Blood groups and reproduction in Hebridean women.

The distribution of ABO and RH blood groups was studied in 910 fertile women from the Isles of Lewis and Harris. The frequencies of group O mothers fell and of RH-mothers rose with increasing parity, but there were no significant changes in the ABO and RH blood group distributions among their offspring when they were subdivided according to their mothers' parity. When mother-offspring pairs classified according to their ABO and RH compatibility status it was found that RH incompatibility was associated with greater number of previous pregnancies and of abortions. Regression coefficients of birth weight on maternal stature and maternal age were significant only for doubly compatible female offspring and on number of previous pregnancies only for doubly compatible offspring of both sexes. When offspring were classified according to their own and their mothers' ABO blood groups, considerable variations in sex ratio were found. However, the only statistically significant finding was a deficit of A females among the offspring of O mothers. It is suggested that for the ABO blood groups system incompatibility selection, which favours homozygotes rather than heterozygotes, can be counterbalanced by selection for greater maternal tolerance of incompatible offspring with increasing parity. Hence the loss of heterozygotes in lower parity mothers may be compensated by an increase production in higher parity mothers. For the Rh system the same mechanism may occur, but there is evidence additionally of reproductive compensation.