There is a such a thing as an affirmative defense, though. An affirmative defense allows a person to commit an act that would otherwise be illegal under certain circumstances. However, as the name implies, an affirmative defense has to be argued by the defense. The burden is on the defense to prove that they acted under the circumstances permitted.
Consider murder, for instance. Self-defense is usually an affirmative defense. The prosecutor's only burden is to prove that you killed someone. You have to demonstrate that you were acting in self defense in order to avoid the guilty verdict for murder.
So @vettnerk is asking a good question: will it be assumed that the doctor acted in good faith, or bad faith? Does the defense have to justify the abortion, or does the prosecutor have to demonstrate that it wasn't necessary?