Comments are coming fast, so I'm going to paste: RepubRob, are you a social conservative with an extreme pro-life position?
Because if you were, you would surely have heard this argument.
Here's an essay, with footnotes worth following:
It lays out the argument as I've heard it from childhood. First, that you should punish the rapist, not the unborn fetus. Second, that rapes rarely if ever cause pregnancy due to reactions in the female body during the trauma, hence that rape is brought up by pro-choice folks to make pro-life folks look insensitive.
This is actually the position of the hardline pro-lifer. Not some gaffe or mistake on Akin's part.