Beyonce & Jay-Z -- Prego and Ballin' for 30th Birthday Bash in Venice

Beyonce celebrated her 30th birthday just like every other expectant mom -- ballin' her face off on a massive yacht in Venice, Italy!

B and Jay-Z held court aboard the super luxurious yacht "Odessa" ... for her big 3-0 celebration -- which included visits from celeb pals Gwyneth Paltrow and Erykah Badu.

Beyonce was seen on-deck checking out her not-yet-visible baby bump.