Angel Baby
The Winchesters find out that angels do not reproduce the same way as humans. Castiel explains that his vessel is not pregnant, his grace is simply splitting to make a new angel. But, even the best laid plans sometimes go awry, especially when Heaven is involved. When the Apocalypse is nigh and Castiel is running out of grace, Dean takes it upon himself to take care of Castiel and the new angel and ends up learning how much the angel means to him. Sam on the other hand, must make amends with his own angel.