New growth curves for Norwegian children aged 0-19 years

INTRODUCTION

Children’s growth and development are dynamic and reflects in part the environment. Secular trends in children’s height and final height has been described over the last hundred years, and reflects changes in the environment with improved nutrition and fewer infections. Over the last 20-30 years there has been a rapid and large change in weight in older children and adults.

The first Norwegian growth curves were drawn in Bergen in the 1950s and the data for the second generation of curves (1971-4) were also collected in the same city.

In the period of November 2003 to December 2006, children at the age of 0-19 years were measured at well-baby clinics, kindergartens and schools in the city of Bergen. Above 8000 children were measured and included in the study with the intent to obtain updated information on growth and weight parameters in children in Norway. Measurements of length/height, sitting height, armspenn, under arm- and leg-length, head circumference, weight, waist circumference and skin folds (triceps and subscapularis), were performed. In 2006-2007, questionnaires to more than 7000 parents were sent out. The purpose of the questionnaire was to identify factors that may affect growth and weight trends. Approximately 67% answered the questionnaire.

Results from the study has made it possible to compare measurements of weight against height and skin folds (triceps and subscapularis) against age, of 4115 healthy children measured during 2003-2006, with similar surveys conducted by Per Erik Waaler in Bergen in 1971-1974. Only ethnic Norwegian children were included in this analysis to make it comparable to the Waaler study. In the material from the Bergen Growth Study, at the age of 4-15 years, 18% of the boys and 20.1% of the girls exceeded the 90th centile, for weight to height in the Waaler study. When it came to skin folds, changes were slightly greater. For the triceps, 30% of boys and 28% of girls exceeded 90th centile of the Waaler study. The same number off subscapularis were respectively 26.5% and 25.9%. The prevalence of overweight and obesity was 12.5% and 2.1% in boys and 14.8% and 2.9% in girls (IOTF cut-off’s).

Growth curves for Norwegian children aged 0-19 years are developed on the basis of data from the Bergen Growth Study. These are published in the Journal of the Norwegian Medical Association and compared with current Norwegian curves and WHO growth standards (Juliusson PB, Roelants M, Eide GE, Moster D, Juul A, Hauspie R, Waaler PE, Bjerknes R. The growth curves for Norwegian children. Tidssr Nor Doctors Unite 2009; 129:281-6). The length and weight in children aged 0-4 years has only slightly changed from the growth curves that are in use. Norwegian school children, however, are higher today than in the 1970s, with an increase in the median (50th percentile) for height up to 3,4 cm for the boys and 2,5 cm for the girls, compared to the Waaler study. The new curves are above the WHO growth standards for birth weight and length, and weight and head circumference at age 6 months-5 years. In the autumn 2010 new charts for weight-for-length/height for 0-5 year-old children were released.

BACKGROUND

Compared with the previous Norwegian growth curves, the Bergen growth study shows that the Norwegian children have become higher and the heaviest have become heavier than 25-35 years ago. This makes it necessary to develop new growth curves.

The curves that are presented here(1), are basis-curves that are needed in all health work with children. Later, special-curves will be made for the assessment of growth proportions (for example, sitting height, arm spen, forearm length, leg length and stomach scope.)

THE CHILDREN POPULATION

In 2003-2006, 8299 children aged 0-19 years were measured at randomly selected health centers, kindergartens and schools in the Bergen municipality. The data for growth curves of 936 children in whom one or both parents were from countries outside the Nordic region, and 85 children with chronic illness or prematurity were excluded. Measurements from a total of 7278 children were therefore used in the work of these curves, including data from 3756 boys and 3535 girls. 2736 children were younger than 5 years of age. Data from the birth length, birth weight and head circumference at birth was obtained from the Medical Birth Registry. Data from pubertal development is taken from the latest available study in Scandinavia and is based on surveys of 1925 Danish boys and girls aged 6 to 19.9 years from the Copenhagen area, collected in 1991-1993 (2).

MEASUREMENT TECHNIQUES

Standardized measurement techniques were used, that you can read more about here.

STATISTICAL ANALYSIS AND PREPARATION OF THE CURVES

The percentile curves were developed by using the growth model LMS developed by Cole and Green (3).

GENERAL DESCRIPTION

The new percentile curves presents the length / height, head circumference and body mass index (BMI) and is divided into three age intervals, 0-1 years, 1-5 years and 4-19 years. In addition to the percentile lines, gray fields are added representing respectively the area between median +2.0 to
+2.5 standard deviation (s) and median -2.0 to -2.5 (s). The axes are linear. On the curves, there is a table to insert the current measurements and space to write the name and date of birth. Furthermore, the curves have some relevant definitions and explanations inserted. Separating the reference material in the three age intervals provides good distance between the individual percentile lines and thus functional workplace on the charts.

The curves of respectively 0-1 and 1-5 year old children have the same template. At the top of the sheet, the head circumference against age is inserted, in the middle, length / height against age and lower weight against age. Curve sheet for children and adolescents aged 4-19 years, shows the top elevation mt age and weight against age at the bottom. BMI curves are designed for ages 2-19 years. Here, a gray area at the top is added representing the International Obesity Task Force (IOTF) their values for overweight and obesity (4). These are based on the BMI of at least 25.0 kg/m2 (overweight) (iso-BMI 25) and 30.0 kg/m2 (obesity) (iso-BMI 30) at age 18 years and is extrapolated down the ages. This was done that the same standardized deviation scores were followed up from the representative point by age 18 years and under (4). The BMI curves are also entered in the World Health Organization (WHO) limits for grade 1 and grade 2 under-weight. These are based on BMI 18.5 kg/m2 (grade 1 under -weight) and 17, 0 kg/m2 (grade 2 under-weight) at age 18 years, also extrapolated into the age (5).

On the growth curves for ages 4-19 years, references to pubertal development is added. On the curves for boys, percentiles for “T4 ml” (i.e. testis size 4 ml), which is consistent with puberty start and P2-P4 (Tanner-stages pubs minority hairs (6)) is included. On the curves for the girls, percentiles for B2-B4 (-Tanner stages for breast development (7)), P2-P4, and percentiles of menarche is added.

The growth curves are based on a cross-sectional- study and therefore gives a description of how the length / height, weight or head circumference of the individual child is compared with other children, each time. The growth curves say less about the expected growth development of the individual child.

Due to a long tradition in Norway using percentile-curves, also the new curves have percentile lines. The percentile-curves are believed to be easier to explain and are easier understood by parents and children. Standard deviation is still better suited for research purposes with the calculation of standardized deviation scores. National and international definitions and event terms are used in increasingly critical values where the standard deviations are included, for example, the definition of children born small for gestational age (8), and the risk of children with short stature applies(9). The new carts combine both approaches, using percentile lines with the gray areas that represent the median of 2 to 2.5, and -2 to -2.5 s.

THE HEAD CIRCUMFERENCE-FOR-AGE CURVE

The curves show the development of head circumference from birth up to 5 years of age. There are some differences, like when comparing with the earlier reference the new curve is slightly below. The difference has no clinical significance, and is believed to be caused by differences in measurement techniques and standardization. SYSBARN- registration in the 1980s routinely collected measurements from the health centers, while the measurements were performed in the Bergen Growth study by a limited number of nurses and nurses with special training in standardized measurement techniques.

Head circumference should be measured by all the Health centers the first 12 months of life (10). At the intersection of 1-2 percentile channels without suspect findings, repeated measurement should be made after 4 weeks. By clinical findings or intersection of 3 percentile channels the child should be assessed, but healthy children with head circumference below 3 percentile should only be referred if the head circumference blunt from the 3 percentile (10).

THE LENGTH/HEIGHT-FOR-AGE CURVE

The new curves have the same reference for children 4 years and younger as the once that have been used until now. For older children, there is however an increase in the 50 percentile for height up to 3,4 cm in boys and 2.5 cm in girls, in “line” with Norwegian children in these age group has become higher.

At the transition from the measurement of length while lying (from birth up to 2 years of age) to standing height (after 2 years) the difference is slight, usually less than 1 cm. Compared to how it has been done in some other countries (11.12), and in the former Norwegian growth curves, the new curves are smooth with no “notch” at 2 years of age.

Experience has shown that crossing of lines are normal the first 2 to 3 years of life, a phenomenon that can also normally be seen around puberty. It is customary to define short stature as height <2.5 percentile for the age. Most children who are below the 2.5 percentile are healthy and do not need investigation. Height should be assessed against the genetic potential and the current level is well below mid-parental height adjusted for age, the report indicated. Flattening of length or height should be investigated if the child has crossed two percentile lines between measurements and are under 5 years of age (10). Children who are below the 2.5 percentile, above the 97.5 percentile, or has crossed a percentile-line and are more than18 months, should be followed up with annual length / height measurements until clarification (10).

THE WEIGHT-FOR-AGE CURVE

In children younger than 4 years the new curves are consistent with earlier references. In children aged 4-19 years, however the highest percentile has increased in line with the increase in prevalence of overweight and obesity.

Assessment of overweight and obesity should include calculating and plotting of the body mass index (BMI). In children under the age of 5 years, weight against age should always be considered against weighed against the length / height to age. Children under 2.5 percentile or above the 97.5 percentile should be considered in terms of morbid weight deviations.

THE BMI-FOR-AGE CURVE

BMI curves for children is now made for the first time in Norway and onto these are included internationally recognized limits, respectively, overweight and obesity as proposed by the IOTF (4). This is rugged lines that can not be changed with secular trends. Therefore there is a clear indication in the curves when a child is at risk for developing overweight or obesity. The stated limit value for overweight, obesity and underweight is still primarily a screening tool and not a diagnostic tool for the individual child (13). It is important to keep in mind that BMI has its limitations, and that it should be used as an addition to clinical examination (14,15). BMI tells nothing about factors like fat distribution in the body (central versus peripheral), which in turn is linked to health risks (16). Furthermore, BMI differs poorly between muscle and fat, and BMI may therefore be unchanged when muscle mass increases and fat mass decreases, as during exercise (17). On the BMI curves, there are also added limit values for grade 1 and grad2 underweight (5).

Upon suspicion of weight discrepancy, over or under weight, BMI should be calculated and plotted. If iso-BMI exceeds 25, it should be perceived as a danger signal in terms of obesity development, especially if there is a preponderance of the family. If iso-BMI exceeds 30, there is considerable overweight and measures should be implemented. Warning signs of the under weight is if the child’s BMI is below level 1 (ref guidelines).

THE REFERENCE FOR PUBERTAL DEVELOPMENT

An overview of the pubertal development of Danish children were included when the new curves were recorded in the years 1991 to 1993 (2). It shows no change in pubs minority point compared to a Danish study from 1964. Menarche age registered in the Bergen growth study (data not shown here), shows good correlation with the Danish puberty reference. Median (50 – percentile) in both cases is slightly above 13 years (18), suggesting that the secular trend in growth and weight are not accompanied by a corresponding secular trend in puberty. Previous growth curves adopted Tanner percentiles in pubertal development, based on surveys of English children in the 1950s (19). The Danish reference shows no systematic differences from the English, and the largest deviations for the different percentiles are up to six months.

Percentile for testicular volume is presented for the first time within the new curves. The background is that the first clinical signs of puberty in normal puberty in boys are testicular volume of 4 ml.

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2. Juul A, Teilmann G, Scheike T, Hertel NT, Holm K, Laursen EM, et al. Pubertal development in Danish children: comparison of recent European and US data. Int J Androl. 2006;29:247-55.

3. Cole TJ, Green PJ. Smoothing reference centile curves: the LMS method and penalized likelihood. Stat Med. 1992l;11:1305-19.

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9. Consensus guidelines for the diagnosis and treatment of growth hormone (GH) deficiency in childhood and adolescence: summary statement of the GH Research Society. GH Research Society. JCEM 2000;85:3990-3.

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11.http://www.cdc.gov/nchs/about/major/nhanes/growthcharts/clinical_charts.htm#Clin%201

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14. Dietz WH, Robinson TN. Use of the body mass index (BMI) as a measure of overweight in children and adolescents. J Pediatr 1998;132:191-3.

15. Chinn S. Definitions of childhood obesity: current practice. Eur J Clin Nutr. 2006;60:1189-94.

16. Freedman DS, Serdula MK, Srinivasan SR, Berenson GS. Relation of circumferences and skinfold thicknesses to lipid and insulin concentrations in children and adolescents: the Bogalusa Heart Study. Am J Clin Nutr 1999;69:308-17.

17. Hall DM, Cole TJ. What use is the BMI? Arch Dis Child 2006;91:283-6.

18. Liestol K, Rosenberg M. Height, weight and menarcheal age of schoolgirls in Oslo–an update. Ann Hum Biol 1995;22:199-205.

19. Tanner JM, Whitehouse RH. Clinical longitudinal standards for height, weight, height velocity, weight velocity, and stages of puberty. Arch Dis Child 1976;51:170-9.

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