My baby has been since she was a little over a week old. (awake or asleep) I personally don't believe it's due to gas, only because if they can be sad and cry, why can't they smile and be happy.
I put Lily on the changing table yesterday and looked down at her....she looked up at me and gave me the most beautiful big smile...then let one rip LOL it was hilarious!! I was like "awww how cute...oh gross that stinks" LOL
Kaelynn smiles when I talk to her. She will look right at me and smile. The first time she made eye contact and smiled I cried.
Now that I know is untrue because if your baby isnt smiling by 3 months doctors consider it a developmental issue. Look it up.
Babies start to smile around 3-4 months.
I believe by 4 weeks babys start actually smiling rather than smiling from gas. Not 100% sure though.
Most smiles at that age a due to wind in their tummy after a feed, or anytime lol. They are still so cute tho. This is only from my experiance with my children.