Greater loss of female embryos during human pregnancy: A novel mechanism

Summary Given an equal sex ratio at conception, we can only explain the excess of human males at birth by greater loss of females during pregnancy. I propose that the bias against females during human development is the result of a greater degree of genetic and metabolic “differentness” between female embryos and maternal tissues than for similarly aged males, and that successful implantation and placentation represents a threshold dichotomy, where the acceptance threshold shifts depending on maternal condition, especially stress. Right and left ovaries are not equal, and neither are the eggs and follicular fluid that they produce, and I further hypothesise that during times of stress, the implantation threshold is shifted sufficiently to favour survival of females, most likely those originating from the right ovary, and that this, rather than simply a greater loss of males, explains at least some of the variability in the human sex ratio at birth.