(found) Stanford Era Pregnancy Spell
I read it a few years ago but can't for the life of me find it!
It was kind of mpreg but not the typical deal. For starters there was no romance, no smut or any slash; the only relationship in it was platonic between brothers. Sam was at Stanford when he was hit with a spell by a woman who couldn't have children (i think she found the spell at some kind of new-agey shop?) I forgot some of the details but it somehow targeted Sam resulting in a fake rapid pregnancy along with all of the symptoms. Dean hadn't heard from Sam for quite awhile but he got a message from him asking Dean to come to Stanford to help with his "issue".