Sex ratio of triplet births in Japan.
The effects of paternal and maternal ages, gestational age and the socioeconomic factor of occupation on the sex ratio of triplet individuals were analysed using data on 4,290 triplet individuals which occurred in Japan during the period from 1955 to 1967 and in 1974. The sex ratios among live births and fetal deaths slightly decreased with time. The decline of the sex ratio was related to the decline of age-specific fertility in older mothers. The mean sex ratio was 0.479 during the period. The sex ratio is not high but similar to those of Europe and the USA. The sex ratio was lower in triplets than in twins or the general population. It seems that predominant male triplets may be selectively aborted in early gestational age compared with that of twins or the general population.