Geographical differences and time trends in the seasonality of birth in Japan.

Seasonal variations of births in six different prefectures in Japan are analysed using Vital Statistics from 1974 to 1983. First births show a bimodal pattern with two peaks, one in winter (December-February) and the other in summer to early autumn (August-September). The seasonality of first births is statistically significantly correlated with the seasonality of marriages with 9 + 12*n (n = 0, 1, 2, 3) lag months. For subsequent births, the seasonal pattern is unimodal and there is also a geographical trend in seasonality of these births with a late spring peak in the northernmost prefecture of Japan giving way to an early autumn peak in the southernmost prefecture. These findings suggest that seasonal variations in marriage play some role in the seasonality of first births, while other features such as environmental factors could be associated with the seasonal variations of subsequent births. Changes in the seasonality of both births and marriages over time suggest that the modernization of Japanese society after the second World War could be responsible for the drastic changes in the pattern and degree of seasonality of births.