Seasonal effects on fetal selection related to AB0 blood groups of mother and child.

An association of AB0 blood groups and month of birth was examined in two groups, students and newborns in Tokyo. Among 4919 students born mainly in the 1960s, an apparent seasonal variation was shown in births of blood group B students. Among 3592 newborns at an obstetric clinic in the 1980s seasonal variation was also observed in all four blood groups. The ratio of 0 group to A group newborns (0/A ratio) from 0 mothers was lower than that from A mothers among the 0 and A pairs contrary to the expected effects of 0-A incompatibility. The 0/A ratio was apparently different by season and the 0/A ratio from 0 mothers was significantly lower than that from A mothers among those born during August-January. It is assumed that an association of birth season with AB0 blood group may be caused by some seasonally and secularly changing environmental "infertility factors" such as infectious microorganism which may have some common components with a particular blood group substance respectively and induce a loss of embryos selectively at a particular season.