sibukh,
You can search for the details on this forum.
the jist of the matter is this :
As long as the baby comes back to US along with PARENTS 1st trip back to US, the baby will get the GC stamp at the airport.. The parents 1st trip can be within 2 yrs of birth of the baby.
The key word is the 1st trip of Parents, so dont screw up on that...