But we know a little bit about murder, and the differences between murder and other forms of killing. And murder, in the way that we define it in all other instances, involves the intentional death of another human person. Even if we were to assume that every embryo or fetus were just as sentient and just as much of a person as any other human being, an argument that is not supportable on the basis of scientific evidence, then the lack of knowledge and intent would still be enough to classify abortion as something other than murder.
Let's imagine a scenario in which two men go deer hunting. One man mistakes his friend for a deer, shoots him, and accidentally kills him. As long as we all agreed on the facts of the case, it's hard to imagine that any reasonable person would describe this as murder--even though we would all know for certain that a real, sentient human person was killed. Why? Because the shooter thought he was killing a deer--something other than a real, sentient human person.
Now consider the example of abortion. If a woman and her physician think they're killing a non-sentient organism, then--even if the embryo or fetus were, unbeknownst to them, a sentient human person--they would not be committing murder. At most, they would be guilty of involuntary manslaughter. But even involuntary manslaughter involves criminal negligence, and it would be very hard to judge someone criminally negligent for not personally believing that a pre-viable embryo or fetus is a sentient human person when we don't actually know this to be the case.
From the point of view of someone who believes that every fertilized egg is a sentient human person, abortion would be horrific. Tragic. Lethal. But it would be no more murderous than any other kind of accidental death.