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Greater Loss of Female Embryos During Human Pregnancy: A Novel Mechanism
Author(s) -
Mulley John F.
Publication year - 2019
Publication title -
bioessays
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.175
H-Index - 184
eISSN - 1521-1878
pISSN - 0265-9247
DOI - 10.1002/bies.201900063
Subject(s) - placentation , pregnancy , embryo , biology , ovary , sex ratio , andrology , follicular fluid , fetus , endocrinology , genetics , demography , medicine , oocyte , population , placenta , sociology
Abstract Given an equal sex ratio at conception, the excess of human males at birth can only be explained by greater loss of females during pregnancy. It is proposed 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 it is further hypothesized that during times of stress, the implantation threshold is shifted sufficiently to favor 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.