I don't know if anyone has ever done a story from this idea. I'm not up to date on all the Buffy fanfiction out there. But I guess it won't hurt to put this out there.

The idea is that Angel was turned human in LA and had his time with Buffy in the sun. But as normal he had everything changed back. Except one little fact. Turns out he got Buffy pregnant then and with all the time changing and memory erasing that didn't get changed.....or couldn't be changed.

So unbeknownst to Buffy when she went back to Sunnydale she was pregnant.When she finds out weeks/months later she's in shock. Not knowing how it happened Buffy is freaked out but Willow test show the baby is 100% human.

But here's the weird thing.....with Angel being the bio father Spike's demon subconsucily picks up the family bond. So for unknown reasons to him Spike is very protective to pregnant Buffy. 

Plus a funny thing would be while in the womb the baby can pick up Spike's family vibes also. So the baby is more calm when Spike is around making Buffy be around him to keep the baby still. Plus when born everyone else is jealous when the baby/toddler makes Spike his/her most favorite person.