Impregnation as a paraphilia is the particular interest in the idea of unprotected sex and the possibility of knocking/being knocked up. Maiesiophilia, a sexual interest in pregnant women, is usually coupled with it.
It isn't 'just' the biological imperative, as many people don't find the idea of impregnation itself to be hot. But some do, and fixate on it like any other fetish to get their jollies off.
If you find the idea of a girl whining, "Don't cum in me. I will get pregnant…" right before the guy dumps a load into her bare pussy, you might have an impregnation fetish.
Post your sloppy creampies, your rounding bellies, your naughty captions, your internal webm's and gif's, and all manner of sexy sites concerning impregnation risk and fantasy.
Starter Materials:
http://impregnationerotica.blogspot.com/
http://imgur.com/r/PreggoPorn
http://www.xvideos.com/profiles/skyliberty
http://www.asstr.org/files/Collections/impregnorium/www/stories/archive/storyindex.htm