The 28-year-old showed off her fake bump in a bright yellow mini dress as she filmed a number of scenes.
Joined by co-star Emile Hirsch, the actor was seen cradling her bump and ranting at a co-star, before fiercely flicking her hair as she waltzed off in the dramatic moment.
Once Upon A Time is Tarantino’s hotly tipped new project, following the success of The Hateful Eight in 2015.
The film is a semi-fictional interpretation of ’60s Hollywood, and tells the story of the murder of pregnant actress Sharon Tate and four others by the Manson family.
DiCaprio plays Tate’s neighbour Rick Dalton, a fading former star of a western TV series, while Pitt plays his stunt double, Cliff Booth.
The film will be out in Australian cinemas in July next year, just shy of the 50th anniversary of the murders back in 1969.