth charts, replacing the 1977 version. Modifications since then have not changed the data points. Set 1 includes the 5th to 95th percentiles; set 2, the 3rd to 97th percentiles. These charts contain data from national surveys conducted by the National Center for Health Statistics between 1963 and 1994. Data are representative of the U.S. population, both demographically and in terms of breast-feeding prevalence. Methodological steps have assured that the increase in the prevalence of obesity has not unduly raised the upper limits of normal. Several deficiencies of the older charts have been corrected, such as the over-representation of bottle-fed infants and the reliance on a local data set for the infant charts. The disjunction between length and height, when moving from the infant curves to those for older children, no longer exists. The charts include curves for plotting BMI for ages 2-20 yr rather than weight for height, facilitating identification of obesity.

The data are presented in 5 standard gender-specific charts: (1) weight for age; (2) height (length and stature) for age; (3) head circumference for age; (4) weight for height (length and stature) for infants; and (5) BMI for age for children over 2 yr of age (Fig. 13-1; also see Figs. 9-1 and 9-2). The charts are available at www.cdc.gov/growthcharts/.

Each chart is composed of percentile curves, representing the cross-sectional distribution of weight, length, stature, head circumference, weight for length, or BMI at each age. The percentile curve indicates the percentage of children at a given age on the x-axis whose measured value falls below the corresponding value on the y-axis. On the weight chart for boys 0-36 mo of age (see Fig. 9-1A), the 9 mo age line intersects the 25th percentile curve at 8.6 kg, indicating that 25% of the 9 mo old boys in the National Center for Health Statistics sample weigh less than 8.6 kg (75% weigh more). Similarly, a 9 mo old boy weighing more than 11.2 kg is heavier than 95% of his peers. The median or 50th percentile is also termed the standard value, in the sense that the standard height for a 7 mo old girl is 67 cm (see Fig. 9-1B). The weight-for-height charts (see Fig. 9-2) are constructed in an analogous fashion, with length or stature in place of age on the x-axis; the median or standard weight for a girl measuring 110 cm is 18.6 kg.

For infants, the revised charts represent observed but not necessarily optimal growth because they still incorporate data from many bottle-fed infants. Rates of initiation of breast-feeding in the USA have more than doubled from 26% in 1970 to 74% in 2005, but only 12% of infants are exclusively breast-fed for 6 mo and only 21% of infants receive breast milk for a yr. Compared with current standards, an exclusively breast-fed infant would be expected to plot higher for weight in the first 6 mo, but relatively lower in the second half of the 1st yr. Awareness of this growth difference should prevent overidentification of growth problems in breast-fed infants.

In an effort to set an internationally usable standard for optimal growth in young children, in 2006 the World Health Organization released growth charts based on the Multicenter Growth Reference Study (MGRS). Rather than describing the growth of typical children, the MGRS describes the growth of children who are predominantly breast-fed and raised under optimal conditions. Six study sites representing 5 continents were included: USA, Brazil, Norway, Ghana, Oman, and India. Use of the new charts in developing nations results in identification of many more children as malnourished and eligible for therapeutic feeding programs. Their use in the USA results in many fewer infants being identified as underweight (comparison of curves shown in Fig. 13-2). Charts are available online at www.who.int/childgrowth/standards/en/.

Figure 13-2 Comparison of the WHO and CDC length/height-for-age z score curves for boys.

(From de Onis M, Garza C, Onyango AW, et al: Comparison of the WHO child growth standards and the CDC 2000 growth charts, J Nutr 137[1]:144–148, 2007.)

For adolescents, caution must be used in applying cross-sectional charts. Growth during adolescence is linked temporally to the onset of puberty, which varies widely. By using cross-sectional data based on chronological age, the charts combine youth who are at different stages of maturation. Normal variations in the timing of the growth spurt can lead to misdiagnosis of growth abnormalities. The data for 12 yr old boys include both early-maturing boys who are at the peak of their growth spurts and late-maturing ones who are still growing at their prepubertal rate. The net result is to artificially level off the growth peak, making it appear that adolescents grow more gradually and for a longer period than they do. When additional precision is necessary, growth charts derived from longitudinal data, such as the height velocity charts of Tanner and colleagues, are recommended.