12 weeks seems quite short to me, especially if you are using the metric of the fetus' ability to survive outside of the womb, even with the most up to date medical intervention, which is very low before 24 weeks.
https://en.wikipedia.org/wiki/Fetal_viability
The problem here is that medical intervention will likely carry on improving (I guess, taken to the extreme, at some point in the future we might even be able to develop and artificial womb etc..) so if a cut off were based on it then that cut off would, in theory, be potentially lowered and lowered...I mean I guess you could even get to some silly point in the future where instead of advocating for no abortions you'd perhaps instead have some new pro-life position involving the removal of the foetus from the mother and external medical care of it.
I think abortions should be allowed, I do thing that when it comes to very late stage then that should only be some medial issue where the life of the mother is directly at risk a result of a physical medical issue - so long as abortions are freely available then there is an element of personal responsibility too in order to make that choice earlier on.
For the previously mentioned reasons I'm not sure that the whole "well technically we can, in some cases, with expensive intensive care facilities, keep a very premature baby alive at 24 weeks etc.." is necessarily a good base for a cut off, but I do think that when it gets much more than that then it is probably around the area that a cut off needs to be drawn. This is inherently arbitrary... that might well be a head **** for some people in the same way that creationists don't seem to get that we can have separate species without really having any point in evolution at which some creature went from being one species to another. Likewise most people are, at one end, happy that a bag of cells isn't a person and at the other that a newborn baby is a person... at some point we might draw a line and that line is going to be entirely artificial.