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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.