Abortion according to the ramba is a murder without a punishment just like killing a trepha. Remember that there was the famous machlokes with the node biyuhda over if killing a trepha is yehorag veaal yavoer, and we pasken that it is yehorag veeal yaavor.
Based on this Menachem Ziemba paskened that when a Nazi ordered a Jewish Doctor to preform an abortion or he will be shot, that he was michuyav to give up his life because the Nazi was the rodef not the baby, and abortion is yehorag veaal yavoer. (this had nothing to do with shaas hashmad)