The mishna states that a baby of one day can be ma'achil t'ruma. Rashi and the Ra'av both say that this refers to his mother who is the widow of a kohen, and can cause his mother to be permitted to eat t'ruma that very day. She is, of course, a yoledes, and cannot eat t'rumah that day. Rabbi Akiva Eiger answers that the baby was a yotze dofen, with no at delivery, and that this mishna reflects the opinion that she would be tahor in this case. Why not say that the "mother" is really the grandmother, and that the mother died in childbirth, leaving the newborn as the only surviving descendant of this grandmother, who is a kohen's widow? This case (lo aleinu) would seem to be more sh'chiach for most of obstectrical history. Perhaps since there was a direct transition from the baby's mother's existence to the baby's existence, the baby cannot be said to be ma'achil in the same sense as in Rashi's case?
Marc Diamond, Providence, RI
The Chochmas Betzalel cites your [original] answer (that the mother is the grandmother) in the name of Rav Leib Fiheles. He says that he presented it in public and it was well accepted.