Geographic differences in seasonal variation of mean birth weight in Japan.

Seasonal variation of mean birth weight (MBW), mean gestational period (MGP), and MBW at 40 weeks of gestation (MBW40) in the 15 different prefectures of Japan were analyzed using vital statistics from 1974 to 1983. MBW generally shows a bimodal pattern with two peaks in spring and autumn and two troughs in summer and winter. Seasonality exhibits a general geographic trend: The autumn peak loses its height and the spring peak increases its amplitude as one moves from north to south. This trend suggests that environmental factors are associated with the creation of seasonality in MBW. For MGP the pattern is relatively unimodal with a deep trough in winter; however, a late summer small dip is observed in most cases. According to an analysis of coincidence between the two time series (MBW and MGP) and correlation analysis, the seasonal variation in birth weight basically depends on the gestational period. For MBW40, which represents the fetal growth rate, the seasonality shows a pattern similar to that of MBW. A statistically significant coincidence and a correlation are observed between the time series of MBW and MBW40. These results suggest that the two processes of fetal growth (gestational period and fetal growth rate) are associated with the creation of seasonality in MBW.