Secular trends in height, weight and body mass index of 6-year-old children in Bremerhaven.

Secular trends in growth processes of children can be important indicators of changes in public health. Common to studies on secular trends in children is that evaluation is based on comparison of data collected at two (or more) distinct points on a time scale. The quantitative characteristic of the secular trend is estimated by linear interpolation between the two end points of the underlying time interval, which in studies of children are usually at least 10 years apart. The purpose of the present paper is to analyse secular trends in height, weight and body mass index (BMI) of 6-year-old children from Bremerhaven over the period 1968-1987 (the year refers to the birth cohort). The results are based on data drawn from health records of the City Health Centre, where all 6-year-old children are routinely measured in a school entrance examination. Thus the data represent complete birth cohorts of children entering school in Bremerhaven and not selected samples. The data reported here refer only to children of German origin. The sample sizes vary from n = 313 (girls born in 1982) to n = 737 (boys born in 1968), and total sample size is n = 7601. Regression of the arithmetic means of height on year of birth showed that the trend in stature for children born between 1968 and 1987 was 0.67 cm/decade for boys and 0.49 cm/decade in girls. Both trends are statistically significant (p < 0.05). Although there was an increasing tendency for weight as well, which was more marked for the 95th percentile than for the median, neither of the trends in both sexes was statistically significant. While the BMI in both sexes showed no trend at all for the median and the 5th percentile, there was a significant linear increase of the 95th percentile. Furthermore, the results for height show that an evaluation of secular trends under qualitative and quantitative perspective critically depends on the selection of points on the time scale.