Brian MPREG/Young!BJ
I'm trying to find a fic I read awhile back where Brian and Justin are at a camp or something, Brian is pregnant but planning to give up the kid. He eventually decides to keep it and then, possibly in a sequel, gets pregnant with twins.
This isn't Liberty Ranch. I recall Brian not wanting anyone to touch his belly while he was pregnant. At one point, he has a dream that he and Justin have a house filled with kids and it freaks him out. In the dream, Justin touches his belly and tells him he can't wait until those babies are out so they can start making more.