Avram: An Ox (or any other animal or object) that is in the process of endangering human life may be destroyed in order to preserve the aforementioned human life.
That being said, I found a Rambam that is Mashma like you:
Rotzeach 1:8 & 1:9:
? ??? ??????, ???? ?? ??? ??? ?? ???? ?????, ??? ???? ???? ?? ?????–????? ????? ??? ????? ????? ???? ???? ?????? ????, ?????? ?? ????? ???? ?? ????; ??? ???? ??????–?????? ???? ?? ?????, ????? “?? ????, ????” (????? ??,??): [?] ??? ?? ???? ?? ????, ??? ???? ?? ??? ?????.
? ????? ???? ????? ??????? ???? ???? ????–???? ????? ????? ?????, ??? ??? ??? ???: ???? ???? ????? ????? ??????. ??? ??????? ????, ??? ?????? ??, ???? ????? ??? ???? ???, ??? ??? ???? ?? ????.
The language of “?????” ties in the previous Halacha, and says to me that even though the fetus is given some level of personhood (although less than a born person), none the less as it has a status of Rodef you are required to kill it. Once it’s head passes through, they have the same level of personhood and therefore they are both/neither considered to have a din of Rodef.
I would still like to see clearer evidence in the Meforshim if anyone could provide it.