Fetal death in those cases corresponds (or ought to correspond) to a projection of the actual victim’s wishes: harm to the fetus is exactly coextensive with harm to the pregnant person’s future plans for it.
That’s one of many sufficient conditions. Another is fetal viability, which is exactly why Roe allowed states to enact restrictions on late-term abortions.
Overall, we’re only having these conversations because a fetus has two simultaneous qualities: it bears visual resemblance to a human being, and it has the future potential to be a human being. We don’t concern ourselves nearly as much with sperm (no visual resemblance) or cakes shaped like babies (no future potential). Together, they deserve concern, but not overriding concern; that is reserved for the sole person in the equation.
But if I give birth to a 26 week premature baby, then kill it. That’s murder, but if I do while still in the womb I didn’t kill anything because the mother didn’t want it?
The logic fails, it literally the exact same life you’re ending. The difference is only the location and whether someone wants it.
What you’re identifying falls under viability as mentioned above, as well as basic independent self-regulation.
They’re not the same life, because they’re two different things: one is a premature newborn that we know can survive outside of a continuing pregnancy, and the other is a fetus that might survive. Unless you propose that we make all pregnant people deliver the moment their doctor believes that the fetus is viable (that seems like a bad state of affairs?), the two will remain different.
That makes no sense. A 26 week old premature baby that is born is still at a high risk of death. We don't know if it will survive. Yet we give it the full protection of any human being, but if one hour ago it was in the womb, we don't.
