Jennifer Aniston -- Is THAT a Baby Bump???

Jennifer Aniston Is THAT a Baby Bump?!?!?

Here it is ... undeniable proof that Jennifer Aniston is finally PREGNANT ... or not pregnant ... depending on if you can see a baby bump in this photo!!!

The former "Friends" actress was all smiles on her way into a spa in Beverly Hills this weekend ... with what appears to be signs of a "stomach" showing underneath her shirt.

Question is ... is her belly full of fetus??? Or perhaps she had a big lunch?? Or is she still really super-skinny and it's all just an optical illusion created by a couple of ill-placed wrinkles in her shirt?

Only Jen knows for sure ... but we should totally guess anyway -- so, we gotta ask ...