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Is there a reproductive compatibility or incompatibility regarding the blood groups?
Studies have shown a significant excess (87%) of ABO incompatible couples in 102 persistently sterile marriages. The same researchers also found that in 7 couples with markedly delayed fertility, the 9 children that did result were all blood type O, and hence would have been compatible with the mother. The authors suggested that the infertility was due to the presence of antibodies in the secretions of the mother's genital tract, or incompatible sperm from the father..
Several older studies have shown childlessness more frequent among blood type A and B women than among AB or O, while a 1965 study showed an increased proportion of type O children in families with more than one child. All these studies consistently showed that if the father possesses an A antigen which the mother does not, there is a marked selection against the survival of the A offspring.
There is a marked tendency of blood type O babies of blood type O mothers to be male as opposed to blood type A babies of type A mothers, who tend to be female. Blood type B and type O offspring have a greater chance of being male if their mother is the same blood type as their offspring. While other studies showed that blood type AB offspring tended to be female more often than expected (as in blood type A) which led the investigators to conclude that the presence of the A antigen may have some effect in skewing sex ratios in favor of higher percentages of female offspring.
A recent study compared 589 ABO compatible (that is, the husband and wife had blood types which could be exchanged) against 432 ABO incompatible mating couples. The mean number of living children presents a significant difference. In the incompatible couples, there was 21% deficiency of blood type A children and 16% deficiency of blood type B children when compared to couples which were ABO compatible. This has led some researchers to theorize that ABO incompatibility results in 'cervical hostility' between the man's blood type antigen on the sperm, and the woman's opposing antibodies in her cervical mucus.
In general, the levels of opposing blood type antibodies are higher in blacks than in whites. Whites typically have higher anti-A levels than anti-B levels and the levels were higher in females than in males. This is not true in blacks, where the anti-B levels are almost as high as were the levels of anti-A in both men and women.