A 10-week fetus, ie 20 weeks of gestation, can hear [1], suck, swallow. I would not say it's obvious they can't experience "mental states"; especially not as you appear to be arguing almost all animalkind can experience those states.
Still, physiological responses are not the best indicator of mental states - plants and primitive animals (from sponges to bivalves) have them too, despite lacking the neurological development necessary for (or evolutionary need for) "consciousness".
And I agree that physiological responses aren't the best way to show mental states. I cry sometimes for no reason whatsoever, my eyes just leak; you can't infer fear just because a creature backs away from fire, either.