Abortion and rape

Liberals sometimes claim to not understand why some people who generally think abortion should be illegal are willing to make an exception in rape cases. For example Kevin Drum :

... I still don't get the rape and incest exception, though. If it's murder, why is it OK to murder children born of rape or incest?

This is a bit muddled, a fetus doesn't become a child until it is born. And if abortion is legal in rape cases then it isn't murder as murder is a legal term for certain unlawful killings. Statements that abortion is murder are really claims that killing a human fetus is analogous to killing a human child or adult and should be illegal for similar reasons. But these reasons are not sufficient for society to make all killing illegal, in exceptional cases such as self-defense killing is allowed. So there is no logical reason society cannot allow some abortions to be legal as well when outlawing them would also impose an unjust burden as is arguably the case for rape.