If your baby isn't hitting these marks, or at any point seems to be losing skills, talk to your doctor. She may refer you to an expert trained in evaluating developmental delays.
By 3 months: Your baby should begin to develop a social smile, smile at the sound of your voice, watch faces intently, coo, and make other noises.
By 6 months: Your baby should recognize you and seem happy to see you, seem interested in different sights and sounds, and begin to babble.
By 9 months: He should engage in and enjoy back-and-forth interactions, smiles, and other facial expressions — and respond to his name.
By 12 months: Your baby should point to and reach for things, wave, and say one word in addition to "mama" and "dada".
By 18 months: He should say 10 to 25 single words, point to objects that interest him, and bring things to show you.
By 24 months: Your baby should say at least 50 words and use two-word phrases ("Doll mine," "Daddy go").