New data from the Bergen Growth Study show clear tendencies towards increased height and weight of school children during the last 20 years; but for children between 0 and 4 years weight-for-height, birth weight and weight in general have not changed much. Norwegian children are clearly heavier at birth than those included in the total WHO material; with few exceptions they were also taller and heavier in general and had a larger head circumference.
The observed trend in height of Norwegian school children is in accordance with that observed for children in other European countries in the same time period (2, 23). The secular trend in height has been linked to improved nutrition and socio-economic conditions and fewer infections. Such a steady increase over time may continue for many generations before it levels out (2). Height stagnation of Norwegian draftees over the past two decades (24), indicates that the country´s population may be about to reach its genetic potential for growth. The trend towards increasing height – observed since the 1970s – still reflects that growth charts should be updated for use in Norway.
In the SYSBARN study, head circumference was found to be somewhat larger than in the Bergen Growth Study. Different measuring techniques may explain this observation, as weight-for-height and weight were found to be similar in the two studies. In the SYSBARN study, many different investigators measured the variables according to their own routines, while in the Bergen Growth Study a limited number of people used standardized measurement techniques.
The Bergen Growth Study concluded that the average age for onset of menstruation (13.25 ± 1.05 years) had not changed since the earlier Norwegian studies (3). Even though data from American studies have raised suspicion of an earlier onset of puberty (25), European studies have so far not been confirmatory (14, 26). The fact that Norwegian children’s age at onset of menstruation has remained the same, indicates that increased height is not caused by earlier maturation – although an earlier onset of puberty and possibly slower progression cannot be ruled out. Data for puberty development of Danish children, which have been incorporated into our charts, are at the same level as those found in a Danish study from 1964 (14).
Weight-by-age in the Sundal Study (from the 1950s), was only marginally different from that in the Waaler study from the 1970s (18). Over the last couple of decades, child overweight and obesity has risen alarmingly worldwide (5). However, our numbers reveal that an increase in weight only occurs in children older than 5 years – also, it is mostly the heavy children that have become even heavier, which means that the highest percentiles are most affected (21). Therefore, the increase in overweight in the child population mainly affects one subgroup.
This development calls for action at many levels; a sensible general health policy being a good starting point (27). The findings in our study indicate that goal-directed actions targeted at special groups in the child population are the actions most urgently called for.
The observation that older children are becoming taller, renders updating of the charts important; but the increase in weight also stresses that the reference charts should be functional in relation to specific problems of overweight and obesity. In Norway, a rational approach is to use BMI curves with cut-off values for overweight and obesity. Based on that approach, charts have been made with internationally recognized cut-off values for both overweight and obesity, as suggested by the International Obesity Task Force (IOTF) (28). These charts are robust and are not affected by slow trends.
We consider the differences between the new Norwegian growth reference and the WHO growth standard to be of clinical relevance. The average birth weight of Norwegian boys is currently 410 g and that for girls is 380 g higher than the birth weight in the WHO standard. The Norwegian children in the WHO study had – not surprisingly – the same average birth weight as those in our study. The low birth weight in other countries impacts on and reduces the average birth weight in the WHO study (20, 29).
The new growth reference for Norwegian children uses a height that is somewhat higher than the WHO standard. Length growth during the first two years for the Norwegian children who participated in the WHO study, is still more similar to that in our growth references than to that in the full WHO study.
It is difficult to compare weight data from the Bergen Growth Study with those in the WHO study, as weight data from individual participating countries were not published separately, and because the heaviest children were excluded from the WHO material (22).
Percentiles for head circumference in the WHO study are at all times lower than those in the new Norwegian growth reference, but the WHO data differ even more from those in the SYSBARN study. Head circumference was measured in the same way in the Bergen Growth Study and the WHO study.
Differences in length growth, head circumference, and possibly also weight challenges the WHO’s position that children who grow up under beneficial conditions will follow the same grow pattern (11, 30). This relates to the ongoing discussing on whether WHO growth standards can replace national references (30).
The new Norwegian growth reference – as the earlier ones – is targeted at children with a Norwegian ethnic background. It has yet to be compared with children from other ethnic backgrounds in the Bergen Growth Study. We chose to restrict the charts to children from a Norwegian ethnic background because the differences between ethnic groups are significant, with an increase in final height of up to 7 cm for population groups with beneficial living conditions (31, 32).
It is important that growth charts – representative for a healthy child population – are updated and available at all times. Percentile placement – especially the highest and lowest percentiles – are of consequence for definitions and assessments. In our opinion, several of the differences documented in this article-through comparison with earlier Norwegian growth references and WHO’s international growth standards – are clinically relevant. To avoid subjecting children to unnecessary assessments and worries it is important that updated professional guidelines are made available in this field in Norway.