 Interpreting Growth and Growth Standards . . . page 2 Choice of reference values Current evidence does not show large genetic differences in birth weight among various populations and therefore does not support the use of separate, race - specific reference curves, even in situations where race is associated with other risk factors, such as poor nutrition or low socioeconomic status. The issue of which reference population to use in assessing the adequacy of growth during childhood has received considerable attention in the last decades. The WHO adopted the reference curve s of the NCHS(6)for international use, based on the growing evidence that growth patterns of well-fed, healthy preschool children from diverse ethnic backgrounds were surprisingly similar. Data for the NCHS charts are a composite from two American sources. For children younger than 2 years of age, longitudinal data from the Fels Research Institute in Yellow Springs, OH, were used. These data were collected between 1929 and 1975 and included 867 infants from a white, predominantly middle-class population. For older children, cross-sectional data we re used from three nationally representative surveys: the NCHS Health Examination Survey cycles II (1963-1965) and III (1966-1970) and the subsequent Health and Nutrition Examination Survey cycle I (1971-1974). This data set covered large numbers of children, approximately 1,000 for each age group and included all ethnic groups and social classes. Concern has been expressed that important technical limitations in the NCHS reference data complicate their use in interpreting growth data(7):
These changes in infant feeding practices, together with reports of altered growth patterns in breastfed infants, have sparked new interest and controversy regarding the appropriateness of the currently used NCHS growth data for infants who are predominantly breastfed. A WHO Working Group on Infant Growth recently combined data from 7 longitudinal studies in Canada, Northern Europe and the US to examine: 1) the consistency in growth patterns of breastfed infants in different geographic areas; 2) the influence of the duration of breastfeeding, supplementation with infant formula or other milks, and the age of introduction of solid foods on growth ; and 3) the magnitude of the discrepancy between growth of breastfed infants and the current WHO reference data(8). Their pooled analysis demonstrated that growth patterns of breastfed infants are similar across different populations from Europe and North America but differ markedly from current WHO reference data and appear to be faltering on the reference curves. This deviation from the national reference data can lead to excess monitoring and testing of infants, as well as unnecessary parental concern and anxiety. Although it is apparent that infants who are breastfed gain weight at a slower rate than infants who are formula-fed, it remains to be established which growth pattern is normal or optimal. Evidence indicates that the slower weight gain of breastfed infants is not associated with any harmful functional outcomes in activity, behavioural development or morbidity. In view of the significant technical drawbacks of the current NCHS/WHO reference, an Expert Committee convened by the WHO to reevaluate the use of anthropometry and reference data has recommended the development of a new reference for weight and length/height for infants and children.(7) The creation of such reference data, with representation of a wider range of ethnic backgrounds and parental stature, is currently underway. The intention is for the new reference to reflect patterns consistent with WHO feeding recommendations (i.e. exclusive breastfeeding from birth to 4 to 6 months of age, with continued breastfeeding, while receiving appropriate and adequate complementary foods, up to 2 years of age or beyond ). Other growth standards Velocity charts Growth data can be represented in a different way by converting longitudinal data to height velocity in cm/year or weight velocity in kg/year. Growth velocity is calculated by dividing the difference between two measurements, as close to 1-year apart as possible, by the exact time elapsed between them. The calculated velocity is then plotted at the midpoint of the time period over which it is measured , to produce the velocity curve. For a child growing normally, height velocity should waver around the 50th percentile. Velocity consistently 25th percentile over successive years is indicative of a very short final height. Velocity charts are used less often than height - or weight - for- age charts. They are more likely to be used in the clinical setting, particularly by endocrinologists and nutritionists, in describing and evaluating the growth velocity of children who fall in the extremes (outside of the lower and upper centiles), exhibit marked changes in percentiles, or have a disease or treatment that affects growth . Average daily increases in length and weight For assessing short - term changes in growth, gender- specific percentiles for daily increases in weight and length during the first 24 months of life are available. These centiles are useful for early detection of failure - to - thrive or excessive weight gain(9). These are most useful in the clinical setting where weight changes over approximately 7 days, or changes in length over approximately 4 week s, can be expressed as a daily ave rage and compared to the references. Catch-up growth Apart from the normal pattern of growth observed in healthy infants, there is the condition known as ‘catch-up growth’. Catch - up growth is a period of compensatory growth which occurs at an above average rate. It occurs after conditions causing growth delay, such as illness or undernutrition, have been corrected. An infant’s ability to fully catch up to his/her genetic growth potential depends on the age at which the insult first appears, as well as the duration and severity of the problem. Infants who are wasted (low weight - for - height) but not stunted (low height-for-age) experience catch-up growth in weight at a faster rate than children who also have stunting. As normal size approaches, the velocity tends to gradually fall to normal and the infant returns to his/her normal growth channel. Overgrowth does not occur. Growth monitoring for children with special needs Children with mental handicaps and developmental, genetic or other disorders often have growth curves that are different from the reference standards. For some of these disorders sufficient numbers of patients have been followed to allow for the development of growth curves specific for these patients. In the case of Down syndrome, charts were based on data from 4,650 observations on 730 children(10). When compared with NCHS data, these data indicate that stature and growth rate for stature are reduced. Their weight, however, is not as greatly affected, and consequently a significant number of children with Down syndrome are overweight by late infancy. Growth charts also exist for children with a number of different special conditions, including cerebral palsy(11), myelomeningocele, and syndromes such as Prader - Willi, Marfan, Noonan, Turner, and Williams(12). Conclusion Understanding growth and the measurement and interpretation of growth data is essential for monitoring and optimizing the health and well-being of infants. Proper growth monitoring includes accuracy and precision in measurement as well as thorough assessment and interpretation of results. This is particularly important in early infancy during the period of greatest change and vulnerability. References:
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