Amanda Seyfried -- OMG! Alec Baldwin's Baby Is ...

So, Alec Baldwin and Amanda Seyfried run into each other on the streets of NYC ... and she catches a severe case of shock-face -- question is: why?

Here's the scene ... Amanda and her bf Justin Long were walking their dog today ... when they bumped into Alec, his wife Hilaria and their newborn daughter Carmen.

It could be ... "Wow, your baby is adorable!"