title: A study of the sex ratio of first-born according to the mother's month of birth.
abstract: The male-to-female sex ratio at birth of the first-born babies delivered in Japan to mothers aged 25 years or less was investigated in two groups: a hospital group and a questionnaire group. The sex ratio fluctuated according to their mother's month of birth. Between 1924 and 1928, the sex ratio of the offspring of the mothers born in March-May in both groups was significantly higher than that of the other mothers. We speculate about the relationship between the sex ratio at birth and the mother's month of birth.
mesh_terms: D000328:Adult; D001722:Birth Order; D005260:Female; D006801:Humans; D007564:Japan; D008297:Male; D012621:Seasons; D012744:Sex Ratio; D011795:Surveys and Questionnaires