This is not "self-defence" and you know it. The baby isn't trying to kill anyone, it has no capacity for malice or conscious control over the mechanism by which it is generated.
I didn't say "abortion is self-defense", I'm saying intent is important to the nature of a crime, or whether an action even is a crime to begin with. Self-defense is a relevant comparison in this sense, as the intention is not "I want that person dead", it's "I don't want to die." The vast majority of abortions are done purely to make the fetus dead. That is a very different than someone who would rather the fetus survive, but sees a substantial probability of their own death if they don't intervene.
Also, I don't think you understand why a homicide in self-defense is justifiable. It has nothing to do with the malice or intentions of the person who is killed in self-defense. Nothing at all. I'm not going to say you kill a fetus in self-defense, as there are other distinctions, but the lack of malice isn't it. We don't allowing killing in self-defense because the killer thought the deceased was mean and scary and deserved it, we consider it justified in the preservation of ones own existence, regardless of whether they deserved it.
What this is is the easiest variation on the trolley problem ever created. There are two people, one has to die, and the choice is whether to intervene and murder one of them so that the other will live.
Not really, most of the time if the mother dies, the child will also. Maternal mortality corresponds to death of the infant in like 75% of cases. So your trolley problem is one of the most solvable you could imagine. It has a split rail with the mother on one side, nothing on the other, and then the lines converge at the child who gets hit either way.
If you actually believe the personhood of that unborn baby is equal in value to that of its mother and that allowing that baby to die by inducing premature birth is murder, there is no argument that would ever justify intervening.
The argument is called the Principle of Double Effect, which argues that even if you know your actions will lead to a bad result, the actions can be morally justified if:
A) The action isn't inherently bad (inducing labor is not inherently bad)
B) The bad result must not be the means to the good result (the death of the fetus is a parallel consequence of inducing birth, not the means of saving the mother, thus not the same as harvesting organs from one person to save another)
C) The intention must be to achieve the good effect, with any feasible attempt to mitigate the bad effect taken (what I'm talking about)
D) The good effect must be proportionately greater than the bad (see trolley problem above)
You probably have no response to that argument other than something like "that's bullcrap", but you don't follow the same moral philosophy, so why would you?