The human sex ratio: increase in first-born males to parents with shared HLA-DR antigens.

Maternal-fetal HLA-DR antigen sharing has been reported to affect the sex ratio of first-born. We therefore studied offspring sex ratios and birth orders in 66 families in which parents shared one or more HLA-DR antigens as compared to 61 families with no parental HLA-DR sharing. A significant excess of males was found among first-born children who were HLA-DR compatible with their mothers compared to first-born HLA-DR incompatible children of couples sharing HLA-DR antigens and couples not sharing HLA-DR antigens. Increased numbers of males may persist among children of higher birth orders in families where parents share both HLA-DR antigens, but not among couples sharing only one HLA-DR antigen. We hypothesize that the presence of the H-Y antigen in the male fetus may provide the necessary stimulus for a successful pregnancy in HLA-DR compatible pregnancies and may explain the excess of male births in the general population.