Association of the Type and Time of Insemination within the Menstrual Cycle with the Human Sex Ratio at Birth

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Abstract
With use of the shift in the basal body temperature as a reference point, a study of the effects of time and type of insemination was carried out on 1318 conception cycles. The sex ratio was found to change significantly during the menstrual cycle both in natural and in artificial insemination, although the trends were opposite. In natural insemination the proportion of male births diminished from 68 per cent six or more days before, to 44 per cent on the day of the shift. In artificial insemination the proportion increased from 39 per cent three or more days before, to 62 per cent the day of the shift. The results help to explain previous conflicting reports and suggest that environmental differences may have a differential effect on the X-bearing and Y-bearing spermatozoa. (N Engl J Med 291: 1056–1059, 1974)