I read or heard from several ambiguous Catholic political pro-choice advocates that the Catholic Church did not definitively rule or state the beginning of life begins at conception until approx 50-200 years ago. Purportedly a Pope ruled that a priest who tried to obtain an abortion did not commit this particular sin because life only began at the quickening of the fetus (when he/she first moves).
What is the CC historically definitive stance on when life begins with regards a human life?
It is very disingenuous of those campaigners. What they refer to is a time when, without today’s medical understanding, microscopes, ultrasounds, pee-on-a-stick pregnancy tests, there was no knowledge of the development of the foetus and how it is a distinct live human even from conception. Put another way, the Church could not state that “life begins at conception” until it had medical knowledge of conception itself. Nevertheless, the Church has always opposed in the strongest terms any action to prevent an act of sexual intercourse leading to birth, even if medical understanding of the time thought of it as contraception rather than abortion.
In that particular case (though I have not heard of it before), it could be that the Pope was refusing to
punish the priest, when there was not the moral certainty of pregnancy before quickening, and therefore he could not have been proved to have procured an actual abortion.
It is similar today. Actions leading to punushment must be interpreted strictly, not broadly. Therefore the canonical penalty of excommunication
laetae sententiae for abortion is only for someone who actually succeeds in procuring an abortion, not merely someone who attempts to do so. Because, in the example you gave, pregancy could not be proven until quickening, it may have been seen as difficult to give a punishment for abortion if it was not clear that one had taken place.
Of course, in my opinion, attempt at abortion renders one morally unable to receive holy communion until sacramental confession … but this is not the same as the canonical penalty of excommunication.