I've actually answered it repeatedly before.
Morally, I believe that the child should not be killed. However, morals are only one way to determine law. Law should primarily be determined by responsibility, and the mother would not be responsible in terms of becoming pregnant in the case of rape (less than 0.5% of abortions are from rape/incest as of
2004 data).
So, from a moral point of view, I would believe that this should not be a reason to kill the child.
From a legal point of view, I believe this would be the case where it's an option. When the health and life (physical) safety of the mother is in real jeopardy, the choice should be up to the doc, mom, and dad. The other 95+% should be illegal, like all other forms of murder.