Don't know if this is the one you want but it sounds a little like William's Girl fic,
As You Wish.Buffy does get pregnant in the fic with Spike's baby. She works in the hospital with Parker (not Riley) who is jealous of Spike. Riley is in the story but he is mentioned as someone with a crush on Dru. A crazy mental patient that thinks he's a vampire. Dru is Spike's sister and she's psychic in it. There is even a scene where Parker tries to give Buffy a bottle of prenatal vitamins but Dru knocks it out of his hands.
If you think this could be the story you want its at :
AS YOU WISH----by: WILLIAM'S GIRLhttp://web.archive.org/web/20030623232024/http://www.spuffyfantasy.com/or
http://web.archive.org/web/20041012014930/members.cox.net/williamsgirl/AsYouWish1.htm lSummary: Buffy is afraid of relationships after her marriage ends in disaster. William is hired to make her birthday special. When they meet sparks fly and Buffy struggles to keep her committment to never love again.Drama, intrige, lots of Spuffiness and even a crazy Dru!