Babies are capable of smiling at birth. At first, these expressions are not truly “smiles” in response to social interactions or events. During the first 2 months babies are learning the meaning of these smiles, and by 3 months most will engage in a type of “smile talk” by starting little “conversations” with a smile (and other enthusiastic gestures).
Smith, P. K., Cowie, H., & Blades, M. (1998). Understanding children’s development (3rd ed.). Malden, MA: Blackwell Publishers.