MILA KUNIS is either pregnant or should've known better than to wear a horizontally-striped shirt after eating a big lunch.

--On Saturday afternoon, paparazzi took some photos of Mila and ASHTON KUTCHER out walking around . . . and Mila looks like she's got a belly.  And so, of course, everyone immediately jumped on the "Could she be pregnant?" train.

--We're a little more skeptical.  On October 19th, Mila was photographed out wearing a shirt that showed her stomach . . . and it looks completely flat.  Would such a significant baby bump appear in just eight days?

--We're going with the "she ate a big lunch and wearing horizontal stripes" theory.