I've described this fic a number of times to people, as an example of the worst kind of horror that could come out of the GW fandom, but I finally tracked it down: Te Amo. Also known as "the fic where Heero is pregnant with Duo's baby but evil Hilde steals it with a butter knife."
Warning: Not work safe. Or brain safe. May cause brain splodey.
Warning: Not work safe. Or brain safe. May cause brain splodey.