The changing sex ratio in Japan

Abstract

The simultaneous effects of maternal age and birth order on the secondary sex ratio have been examined using data on about 59 million births which occurred in Japan during the period from 1947 to 1978 except 1950. Non-linear negative association between the sex ratio and birth order was observed. The secondary sex ratio in Japan has been increasing during the period of 1900 through 1978. The change in birth order distribution can explain only a part of the increase in the sex ratio. The decrease in stillbirth rate was not only negatively correlated with the increase in the sex ratio at birth but also with that in fetal deaths. Therefore, it seems that there are some unexplainable factors associated with the increase in the secondary sex ratio other than the decrease in the stillbirth rate. The present results strongly suggest that the true cause of the secular trend in the secondary sex ratio in Japan will be found in the very early stage of pregnancy or at the time of conception. © 1981 Japan Society of Human Genetics.