The relation between seasonal birth variation and the season of the mother's birth.

Seasonal birth variation was investigated in relation to the season of the mother's birth by examining: (1) 21,517 case records of a maternity hospital in Tokyo, Japan, for the period between 1925 and 1960 and (2) 2,348 questionnaires on mothers and their offsprings. In general, the birth rate was the lowest in the May--July season but mothers born in May--July of 1896 through 1915 and February--April of 1916 through 1935 had their births more evenly distributed throughout the year. The less season-sensitive mothers were more fertile than the others. The existence of some epidemic and immunogenic "seasonal infertile factors" is put forward as an explanation for these phenomena.