Over the past hundred years in industrialised countries, and recently in some developing nations, children have been getting larger and growing to maturity more rapidly. This has been referred to as ‘the secular trend’ in growth. In other words, the acceleration or retardation of growth and maturation as indicated by changes in height, weight or other variables over time is called secular trend. Many factors such as improved nutrition, control of infectious diseases through immunizations and sanitation, widespread health and medical care, better living conditions, changes in environmental and socio-economic factors, population mobility (both geographically to urban areas and socially upward) may account for increase in body size and biological maturation resulting in secular trends. On the other hand wartime deprivations or natural calamities like famine, tsunami may cause a decrease in growth.
The occurrence of a secular increase in height and weight has been well documented from many European countries like, Sweden, Finland, Norway, France, United Kingdom, Italy, Germany, Czechoslovakia, Poland, Hungry, the Soviet Union, the Netherlands, Belgium, Switzerland and Austria. From the rest of the world there are reports from Canada, the United States, Jamaica, Chile, Australia, New Zealand, Japan, Hong Kong, China, Sechychelles and India showing increase in height and weight over decades. Even the adult height of Kalahari Bushmen in South Africa and Australian Aborigines has shown an increase due to a more settled existence from the traditional hunting and gathering life. The average secular increase in height in Europe and North America is greatest during adolescence (2 to 3 cm. per decade), less during childhood (1 to 2 cm. per decade) and least for adults about 1cm per decade or less. Comparable changes have been occurring in weight and other body dimensions. Secular trend in birth length has also been observed in new born babies. Studies have noticed rising trends in 11 European countries. Rates of 30 mm. per decade have been achieved in Eastern Europe and Japan.
There has also been a secular change in the tempo of growth as is shown by an advancement of age at menarche and age at peak height velocity. Maturation hasbeen getting earlier during the last hundred years by 3 to 4 months per decade in most European countries. This trend is slowing down now in developed countries, both in body size as well as in maturation. Recent studies have shown that the increase has reached a plateau in countries like Germany and Poland due to the fact that the corresponding populations had achieved their full genetic potential or that their socio-economic conditions had ceased to further improve. Its magnitude is such that in Europe, America and Japan it has dwarfed the differences between occupational groups. In developing countries due to continuous improvement of living standards, nutritional and health care, the secular trend in various biological parameters is still observed. In India studies have reported positive secular trends in height among high altitude Himalayan populations over last three decades. A comparison in the heights and weights of Punjabi boys from Patiala between 1950 and 1975 showed a negligible average increase in magnitude of stature from 1950 to 1955, from 1955 to 1965 there is an increase of 2.20 cm. and from 1965 to 1975 it is 4.90 cm. per decade. In the total period of 25 years, an overall increase of 7.45 cm. has been noticed giving an increment of height of 2.98 cm. per decade and for weight of 1.48 kg. per decade.
In three decades i.e., from 1962 to 1991 the age at menarche in Maharashtrian girls has lowered by two years. Among Bengali Hindu girls a decrease of 5-7 days per annum was observed. In general, girls from upper socio-economic group experience menarche earlier than the girls from lower socio-economic status. Reports on stature and age at menarche of Punjabi Arora mothers and daughters from Delhi also show a substantial increase in stature of daughters and an advanced age at menarche as compared to their mothers indicating secular trends towards increase in height and decrease in age at menarche. Secular trends in growth in terms of the narrowing of ethnic differences in stature have been discussed in some studies. Asiatic populations’ seen since 1990 are much more comparable in stature to their counterparts to a different place in the world. A comparable evaluation among affluent adolescents illustrates Asiatic populations to experience earlier onset of pubertal growth spurt in stature than other major populations but have similar peak height velocities. Studies on secular changes in height and weight in populations are invaluable as they give information on nutritional status in early life, evaluating the growth reference standards and providing perception with regards to epidemiological trends of lifestyle diseases.