The male to female ratio of newborn infants in Japan in relation to climate change, earthquakes, fetal deaths, and singleton male and female birth weights

摘要

We have updated our work regarding climate extremes in Japan and the consequences of the huge earthquake in the Fukushima Prefecture in year 2011. We have interchangeably used the term sex ratio at birth, sex ratio of births. This ratio describes the secondary sex ratio, whereas the primary sex ratio designates the male/ female ratio at fertilization. The underlying mechanisms of declines in sex ratios at birth may be related to decreased fertilization of XY embryos by reduced motility of Y spermatozoa from male factor approach and increased cortisol secretion prior to conception from a female factor approach. We have shown that the declines in sex ratio at birth were observed 9 months after the Kobe earthquake but 10 months after the Tohoku and Kumamoto earthquakes. The temperature difference may be associated positively with sex ratio of fetal deaths and negatively with sex ratio of births and with singleton male and female birth weights.