Seasonal variations of the mean gestational period (MGP) and mean birth weight (MBW) are examined by the traditional method of time-series analysis during the period from 1979 to 1983 in Japan.
The results of the present study support the existence of a seasonal rhythm of MGP. Generally MGP showed a deep trough in winter and a shallow one in summer to early autumn. This pattern was similar to that of MBW. The two time series showed a statistically significant coincidence in their monthly fluctuations, and a statistically significant correlation was also observed for their amplitude. According to these results, we concluded that the seasonality in MGP plays an important role in creating the seasonality in MBW. However, the seasonality in MGP can not be the only definitive factor that totally regulates the seasonal variations in MBW. The relationship between the two time series is considered to be different among different seasons as well as regions. For example, we observed a decrease of the seasonal index in winter in the same month for both MBW and MGP, but not necessarily in summer. This suggests that the etiology of the decrease in seasonal variations is different between summer and winter.
