[Secular trends of sex ratios at birth in Japan]

To describe secular trends of change in sex ratio at birth in Japan during the 100 years from 1899 to 1998.

Data on sex ratios (number of males to females at birth by prefecture) from 1899 to 1998 were obtained from the vital statistics of Japan. We used moving averages (9-years averages) to minimize the effects of yearly fluctuation on the sex ratio and calculated simple linear regression coefficients to examine trends in change for the period between 1925 and 1969 and between 1970 and 1998.

The male: female sex ratio in Japan increased during the period through 1925 to about 1970 and thereafter decreased. The increase was the case for all prefectures with few exception, while these but Fukushima, Ibaraki, Fukui, Yamanashi and Kumamoto demonstrated subsequent decrease. The prefectures with strong decrease in the sex ratio from about 1970 at largely distributed facing the Sea of Japan; e.g. Akita, Toyama, Ishikawa, Kyoto, Shimane and Fukuoka.

Our study confirmed a decline in the male: female sex ratio at birth in recent decades in Japan as a whole and in most prefectures. Although we do not know whether this is an effect of environmental pollution due to chemical substances, monitoring of secular trends in sex ratios should be continued with due attention to counsel to government.