Fetal Development of Red Cell Antigens K, k, Lu a , Lu b , Fy a , Fy b , Vel and Xg a

Red cell antigen k occurs in the same frequency and strength as in adults in fetuses of 9 weeks or more and antigens Fy a , Fy b and Vel in fetuses aged at least twelve weeks. Antigen K was encountered twice, in fetuses of 14 and 16 weeks. Lu b appears weaker in fetuses than in adults; it was not detected in a fetus of 9 weeks although both parents were Lu(a–b+). Xg a was found to be yet more poorly developed before birth; it was demonstrated in only four out of 23 fetuses. Its frequency in fullterm newborns was, however, the same as in adults, but with lower scores.