Correlation of month and season of birth with height, weight and degree of obesity of rural Japanese children.

Month and season of birth are thought to influence height, weight and degree of obesity in schoolchildren. A cross-sectional study was designed to measure the height and weight of all children aged 6-15 years attending primary and junior high schools in Tokushima Prefecture, Japan. Data were standardized (z-scores) and analysed separately by gender and age. The mean z-score for height and weight were the highest in subjects born during the months of spring and the lowest in those born during the months of winter (p < 0.0001), whereas the means were significantly higher in children born during the months of summer than in those born during the months of autumn (p < 0.0001). A gradually decreasing trend of height and weight was observed in children of both genders born between May and Mar (from spring to winter). There was no significant difference in degree of obesity among the four seasons of birth for boys and girls. The highest prevalence of obese boys have born during spring (among 6-year-old boys) and summer (among 7-year-old boys), whereas the highest prevalence of obese girls have born during spring (among 6-year-old girls) and winter (among 10-year-old girls). Our findings suggest that month and season of birth influence height and weight of schoolchildren in Tokushima but not their degree of obesity.