Sex ratio of twin births in Japan.

The effects of paternal and maternal ages, gestational age and the socioeconomic factor of occupation on the sex ratio of twin individuals were analysed using data on 326,140 twin individuals which occurred in Japan during the period from 1955 to 1967 and in 1974. The sex ratio in live births slightly increased with time, whereas the sex ratio in fetal deaths slightly decreased with time. The sex ratio in total births significantly decreased during these years. There might be the possibility of a postponement of late fetal deaths of Japanese twin individuals into the early infant period for the younger maternal age group. The mean sex ratios of twin births during the period from 1960 to 1967 and in 1974 were 0.4995 and 0.5605 for live births and fetal deaths, respectively; the difference is significant. The sex ratio of Japanese twin births is not so high compared with those of Europe and the USA in the recent years.