Cycle day of insemination, sex ratio of offspring and duration of gestation

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Abstract
It has been reported that the regression of human sex ratio (number of males per 100 females at birth) on cycle day of insemination is U-shaped. The point is not yet established, so a search was made for possible effects of this phenomenon on the regression of sex ratio on duration of gestation (from last menstrual period to delivery). A previously unremarked feature of this latter regression is that it, too, is U-shaped. It is accordingly suggested that the one regression is responsible for the other, and a simple mathematical model indicates that the magnitudes of the two regressions are compatible with this conclusion.