Variation of sex ratio and ABO blood group composition by month of birth of blood donors in Tokyo

Sex ratio of 17,273 blood donors born during the period between 1925 and 1935 was examined according to their month of birth and ABO blood groups in comparison with 5,810 healthy non-blood donors born in the 1900s to 1930s. The sex ratio of the blood donors and the non-blood donors varied similarly according to their month of birth with a prominent peak in summer births and a trough in winter births. This birth season with a high sex ratio was different from that of the general births during the period between 1921 and 1935, in which a maximum sex ratio was found in November. A possible explanation for the difference is the different rate of male and female infant deaths according to birth month. Variation of the sex ratio according to season of birth was not similar among the four ABO blood groups. Sex ratio of the donors with blood group B showed no elevation among the summer births. Non-blood donors with blood group B, on the contrary, showed a higher sex ratio than the others in the summer births. This difference can not be explained by infant or juvenile deaths. A possibility is that a tendency to become a blood donor is modified by the season of one's birth differently according to gender and ABO blood groups.