#mpreg (maybe)

Sam can't get pregnant after coming back from the cage no matter how hard they try. He starts hiding it after the first "failed" attempt because he won't be able to handle Dean's heartbreak once more. Sam needs to be sure it's going to be certain before telling his brother. Instead, he tells Dean the tests come back negative because that might be the outcome at some point, so why should Sam bother getting his brother's hopes up? It sure is a hassle to hide everything from Dean, and it's going to backfire as all the secrets they keep from each other do. But Sam believes this is better than the truth.