Baby Development: The First Year

The first year of your baby's life is a period of remarkable transformation. In just twelve months, your helpless newborn will grow into a mobile, communicating little person with a distinct personality. Understanding developmental milestones helps you appreciate your baby's growth, know what to encourage, and recognize when to seek guidance—all while remembering that healthy babies develop at their own pace within a wide range of normal.

How Development Happens CDC

Development in the first year occurs across several interconnected domains. Physical development includes motor skills like rolling, sitting, and walking. Cognitive development encompasses problem-solving, memory, and understanding cause and effect. Language development progresses from coos to babbles to first words. Social-emotional development includes attachment, emotional expression, and social engagement. CDC

These domains don't develop in isolation—they build on each other. A baby needs adequate physical control to sit up and manipulate objects, which enables exploration that drives cognitive development. Social engagement motivates language development. The CDC emphasizes that this interconnection is why developmental delay in one area can sometimes affect others. CDC

Every baby follows their own developmental timeline within a range of normal. The milestones described here are based on averages—50% of babies reach them by the listed age, meaning 50% take longer. Some variation is completely normal. However, patterns of delay across multiple areas, or regression (losing skills previously acquired), warrant professional evaluation. AAP

Birth to Two Months CDC

In the earliest weeks, your newborn is adjusting to life outside the womb while their brain undergoes rapid development. This is a period of reflexes, growing awareness, and the beginning of attachment.

Physically, newborns have limited voluntary control but demonstrate important reflexes that will gradually disappear as voluntary movement develops. They can turn their head to both sides while lying on their back, bring hands to mouth, and briefly lift their head during tummy time. CDC

Visually, newborns see best at about 8 to 12 inches away—roughly the distance to a parent's face during feeding. They prefer high-contrast patterns and human faces. By six to eight weeks, many babies begin tracking moving objects with their eyes and may follow your face as you move. AAP

Socially, the most exciting development of this period is the emergence of the social smile around six to eight weeks. This intentional smile in response to faces—different from the reflexive smiles of early weeks—marks the beginning of true social engagement. Babies also begin to coo and make vowel sounds, the earliest precursors to speech. CDC

Warning signs in this period include not reacting to loud sounds, not following moving objects with eyes by six to eight weeks, not bringing hands to mouth, or appearing very stiff or very floppy. These warrant discussion with your pediatrician. CDC

Two to Four Months AAP

This period brings delightful increases in social engagement and physical capability. Your baby is becoming a more active participant in the world.

Physically, babies gain significant head and trunk control during these months. Most babies can hold their head steady when upright and push up on forearms during tummy time. By four months, many babies can push up to elbows and may begin rolling from tummy to back. They swipe at and grasp dangling objects with increasing accuracy. AAP

Socially, this is an engaging period. Babies smile spontaneously and specifically at familiar people. They babble with expression, as if having a conversation. They study faces intently, calm when talked to, and begin showing clear preference for familiar caregivers. The AAP notes that this emerging attachment is a crucial foundation for emotional development. AAP

Cognitively, babies show increasing interest in their environment. They track objects across their visual field, turn toward sounds, and begin to anticipate routines (like becoming excited when they see a bottle or breast). They're discovering their hands—watching them, bringing them together, and using them to explore. CDC

Warning signs to watch for include lack of social smile by three months, not following moving objects with eyes, not grasping or reaching for objects by four months, or not cooing or making sounds. These concerns should be discussed with your doctor. CDC

Four to Six Months CDC

The second half of infancy begins with an explosion of new skills. Your baby becomes increasingly mobile, interactive, and expressive.

Physically, most babies can roll both ways by six months and sit with support (and many briefly without support). They reach accurately for objects and transfer them between hands. Tummy time shows significant strength—babies push up on extended arms and may rock on hands and knees, preparing for crawling. The CDC notes that these motor skills enable new ways of exploring and learning. CDC

Language development accelerates during this period. Babies babble with increasing variety of sounds, including consonants (ba, da, ma). They respond to their name, turn toward sounds, and show understanding of emotional tone in voices. Many babies begin making sounds back and forth with parents—early "conversations" that build language foundations. AAP

Socially, babies show clear attachment to caregivers and may begin showing wariness of strangers. They laugh freely, express a range of emotions, and love face-to-face games like peek-a-boo. They reach for familiar people and may cry when a parent leaves the room. CDC

Cognitively, babies demonstrate growing understanding of objects and their properties. They mouth, shake, and bang objects to explore them. They show interest in mirrors (though they don't yet recognize themselves). They begin understanding simple cause and effect—shake the rattle, it makes noise. AAP

Warning signs at this stage include not reaching for objects, not responding to sounds or voices, not making vowel sounds, not showing affection to caregivers, or difficulty getting objects to mouth. Discuss these with your pediatrician. CDC

Six to Nine Months AAP

This period often brings the most dramatic changes in mobility and independence. Many babies transition from stationary to mobile during these months.

Physically, sitting independently is typically achieved by seven months. Crawling begins for many babies during this period, though the AAP emphasizes that some babies skip crawling entirely—scooting, rolling, or going straight to walking—and this is normal. AAP Babies develop the pincer grasp (thumb and forefinger) for picking up small objects, a major fine motor milestone.

Cognitively, object permanence develops—babies understand that things exist even when hidden. This leads to searching for dropped toys and increased separation anxiety (if the parent disappears, they still exist and might not come back!). Babies imitate sounds and gestures, show preferences for certain toys, and demonstrate problem-solving like removing obstacles to reach desired objects. CDC

Language continues progressing with more varied babbling, often sounding like actual speech in rhythm and inflection. Many babies say "mama" and "dada" nonspecifically during this period. They understand "no" (even if they don't always comply) and respond to simple requests paired with gestures. AAP

Socially, stranger anxiety often peaks during these months, with babies showing clear preference for familiar caregivers and wariness of unfamiliar people. They play interactive games like peek-a-boo and pat-a-cake, show clear emotional attachments, and may have favorite toys or objects. CDC

Warning signs include not bearing weight on legs when held upright, not sitting with help, not babbling, not responding to own name, not recognizing familiar people, or not looking where you point. Discuss concerns with your doctor. CDC

Nine to Twelve Months CDC

The final months of infancy bring your baby to the threshold of toddlerhood. Communication, mobility, and independence all take major leaps forward.

Physically, most babies can pull to stand and cruise along furniture. Some take their first independent steps during this period, though walking anytime up to 15-18 months is normal. The CDC notes that fine motor skills are refined—babies use pincer grasp with precision, can put objects into containers and take them out, and begin using tools like spoons (messily). CDC

Language development often includes first true words during this period—words used consistently and with meaning. Babies understand many more words than they can say. They follow simple instructions ("Give it to me"), point to things they want, and use gestures like waving bye-bye and shaking head for "no." The AAP emphasizes that receptive language (understanding) typically develops ahead of expressive language (speaking). AAP

Cognitively, babies demonstrate impressive problem-solving. They figure out how toys work, imitate complex actions, explore objects in many ways (shaking, banging, throwing, dropping), and show clear preferences. They begin pretend play, like pretending to drink from a cup or talk on a phone. CDC

Socially, babies show strong attachment to specific caregivers and may show distress when separated. They play interactive games, repeat actions that get a response, and show beginning empathy (becoming distressed when another baby cries). Many develop comfort objects like a special blanket or stuffed animal. AAP

Warning signs at this age include not crawling, not standing with support, not searching for hidden objects, not saying single words, not pointing to show things, not using gestures like waving, or losing skills previously acquired. These warrant prompt evaluation. CDC

Premature Baby Milestones AAP

For babies born prematurely, milestones should be evaluated using "adjusted age" or "corrected age"—the age your baby would be if born on their due date. For example, a baby born two months early who is currently six months old has an adjusted age of four months, and milestone expectations should reflect the four-month level. AAP

Most pediatricians continue using adjusted age until about two years, when most premature babies have caught up with their full-term peers. However, some premature babies, particularly those born very early or with complications, may have ongoing developmental differences that need continued monitoring. AAP

When to Seek Evaluation CDC

The CDC's "Learn the Signs. Act Early" program emphasizes that early identification of developmental delays leads to better outcomes because early intervention services are most effective during the brain's most plastic developmental periods. CDC

Trust your parental instincts. If you have concerns about your baby's development—even if you can't pinpoint exactly why—bring them up with your pediatrician. You know your baby best, and that intuition matters.

Specific warning signs that warrant prompt evaluation include loss of skills your baby previously had (regression), not meeting multiple milestones for their age, and specific red flags like no social smile by 3 months, not reaching for objects by 4-5 months, not babbling by 9 months, or not responding to name by 12 months. CDC

Supporting Your Baby's Development AAP

You are your baby's first and most important teacher. Simple daily interactions powerfully support development.

Talk to your baby constantly—narrate what you're doing, describe what you see, respond to their sounds as if having a conversation. This builds language foundations even before babies understand words. The AAP emphasizes that the quantity and quality of language exposure in the first years significantly impacts later language development. AAP

Read to your baby from the earliest weeks. Even before babies understand stories, they benefit from hearing language, seeing pictures, and experiencing this special interaction with caregivers. AAP

Provide tummy time daily, starting from birth. This builds the core and upper body strength needed for sitting, crawling, and walking. If your baby protests, start with short sessions and work up gradually. CDC

Follow your baby's lead in play. Watch what interests them and build on it. Respond to their cues. This responsive interaction supports cognitive, social, and emotional development. AAP

Limit screen time. The AAP recommends no screen time for babies under 18-24 months except video chatting with family. Real-world interaction with caregivers is what builds developing brains. AAP

The Bottom Line

Your baby's first year is an incredible journey of development across physical, cognitive, language, and social-emotional domains. Understanding typical milestones helps you appreciate this growth and know when to seek guidance—while always remembering that healthy babies develop at their own pace.

The most important thing you can do is engage with your baby. Talk, read, play, and respond to them. Your everyday interactions are the richest developmental environment possible.

Clara is here to help you understand what you're observing in your baby's development, troubleshoot concerns, or decide when it's time to talk to your pediatrician. Don't hesitate to ask.