Well there's Niamh's Origins series but that's a long one and not particularly funny. It's really well written though. Also, the baby isn't theirs, it's Angel's. It is also not the main point of the story.
Origins by Niamhhttp://spikeluver.com/SpuffyRealm/viewstory.php?sid=6300&warning=NC-17Baby Love by NiamhAll human baby story. I really liked it, but again it's all human and the baby isn't Spike's.
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=10127There's mine, but it's not finished yet. (forgive me for mentioning my own here, but...). It's also all human though.
The Song Remains the Same by SMachttp://spikeluver.com/SpuffyRealm/viewstory.php?sid=17766I know there are baby stories in canon that are actually Spike's but my memory fails me tonight. I'll keep thinking on it.