Postnatal Rapid Weight Gain of Japanese Infants Based on the World Health Organization Child Growth Standards.

There is limited research directly comparing WHO weight-for-age z-scores with the Japanese national reference values for infants. Furthermore, few studies have examined the prevalence of rapid weight gain from birth to at least three months of age using the standard deviation scores based on the national reference. However, no studies have been reported that investigate postnatal rapid weight gain in Japanese infants and preschool children using the WHO weight z-score. This study aims to convert the national references to WHO weight-for-age z-scores, to assess whether WHO z-scores at birth remain low by 2023 for both sexes, to examine trends in these weight growth patterns up to 2023, and to estimate the prevalence of postnatal rapid weight gain using the WHO weight-for-age z-scores and to evaluate whether this prevalence of postnatal rapid weight gain differs between WHO weight-for-age z-scores and standard deviation scores based on the national weight references. The database in this study used the community-based and cross-sectional national surveys, which comprised 14,115 Japanese infants and preschool children aged 14 days to six years at the time of measurement in 2000, 12,426 in 2010, and 11,194 in 2023. WHO weight-for-age z-scores were calculated using an automated conversion tool based on the Japanese national reference. Postnatal rapid weight gain was defined as a sex-specific increase in weight-for-age z-score or SD score of > 0.67. Across all percentiles in the years 2000, 2010, and 2023, the WHO weight-for-age z-scores remained - 0.6 or less at birth, peaked at one and a half months of age, and gradually declined thereafter. Weight-for-age z-score trajectories for both sexes were the highest in 2000 and showed a steady decline by 2023. The WHO weight-for-age z-scores produced a higher prevalence of postnatal rapid weight gain compared with the SD scores of Japanese references. Weight growth patterns based on WHO z-scores showed a steady downward trend through 2023. The prevalence of postnatal rapid weight gain assessed using WHO z-scores appears to be potentially overestimated. Caution is advised when evaluating research that uses different criteria for postnatal rapid weight gain.