Fic begins while Buffy is dead, but she is resurrected and Spike is the one who finds her when she climbs out of her grave. She becomes very attached to him and they begin a relationship. The gang find out about (canon) Darla's pregnancy and, in researching it, Giles discovers that, before she died, Darla was a potential, and that Aurelian vampires are able to impregnate human women that were slayers or potential slayers, with there being several records of this happening throughout history. Buffy then gets pregnant with Spike's child.
I remember one specific scene in which Buffy tells Spike she's pregnant by adding their names to the list of the recorded resurrected slayers/potentials that were impregnated by vamps, and leaves the list on Spike's pillow.