gavra_at_work,
I thought the argument against person-hood of a fetus was from the Din of Rodef, that we don’t say “Mai Chaziz” about the fetus and it’s mother at any stage, even though we would say it once “Hotze Rosho”.
That was my understanding as well.
The point I’m trying to make is that the secular threshold for abortion (if there is a threshold at all) is not a din of rodef. Therefore, I don’t think we should be using the halachic argument regarding fetal status with respect to the mother as a cover for the secular/pro-choice position. There may be overlap in some instances of real-world application, I grant that, but they are originating from two entirely different moral frameworks, so from that perspective I believe the overlap is coincidental.