Analysis of Geographical Differences in the Seasonal Variation of Mean Birth Weight in Japan (1974-1983)

Geographical differences in the seasonal variation in mean birth weight (MBW) were examined by a traditional time series analysis based on the birth data of 47 different prefectures in Japan for the period from January 1974 to December 1983. The general pattern on the seasonality in MBW has two peaks in spring and autumn, and two troughs in summer and winter. There is a general trend in the seasonality with the autumn-peak-dominant pattern observed in the northern part of Japan (Hokkaido, Tohoku, Kita-Kanto) giving way to the spring-peak-dominant pattern in the southwest. Another interesting finding is that there is high statistical significance as a general tendency, in the seasonality among immensely populated prefectures such as Tokyo, Kanagawa, Aichi, Osaka, and Fukuoka, while in the case of scarcely populated rural prefectures, the time series fluctuates almost randomly. Some socio-cultural and biometeorological hypotheses were evaluated to explain these results.