Hollywood's worst kept secret has finally been confirmed ... Jessica Simpson IS pregnant.

The 31-year-old songstress Tweeted the above photo of her pregnant belly all wrapped up ... and the following statement:

    "It's true – I'm going to be a mummy!"

Aww ... so appropriate for Halloween! For weeks now, the pop diva has been sporting an obvious baby bump, but has refused to comment on whether or not she's expecting.

This will be the first baby for Jess and her fiance Eric Johnson.