CYCLE DAY OF INSEMINATION, SEX-RATIO OF OFFSPRING AND DURATION OF GESTATION

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.