The actor was spotted at a Chuck E. Cheese restaurant Saturday with none other than his on-again, off-again girlfriend, Mia -- who might actually still be his wife, but more on that later -- where she looked very pregnant as they hit the arcade games together.

We're told it was just them there ... no other kids around. But, based on Mia's more-than-prominent bump, we'd guess she's got a bun in the oven -- and at least 4-5 months along. Disclaimer: We ain't doctors.

It's a little strange that 2 grown adults -- and potential parents-to-be -- would be gaming it up at a place for children, but maybe they're just getting ready for what's to come and preparing early. In any case, some congratulations could be in order here.

Shia and Mia have recently reconciled ... being spotted out together and getting cozy since at least last year -- this after his brief stint with FKA Twigs. They've even been seen rocking their wedding bands anew ... something most everyone thought was done with after 2018.

You'll recall ... SLB and MG got hitched way back in 2016, during a Vegas ceremony, but appeared to file for divorce a couple years later ... as each party was seen dating other people thereafter. It's unclear if they ever finalized, though, and quite possibly didn't.

If that is, indeed, the case ... then they could be married right now -- and not just that, may well be on first-time mom and dad duty in the near future. Neither of 'em have kids ... this would be their first -- again, operating under the presumption they are still romantically involved at this point ... which, by most accounts, they certainly appear to be.