Another post from you and another unfounded assertion. Typical catholic apologetics.
I did not assert anything about Mary's blood cells crossing the placental barrier --- I will prescind, of course, from any discussion of feto-maternal microchimerism.
Every natural mother (not speaking of IVF-with-donated-ova and other artificial reproductive technologies) is the genetic and gestational source of her child's body.
Perhaps this discussion is suffering from some weird ambiguity about what "genetic" and "gestational" mean.
Mary is the genetic, gestational, parturient, lactational, legal, social, psychological and sociological mother of Jesus Christ our Lord. Previous to this very thread, I had not even imagined any Christian denying that.