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Abstract
Background: Biological variables in height growth in Japanese singletons are well documented, but there has been less research on Japanese twins.
Aim: The study investigated the biological variables in height growth of Japanese twins and compared them with those of Japanese singletons.
Subjects and methods: Samples of 92 boys and 99 girls from monozygotic twins, and 41 boys and 38 girls from dizygotic twins born in 1960–1973, from the Twin Class in the Junior and Senior High Schools affiliated to the University of Tokyo, Japan, were considered. The Bayes modal estimation method was applied to the triphasic generalized logistic growth model (BTT model). From the fitted model, the biological variables were extracted.
Results: The monozygotic twins, on average, matured earlier than dizygotic twins, but later than singletons. Until pre-adolescence, the height velocity of twin children is lower than that of singleton children. This results in the final stature of twin youths being smaller than that of singleton youths. The twins are, on average, about 2.6 cm smaller than singletons in final height only due to the lower velocity during birth to pre-adolescence. Comparing with the average final stature of dizygotic children, the boys were 3.9 cm and the girls were 2.8 cm shorter than Japanese late-maturing boys and girls, respectively. For monozygotic children, the boys were 3.8 cm and the girls were 3.0 cm shorter than the respective Japanese late-matured singleton boys and girls.
Conclusion: The biological variables of twins differ significantly from those of singletons. It is important to recognize that singleton reference values do not correctly reflect the growth of twins.