Objective: The infancy-childhood-puberty (ICP) growth model divides human growth into three additive and partly superimposed phases: infancy, childhood, and puberty reflecting the endocrinology of the growth process. The childhood phase of the ICP model has been assumed to start during the second half-year of life and slowly decelerates, although it continues into puberty until growth ceases. This phase has been taken to mirror the effect of growth hormone (GH). The age at onset of the phase of growth, normally occurring between 6 and 12 months of age is recognized to be an important time event at postnatal human growth. The aims of this study were to study the age at childhood onset in Shanghai children and its effect on child growth, and to compare these results with the results obtained from Swedish longitudinal studies.
Methods: The study population consisted of 1 720 children born between January 1, 1980, and December 31, 1990 in Fenglin Community, Shanghai, who got a continual monitoring at the clinic from birth to six years of age. The age at onset of the 'childhood phase' of growth was determined individually, in units of one month, by visual inspection of the data contained within the individual, computer-generated, ICP-based growth chart for both length and length velocity. T tests, chi(2) tests and multiple linear regression analyses were used in data analysis.
Results: There was a significant (P < 0.05) difference in the distribution of the age at onset of the childhood phase of growth between Shanghai and Swedish populations. The mean age of the infants at onset of the 'childhood phase' of growth was 11.2 months in boys and 10.7 months in girls. Compared to their Swedish counterparts, these means occurred 1.3 months later in boys and 1.4 months later in girls. Both age at onset of the 'childhood phase' of growth and length at six months of age significantly (P < 0.05) contributed to the attained height from 12 months of age onward; one month delay in the onset of the 'childhood phase' of growth reduced height, at 5 years of age, by 0.4 cm in boys and 0.56 cm in girls. The mean values of length/height among the four groups of the age of the childhood onset, i.e. 6 m-, 9 m-, 12 m-, >/= 15 m, were found to be significantly (P < 0.05) different from 12 month of age onward. However, a significant (P < 0.05) difference in the mean length/height velocities among the four groups was in principle only found during the following intervals: 3 - 6, 6 - 9, 9 - 12, and 12 - 18 months of age. The mean values of BMI among the four groups of the age of the childhood onset showed significantly (P < 0.05) different only in 9 and 12 months of age. The mean values of BMI velocity among the four groups of the age of the childhood onset were found to be significantly (P < 0.05) different in four age intervals: 3 - 6, 6 - 9, 9 - 12, and 12 - 18 months of age. The age at onset of the 'childhood phase' of growth was negatively associated (P < 0.05) with mid-parental height, though positively related (P < 0.05) to height at six months of age. A simple linear regression was applied to the age at onset of the 'childhood phase' of growth and mid-parental height. It was found that the age at onset of the childhood phase' of growth was 1.5 months later in boys and 1.3 months later in girls, for children with a mid-parent height being -2SD below the mean in comparison to the children of tall parents, i.e. a mid-parental height equals to mean +2SD.
Conclusions: The age at childhood onset is equally important when studying children from Shanghai, as it is with their Swedish counterparts.