Hey looking for Fics where Buffy goes back in time and meets William. When she returns to the future Spike remembers it like an event from his past. Mainly looking for fics where she becomes pregnant because of William but that doesn't matter as long as Spike remembers their encounter from when he was still William.

You may have read this one
Legions of True Hearts by Eurydice
Post graduation, a restless Buffy goes to London to escape the memories of Angel, only to lose herself in unexpected dreams. But is William real, or is he a distraction? Is he, possibly...both? Buffy/William
http://www.dark-solace.org/elysian/viewstory.php?sid=431

There's a sequel A Symphony of Echoes

I love William stories and the first two are two of my absolute favourite fics...

Forward to Time Past by Unbridled _Brunette
The victim of a spell gone awry, Buffy finds herself trapped in Victorian London, where she meets a man that is surprisingly familiar to her.

Hearts Breaking Even by slaymesoftly
A very young Buffy jumps through a portal to chase a demon and ends up saving a young boy...in 19th century England. When a certain blond vampire appears in Sunnydale years later, things get interesting...

Only Time Will Tell by dampersandspoons (inc baby)
A year after the Hellmouth collapsed, Buffy and co., who now reside in London, are faced with an interesting mission. They must retrieve an artifact, however, this one was destroyed over a hundred years ago. In order to retrieve it, she and her watcher must travel back in time to the late 1800's, to the precise location where William lived before he was turned. Will she risk changing history to seek out the one she loves or will she be put in a completely different situation than she'd anticipated in the first place?

Imitation of a Man by darkapple (bit different because Spike is with her)
The events of "Dead Things" plunge Buffy and Spike, via time travel, into Spike's human past in an experience that transforms Buffy's view of him. The action begins with Buffy's return to the police station alley where, minutes before, she left Spike beaten and bloody.

Past Present by jamesmarstersjunkie
This is set in S2. Buffy gets the flu but does not face the Der Kinderstaut demon. She instead falls into a state of suspended animation and her seletial body is taken to a place in which she is free to just be her self. There, she finds a familiar face but one who does not know her. He is a young man with striking blue eyes and a kind smile. Will thier meeting change the course of both thier futures in the real world...? Let's find out.

A Wish Your Heart Makes by Alexannah (inc baby)
"No matter how your heart is grieving / if you keep on believing / the dream that you wish will come true"
When Buffy falls for William, a man she meets in her dreams, she has no idea she is about to meet his modern, vampire counterpart. But the line between William and Spike is more blurry than she might think.

It's been a while since I read it and there's no pregnancy, but I think this fits your description.

Things Present – Things Past by Estepheia and Marcee
Two timelines. Two Slayers. One problem! How does Buffy get back to her present time when she's sucked into the body of a Slayer in 1880? Can another Scooby who tries to rescue her survive the past? Plus, meet William and witness Spike's big decision!
Post-Life Serial, Rated R, Spike/Buffy, 141908 words.

William is brought forward briefly in this one so a side step to the request but you may still like it. The aspect of Spike's return is nicely done and although the sequel only has the first chapter, is a WIP, it is a nice future glimpse follow-up as is regardless.

Pursuits of Happiness by facingthesun sequel Founding Father
After being called on by the Council, Buffy is off to Brazil to help a Watcher learn about the 'dreaded' William the Bloody.

Yours, William by Puddinhead is an awesome time travel William/Buffy fic. Very well done.
Guess this doesn't meet your requirements for going back to the future or Spike interaction but still a great read.

Thank you all. I forgot I posted this. and suprisingly I have not read Legions or it's sequel yet but I will start now.