If abortions were actually classified as murder, they'd be handled like any murder case would be.
First Degree Murder: if the mother premeditated the murder of the baby (gave it thought, planned it, and executed it in a cool and deliberate manner.) So if there were evidence that the mother went to a doctor, and had a abortion, it'd be first degree probably, pretty easy for the jury to infer premeditation there.
Second Degree Murder: Second Degree Murder is known as reckless and depraved heart murder. You might not have intended to kill the baby, but you certainly acted indifferent towards it's life. For example, if the mother snorted cocaine and drank heavily, resulting in the death of the baby, it might be second degree murder, if you have the evidence to prove the baby died as a result of drug or alcohol abuse.
Accidental deaths as the result of negligence, would be involuntary manslaughter.
It'd probably be much easier to prove murder in abortion cases than regular old murder, kinda hard to hide the fact you used to be pregnant and now aren't, also hard to hide medical records, or lack there of.
It'd be awfully suspicious to show up one day with no pregnant belly 6 months in, there'd be a lot of explaining to do.