Physical growth charts from birth to six years of age in Japanese twins

BACKGROUND: The purpose of this study was to analyze the characteristics of the physical growth of twins in childhood and to present growth charts of Japanese twins. METHODS: The subjects consisted of 2029 pairs of normally developed Japanese twins. Growth data were obtained by mailed or hand-distributed questionnaires. Factors that affect body weight and height/length at selected ages were analyzed by stepwise regression analysis. Selected percentiles of body weight, height/length, and body mass index were calculated according to sex, and growth curves were drawn using a spline function. The size deficit of the twins compared to the standards for the general population of Japan was calculated. RESULTS: Gestational age, parity, zygosity, and birth order affected physical growth in varying degrees, although the overall effects themselves were small and mostly disappeared by one year of age. Growth charts of the twins present growth at selected percentiles from birth to 6 years of age according to sex. The size deficit of the twins was largest at birth: more than 20% for weight and approximately 6% for length compared to the 50th percentile of the standard for the general population of Japan. These deficits decreased rapidly in the first 6 to 12 months, and were found to be as low as 0-2% at 4 to 6 years of age. CONCLUSION: Growth charts specifically for twins are needed, at least for the first 1 to 3 years of age but not beyond the age of 6 years.