Type and time of insemination within the menstrual cycle and the human sex ratio at birth.

Data that strongly suggest that the timing of insemination affects the human sex ratio at birth are presented. Two studies conducted by the author, including a total of 1,318 pregnancies, in which timing of insemination was related to the shift in basal body temperature are discussed, and previous literature is reviewed. The differential effect of type of insemination-natural versus artificial-is noted. The further question why timing and type of insemination affects the human sex ratio is also explored.