I agree with Ender, about the "Mortal" scene... but, I don't think I have read a fic with that plot... more like an inquiry along these lines. Someone did post an inquiry (though I can't remember if it was in sv_inquiry or at smallville_storyfinders) which was very similar to your plot.

And, about "Lexmas", supercaptian182 has a fic (Clark is pregnant in the fic, I'm afraid) where Lana is replaced by Clark! What Could Be (http://www.smallville.slashdom.net/archive/46/whatcould.html).