Objectives: To determine the age-, period-, and cohort-specific effects on the male proportion in Japanese newborns, we performed an age-period-cohort (APC) analysis in this study. In addition, projections for the male proportion were analyzed.
Methods: We obtained data on live births of newborns for Japanese women in 1947–2007 from the National Vital Statistics. Cohort tables containing data on the male proportion were analyzed using a Bayesian APC model. Projections of the male proportion (2008–2027) were calculated.
Results: The age effect decreased when the mothers were 40–44 years old; however, the effect was relatively limited as compared with the period and cohort effects. The period effect increased from 1947 to 1969 and decreased thereafter. Analysis of the cohort effect on male proportion trends revealed a decreasing slope for birth cohorts born between 1905 and 1945 and a subsequent increase after 1958. The projections for male proportion indicated that the male proportion in 2027 would be similar to that in the 1970s.
Conclusions: The age of the mother hardly affected the male proportion. The period effect started decreasing from the latter half of the 1960s. This may be attributable to the high economic growth since 1965 that promoted industrial development that led to environmental pollution, which in turn may have lead to the deterioration of the intrauterine environment. Cohort effects changed from 1958 and exhibited trends toward increase in male proportion; this may be due to improvements in obstetric care. Our results suggest that the male proportion in Japanese newborns will increase in the future.