Spencer, 29, made his fictional suggestion about the former couple known last night (7 August) while taking part in the Twitter hashtag '#confessyourunpopularopinion'.
It comes after MTV revealed that US fans would be treated to a different ending to The Hills, which will air in the States tomorrow night (9 August).
Spencer, who is married to Lauren's ex-best friend Heidi Montag, tweeted: "Alternate ending for #TheHills = LC pregnant with #theprinceofmalibu @BrodyJenner baby! #confessyourunpopularopinion."
One fan replied: "@spencerpratt HAHA, this would be an amazing ending! The viewers would be greatly shocked!"
Another follower said: "Not gonna lie that would've been amazeballs."
But one Hills fan suggested a twist on Spencer's made-up idea: "Or pregnant with your baby. Now that would have been a shocking ending!"
Lauren - who was arch enemies with Spencer - left The Hills midway through season five and it eventually finished following season six with Brody standing in a studio lot, hinting that the show had been scripted.
Lauren and Brody have not responded to Spencer's tweet.
Heidi Montag's rise to fame in pictures: