I am looking for a series where willow casts a love spell on spike/Buffy which causes spike to become human and Buffy to become pregnant.

Hmmm - in my "Whatever it Takes" Willow sends Spike and Buffy to an alternate dimension where he is human and when they come back, she is pregnant. Is that the one? No love spell involved, though.

There is a unfinished baby fic story on ff.net called: If I Can't Love Her by JennJ


Summary: This takes place a few months after 'Dead Things' Spike and Buffy are still going at it with only Tara knowing. She wants to help Buffy so she casts a spell with unexpected consequences.

In this story Tara turns Spike human for a brief time, one night while Buffy and him were doing it and didn't know it.

Luckily the story ends, where all there is left to do is a chapter or two, of wrapping it up.