 Interpreting Growth and Growth Standards . . . page 1 Donna Secker MSc, RD Clinical Dietitian The Hospital for Sick Children, Toronto Traditionally, growth assessment has been the major screening tool for defining the health and nutritional status of infants and children. This is because disturbances in health and nutrition, regardless of their etiology, almost always affect growth. Growth in Infancy There are three distinct stages of growth: early, rapid growth during infancy; the relatively static pattern of growth of toddlers and school-aged children; and the teenage growth spurt. Growth in the first year of life is the most dramatic because it occurs rapidly and in unison with noticeable developmental changes. After an initial fluid loss of approximately 6% of body weight, a newborn infant generally regains his/her birth weight by 10 to 14 days. Weight usually doubles by 4 to 5 months, triples by 1 year, and quadruples by the end of the second year.(1) Length typically changes more slowly than weight, increasing from birth by 50% by 1 year of age and by 75% by 2 years of age. By the end of the second year infants have reached approximately half of their adult height. Studies have shown that length at birth does not correlate well to eventual adult height, but relates best to maternal size. The influence of the paternal height gradually increases until by 2 years of age the infant's length correlates best to mean parental height. Factors Affecting Growth Size at birth is the product of duration of gestation and rate of fetal growth, and is an important indicator of fetal and neonatal health. Postnatal growth is a complex process affected by many inter-related genetic, environmental and nutritional factors including the nutritional status and health of the mother, and the adequacy of the infant's diet. Socioeconomic status, and cultural and biological factors such as the infant's gender, birth weight, and birth order, also influence growth. It is hard to know how much of the differences in weight and growth of infants would remain if they all lived in a similar environment and received the same optimal nutrition and care. Measuring Growth The most commonly used anthropometric measures for infants are length, weight, and head circumference. These measures are non-invasive, inexpensive, and easy to perform. Length and weight measurements can then be used to derive indices of growth such as length - for- age, weight - for - age, and weight - for - length. Weight is often used as the major parameter of growth, particularly during infancy; however, to evaluate growth accurately, weight should be considered in relation to length. In general, inadequate nutrition affects weight first, but prolonged and severe undernutrition eventually affects linear growth as well . In the clinical setting, measurement of growth is important in detecting and treating chronic illness (e.g. failure - to - thrive, Crohn's disease, renal disease) and disorders of growth (e.g. hypothyroidism , growth hormone deficiency ) . Growth Charts Once reliable measurements are obtained, the interpretation or 'clinical meaning' of the measurements is dependent on their comparison with reference data from normal populations or, when available, with condition-specific reference data. Comparison is made by plotting an individual infant's measurements on a growth chart. Comparison of single, one-time measures of length and weight gives information as to where that infant ranks relative to other infants of the same age and gender (i.e. their size). Plotting serial measurements over time provides information about the infant's pattern of growth, regardless of whether the measurements fall below, between, or above the lower or upper percentiles on the chart. Accurate plotting of age is essential(2). Appropriateness of weight in relation to length can be estimated by expressing an infant's current weight as a percentage of his/her ideal weight for length. Using the growth chart, ideal weight for length is identified by finding the weight which is approximately on the same percentile as the infant's length measurement. For infants whose length is below the bottom centile line, or above the highest centile line on the growth chart, ideal body weight is determined from the infant's height age. Height age is the age at which the infant's height (length) would be on the 50th percentile. The infant's ideal weight for length would be the corresponding weight at the 50th percentile for that height age. Height age is not a perfect indice for determining ideal body weight because using the 50th percentile assumes that the infant has the potential to reach average length/height; genetically, this may not be feasible for that individual infant. Recognizing this deficiency, height age remains a useful tool to help the practitioner identify an ideal weight for the infant whose length falls outside of the growth curves. Current weight expressed as a percentage of ideal weight for length can be used to identify infant's at risk for under- or over- nutrition : The Design of Growth Charts Growth charts are constructed from either longitudinal or cross sectional data which is used to form a series of smooth curves illustrating chosen percentiles of the distribution of the measurement of interest (e.g. height, weight, head circumference) in the reference population, plotted against age. The centiles are symmetrical above and below the median (50th centile) curve. A child of average height and average timing of pubertal growth spurt would follow the 50th percentile throughout life. There are 3 major sets of growth charts most commonly used around the world: British / European (Tanner & Whitehouse), North American (National Centre for Health Statistics) and the World Health Organization International Reference charts. Aside from the 50th centile, the other centiles used on the charts have not been standardized. Until recently, European charts have used the 3rd, 10th, and 25th centiles below the median and the 75th, 90th and 97th above. The National Centre for Health Statistics (NCHS) chart uses the 5th and 95th centiles instead of the 3rd and 97th. The WHO's international reference does not use centiles at all; instead, it makes use of standard deviation scores (SDS; also known as z scores), which can be converted to centiles if the measurement is normally distributed(3). The WHO curves are set at -3-, -2 and -1 SDS below the median, and 1, 2 and 3 SDS above. These correspond to the 0.14th,2.3rd, 16th, 84th, 97.7th and 99.9th centiles respectively. These different approaches to the choice of centiles are partly due to differing requirements for identifying children with poor growth. In Europe and North America, the vast majority of children lie between the 3rd and 97th centiles, whereas in the developing world, where the WHO reference is used, many children lie below the 1st centile, which makes classification based on centiles useless. Because a cutoff that includes more children than can be seen or evaluated is not useful, the goal should be to select a criterion that is practical but not so severe that it would exclude children who may go on to develop more serious problems if we re not identified and treated. In Europe and North America, children below the 3rd or 5th centile, respectively, a reconsidered for referral, but relatively few get referred because the false positive rate is unacceptably high. Cole(4) suggested that a much lower centile than the 3rd or 5th centile is needed as a more realistic cut off for referral in developed countries. Recently, new growth charts for children in the UK have been released to better reflect the current growth of children in the UK, and to utilize a lower cutoff centile(5). These charts utilize the 0.4th, 2nd, 9th, 25th, 50th, 75th, 91st, 98th and 99.6th centiles and they replace the old Tanner- Whitehouse charts. |
