The only Ties That Bind I know is by rainsrabble but there Spike goes to the Scoobs to help stop Dru doing a ritual to return Angelus that would open the hellmouth and it turns out to be a prophecy that binds Spike and Buffy with a mating claim I think. It is Glory timewise rather than initiative and although Dawn appears there is no baby and I don't think she is said to be Buffy/Spike's either. There could, of course, be another by the same title.
I think it is the Origins series you are looking for too.
