Liv, 37, announced last month that she was expecting a baby with her boyfriend, British sports agent David Gardner, a close pal of David Beckham.
She already has a nine-year-old son, Milo, from her relationship with musician Royston Langdon, although he doesn't appear in the pictures the actress chose to upload to Instagram.
David Gardner, also absent from the snaps, has two children from a previous marriage.
We have to admit, our favourite part of the photo - apart from its sheer unexpectedness - is the reaction of the kid behind, to the right of Liv.
It's hard to tell whether he's gasping in shock that one of Hollywood's most beautiful women would allow herself to be photographed in such a dorktastic costume, or just yawning because his Halloween party appears to be taking place at an undecorated sit-down restaurant.