Baby’s Developmental Milestones — Explained

New moms worry. A lot. But if your little one isn’t walking as quickly as her brother did, don't worry too much. “Generally, babies develop at their own rate,” says pediatrician Dr. Tanya Altmann, author of Caring for Your Baby and Young Child: Birth to Age Five. “If your baby is just missing one milestone, that’s usually not a major problem. But if she’s missing more than one, across different developmental areas—social, cognitive, motor skills—then that signals there’s something else going on. Let your pediatrician know, because early diagnosis is important.”

To keep worries at bay, we’ve asked Dr. Altmann to weigh in on some of your baby’s key milestones through the first months of life. Take a breath, and use these as guidelines, not strict rules.

Key milestones: Develops strong reflex movements, moves head from side-to-side while on stomach, will recognize some sounds.

Tell your pediatrician if your baby … doesn’t respond to loud noises, doesn’t move arms and legs much, sucks poorly, doesn’t blink in bright light, has his or her lower jaw trembling constantly.

Dr. Altmann says: “A big milestone at two months is the social smile. They’ll start responding to outside sources, like mommy and daddy’s voice.”

Key milestones: Opens and shuts hands, grasps and shakes toys, recognizes familiar faces, smiles at voices, begins to imitate and turn head toward sounds, enjoys play.

Tell your pediatrician if your baby … doesn’t follow moving objects with eyes, doesn’t grasp and hold objects, doesn’t smile at people, cannot support his or her head well.

Dr. Altmann says: “If she doesn’t start interacting with a smile by three months of age, let your pediatrician know. It could signal a more serious developmental problem.”

MORE: Some Additional Milestones the Books Don't Mention

Key Milestones: Raises head and chest when lying on tummy and supports upper body with arms, follows moving objects with eyes, interacts more with surroundings, vocalizes and babbles.

Tell your pediatrician if your baby … doesn’t pick up head while on his or her tummy, does not turn head or find the source of sounds, has trouble getting objects to mouth, doesn’t show affection toward mom and dad.

Dr. Altmann says: "By four months, babies have usually begun pushing their chests and heads up using their hands when they’re lying on their tummies. It shows they’re gaining muscle control. Give them plenty of tummy time, but never leave them alone on a flat surface, especially if it’s raised. If they’re not picking up their head, tell your doc.”

Key Milestones: Laughs frequently, makes squealing sounds, reaches for objects on his or her own, grasps and shakes toys, captivated by own face in a mirror, responds to others' feelings, finds partially obscured objects.

Tell your pediatrician if your baby … doesn’t start to bear weight on legs, can’t sit up on her own, doesn’t smile unprovoked, does not laugh and squeal.

Dr. Altmann says: "Your baby is growing and developing rapidly by six months. She should be rolling over at six months. If she’s not, you should mention it to your pediatrician. If she’s not sitting on her own by eight months, it could signal a developmental delay and she could need some physical therapy to strengthen muscles.”

Key Milestones: Reaches for and holds small pieces of food, finger feeds, pulls self up to stand, walks a few steps on his or her own, crawls on belly, responds to yes, calls you “mama,” begins to use objects the right way, imitates your actions.

Tell your pediatrician if your baby … doesn’t crawl, cannot stand with support, doesn’t use single words like “mama” or “dada,” doesn’t gesture at objects or images.

Dr. Altmann says: "If a baby can’t grasp or feed himself small amounts of food by 12 months, he may need some occupational therapy. Pulling himself up to stand is another big one. It lets us know everything is developing and they can identify sturdy objects in their surroundings. Let your doctor know if he isn’t doing this.”

Key Milestones: Begins to walk alone, pulls toys along behind body, says first words, walks up and down stairs with help, climbs on furniture, builds with toys, uses few-word sentences, loves other children.

Tell your pediatrician if your baby: Cannot walk by 18 months, does not imitate actions, doesn’t use at least 15 words, doesn’t use few-word sentences, does not recognize common objects’ functions.

Dr. Altmann says: "This age is a huge period for both motor skills and social development. Let your doctor know if your baby isn’t walking or doesn’t seem to interact well with others.”