I'll make it easy for you - the prosecution and the victim state that the pregnancy was a result of the alleged rape. So there's nothing for the defense to "prove."
I have no idea what you mean by the 5% "exception." If you truly believe what you're typing, you'll be going into that jury room and say "I have reasonable doubt that this was an actual rape - since the alleged victim is pregnant, that means there's a good chance she's lying - does anyone else agree?"
And they would state that why???