Our Gemara on Amud Aleph discusses the idea that עוּבָּר בִּמְעֵי זָרָה זָר הוא. A fetus in the womb of a non-Cohen who is married to a Cohen, is not considered a Cohen until it is born. This has halakhic implications in terms of the slaves in the household who ordinarily are allowed to eat Terumah as they are part of the Cohen’s household. However, in this case because they are partially owned by this fetus who is inheriting his father, they cannot eat, as he still is not yet a Cohen until he is born.
The idea that a fetus does not yet have spiritual status as a cohen comes up in another interesting discussion. The Piskei HaRosh in Kiddushin 40 references a nusach for the blessing recited at a pidyon Haben. He has a number of problems with this blessing, and doubts its authenticity. He points out that this blessing is not mentioned anywhere in the Gemara, and he says we have no right to make up a new blessing post the Gemara. Second, the nusach of the blessing begins with, “אשר קדש עובר ממעי אמו who sanctified the fetus from the womb of the mother”, which is factually incorrect. A Bechor only receives its status when it is born (a Cesarean Section baby does not become a Bechor). This is similar to what our Gemara says by a cohen fetus; it is not really a full cohen yet.
However, Bach YD (305:16) defends this nusach, suggesting that the Rosh misunderstood what it meant. It is not referring to the holiness of Bechor, but rather the holiness that is conferred upon every Jewish fetus and soul, as described in detail in Niddah 30b.
As a fascinating aside, the Rosh’s assertion about blessings has been challenged by other poskim. In actuality, there are a number of blessings that we currently make which came from the time of the geomim, after the Gemara. For example, the blessing recited after the first act of intercourse with a newlywed virgin comes from the Geonim (see Derisha EH 63:2), also the woman’s blessing is post talmudic, She-asani Kirzono, see Taz OC 46:4.
In these contentious times where the basic sanctity and worthiness of the life of a fetus is under continuous assault, it is meaningful to learn not only when life begins, but when holiness begins. We will discuss this more on daf 69.
Translations Courtesy of Sefaria, (except when, sometimes, I disagree with the translation .)