As I am using the terms, murder is a class of homicide. Murder entails malice aforethought, or something like that. There is also manslaughter, which is less than murder. And, there is homicide in self-defense. Why don't we just go with homicide for now? A human is killed. We can address the culpability of the actors later. Can we agree that an abortion is a homicide?
If we can agree on that, then we can talk about whether that homicide is justifiable in some way.
Probably the least justification for homicide would be if 2 people decide to have consensual, conjugal sex, to completion, without protection...just because they want to do it. They intentionally engage in acts that make it possible, and maybe even probable, that they create a human being. The question then becomes: Are they allowed to kill that human being? If we are looking to culpability or responsibility of the parent to justify the homicide, then that case is probably the weakest case for justification. In that case, do you think abortion should be allowed? If you do, then we don't have to discuss special cases, because you believe it should be allowed in all cases.