Seasonality of mean birth weight and mean gestational period in Japan.

Seasonal variations of mean birth weight from January 1974 to December 1983 and of mean gestational period from January 1979 to December 1983 in Japan are analyzed by time-series analysis. This analysis decomposes the variation in a series into trend, seasonal variation, other cyclic changes, and a remaining irregular fluctuation. The results of the present analysis support the existence of a seasonal rhythm of mean birth weight and mean gestational period in Japan with statistical significance (p < 0.001). Seasonal patterns of mean birth weight show two peaks in May and October-November and two troughs in June-September and December. Mean gestational periods shows two peaks in October and February-March and a deep trough in winter and a much smaller one in June-September. Two time series demonstrated a coincidence in monthly variations with statistical significance. These results indicate that a part of seasonality in birth weight could be explained by the seasonal variations in mean gestational period.