In developmental terms, infancy is usually defined as which period of life?
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Human Development · Foundations
Infancy
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Infancy The developmental period from birth until about age two, characterized by rapid growth and change (working definition per NICHD). Full entry → is the developmental period from birth to about age two, a stretch of rapid growth and change with no equal later in life. Within roughly 24 months, a mostly helpless newborn learns to roll, sit, crawl, pull up, and take first steps; to use hearing, vision, and touch with growing skill; and to smile, babble, and say First words The first meaningful single words, such as 'mama' or 'dada' used for a specific person, which typically appear around the first birthday. Full entry →. Piaget called this era the Sensorimotor stage Piaget's first stage of cognitive development, from birth to about age two, when infants learn about the world through their senses and actions. Full entry →. Milestones are typical ranges, not deadlines: most children (75% or more, per the CDC) reach each one by a given age, but timing varies.
Why this matters
Every later chapter of development — early childhood, the school years, adolescence, adulthood — rests on foundations laid in the first two years. Knowing what infancy looks like helps parents, caregivers, teachers, and health workers tell ordinary variation apart from something worth checking with a pediatrician, and it gives students the anchor they need for stage theories like Piaget's. Studying infancy honestly also builds a habit that carries forward: describing what most children can do by a given age without turning ranges into deadlines or alarm.
The college version
What Infancy Is
In developmental terms, infancy is the period from birth until about age two. The U.S. National Institute of Child Health and Human Development (NICHD) defines it as the period from birth until age 2 years, a time of rapid growth and change. Everyday speech sometimes uses "infant" for the first months or year, and a few stage theories end infancy at twelve months, but the two-year window is the standard framing in lifespan development; ages one to two are often called the toddler years. Infancy sits between the prenatal stage that precedes it and early childhood that follows it. Attachment, cognitive development, and language development each get their own topic in this course; this lesson maps the whole period.
Growth and Motor Milestones: The Body Leads
The most obvious feature of infancy is physical change: clothes are outgrown in weeks. Motor development follows a predictable but flexible sequence, and the CDC's updated checklists give typical ranges rather than deadlines. Most babies hold their head steady by about 4 months; roll from tummy to back by about 6 months; sit without support by about 9 months; pull up to stand and walk holding onto furniture around 12 months; and take a few steps on their own around 15 months. Crawling is common — MedlinePlus lists it among typical 9-month abilities — but the CDC's current checklists do not require it, since some babies never crawl and move straight to pulling up and cruising. OpenStax makes the same point differently: the average age of first walking is about 12 months, yet 10 months and 14 months can both be normal, and child-rearing practices across cultures shift the timing of sitting, crawling, and walking. The CDC defines milestones as things most children — 75% or more — can do by a certain age: a statistical guide, not an exam.
The Senses: A World That Comes Into Focus
Babies are born with working senses, but they are not equally mature. Per MedlinePlus, hearing begins before birth and is mature at birth — newborns prefer the human voice — and touch, taste, and smell are mature at birth too. Vision is the late bloomer: a newborn sees clearly only within about 8 to 12 inches, roughly the distance to a caregiver's face during feeding; color vision develops between about 4 and 6 months; and by 2 months infants track moving objects and prefer faces. The CDC's checklists show the same pattern in behavior: a 2-month-old reacts to loud sounds; a 4-month-old turns his head toward your voice and watches you as you move; a 6-month-old mouths objects to explore them. With touch and taste mature, the mouth is one of an infant's best research instruments.
The First Cognitive Leap: Piaget's Sensorimotor Stage
Jean Piaget's theory of cognitive development calls infancy's first stage the sensorimotor stage, lasting from birth to about 2 years. During this stage, children learn about the world through their senses and motor behavior — they are not yet thinking in symbols or words. A four-month-old who shakes a rattle, bangs a block, and mouths everything in reach is not being random; she is running experiments on sound, texture, and cause and effect. Piaget's best-known marker is Object permanence The understanding that an object continues to exist even when it is out of sight, which typically develops between about 5 and 8 months of age. Full entry →, the understanding that an object still exists when out of sight, which typically develops between about 5 and 8 months. Before that, a toy hidden under a blanket has, for the infant, effectively vanished. This stage sets up the later leaps — pretend play, language, logical reasoning — that the cognitive-development topic covers in depth. For this lesson: infants think by doing.
Social Beginnings: From Smiles to First Words
The social life of an infant is built from small, momentous firsts, and the CDC's checklists trace the arc. Around 2 months, most babies smile when you talk to or smile at them — a social smile aimed at faces. By 4 months, many smile on their own to get your attention, an early bid for connection. Language begins as sound play: 6-month-olds take turns making sounds and blow raspberries; by 9 months, most babies produce strings of repeated syllables like "mamamama" and "bababababa" — Babbling Repetitive consonant-vowel sound play, such as 'mamamama' or 'bababababa,' typically heard around 6 to 9 months before real words appear. Full entry → that sounds like language but carries no meaning yet. Around the first birthday, most babies call a parent "mama" or "dada" (or another special name) and understand "no." First words open a long road; how words become grammar and vocabulary belongs to the language-development topic. The pattern is what matters: smile, then sound play, then babbling, then words — in a reliable order with timing that varies from baby to baby.
The Reality Check: The Steepest Learning Curve
No other period of human life packs so many firsts into so little time. In about 24 months, an infant who enters the world dependent on reflexes — startle, grasp, rooting — becomes a walker, a talker, a social partner with preferences and a sense of humor. That is why infancy is often called the steepest learning curve in human life. The honest framing matters: first, it is genuinely remarkable — every later skill builds on these foundations. Second, steepness is not a reason to pressure babies or parents. Milestone ranges exist because development is variable; the CDC stresses that milestones are things most children can do by a certain age, and missing one checkmark is not a diagnosis. When a child seems clearly behind in several areas, the right step is a conversation with a pediatrician — early help works best. For everyone else, infancy teaches patience with process and respect for a period of change that never comes again.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Think of infancy as the body's busiest construction site. In the first two years, a baby who can barely hold up her own head becomes a walker, a talker, and a social partner. None of it happens on a fixed schedule: each baby's body and brain build skills in its own order and pace. That is why professionals talk about typical ranges — most children by nine months, most children by twelve — rather than exact due dates. The point of watching milestones is not to grade babies but to notice progress, and to catch early, with a pediatrician's help, the rare child who is not moving forward. Infancy is a steep learning curve, but it is a climb each baby makes at her own speed.
Picture it like this
A baby's first two years are like a smartphone's first start-up: a burst of updates installs in a short window — the sensors calibrate, the motion controls come online, and the system begins recognizing faces and voices. Each device finishes the update at its own pace; one unit's camera app comes online before another's, and a restart is rarely needed.
Where the picture stops working
Unlike a phone update, infant development is not a software install with a fixed checklist. Babies grow through real experience, practice, and warm care from adults, and skills build on each other unevenly — a baby may talk early and walk late. A phone that lags gets a patch; a baby who seems behind usually needs patient support and sometimes professional guidance, and often catches up on her own. And a phone starts with all its hardware ready, while a baby's brain and body are still forming as they go.
Worked example
Mateo Reyes is eight months old. At his 6-month checkup he rolled from tummy to back, and by seven months he sat leaning on his hands. At ten months he sits without support, gets into a sitting position by himself, and has started pulling up to stand. He does not crawl — he scoots on his bottom and rolls toward toys instead. His cousin walked at eleven months; Mateo, at fourteen months, still mostly cruises along furniture. Reading this against the typical ranges: Mateo's sequence matches the CDC's checklists (sitting by 9 months, pulling up around 12 months, first steps around 15 months), his bottom-scooting is a normal alternative to crawling, and his cousin's early walking is variation, not a standard. His pediatrician tracks the same checklists and sees steady progress; nothing about Mateo's timeline is cause for alarm.
Key takeaway
Infancy — from birth to about age two — is the period of the fastest physical and developmental change in human life: motor, sensory, cognitive, and social firsts arrive in rapid succession. Milestones mark typical ranges with variation, not deadlines, and every first skill becomes the foundation for the next.
Quick check
3 questions here, of 5 in this lesson’s practice set. Answers stay hidden until you check.
When the CDC describes a milestone as something 'most children' can do by a certain age, what does that mean in practice?
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At her nine-month checkup, Priya's daughter sits without support, gets into a sitting position by herself, and pulls up to stand, but she has not taken any steps. What should Priya conclude from the typical milestone ranges?
Choose an answer, then check it.
Study tools & related lessonsYou’ll learn to · Common mistakes · Easily confused · Key vocabulary · Related
You’ll learn to
- Define infancy in developmental terms and identify its approximate time span.
- Explain the difference between a developmental milestone and a fixed deadline.
- Describe the typical sequence of motor milestones — rolling, sitting, crawling, walking — with their typical ranges and variation.
- Apply the concept of Piaget's sensorimotor stage to a concrete infant behavior.
- Distinguish early social and communication firsts (smiles, babbling, first words) from later language development.
- Analyze why infancy is described as the steepest learning curve in human life.
Common mistakes
Treating milestone ages as deadlines, so a baby who has not walked by twelve months is assumed to be behind.
Milestones are typical ranges, not deadlines: the CDC defines them as things most children (75% or more) can do by a given age, and walking can start anywhere from about 10 to 15 months and still be normal.
Believing every baby must crawl before walking.
Crawling is common by about 9 months, but the CDC's current checklists do not require it — many babies bottom-scoot, roll, or shuffle instead, and some skip crawling entirely to pull up and cruise.
Reading a single early or late skill as proof that a baby is advanced or delayed.
Development is uneven: one baby talks early and walks late, another the reverse. Patterns across several areas matter far more than any single milestone, and pediatricians look at the whole picture.
Confusing the sensorimotor stage with an intelligence score.
Piaget's sensorimotor stage describes how infants learn — through senses and actions, not symbols — for the first two years. It is a description of thinking style, not a measure of ability.
Treating first words as the end of language beginnings.
Saying 'mama' or 'dada' around the first birthday is a start, not a finish: babbling becomes words, words become two-word phrases, and grammar takes years. That longer arc belongs to the language-development topic.
Easily confused
A developmental milestone vs. A deadline
A milestone is a typical range — what most children (75% or more, per CDC) can do by a given age — while a deadline is a fixed point with consequences for missing it. Babies are not graded against milestones.
The sensorimotor stage vs. The preoperational stage
In Piaget's theory, the sensorimotor stage (birth to about 2 years) is learning through senses and actions, while the preoperational stage (roughly 2 to 6 years) adds words, images, and pretend play — thinking without logic yet.
Babbling vs. First words
Babbling ('mamamama') repeats sounds without meaning, while first words use sounds meaningfully for a specific person or thing. Babbling typically appears around 6 to 9 months; first words around 12 months.
A newborn reflex vs. A learned movement
Reflexes such as the startle or grasp response are automatic and present at birth, while learned movements such as reaching, rolling, and stepping build through practice during infancy.
Key vocabulary
- Infancy
- The developmental period from birth until about age two, characterized by rapid growth and change (working definition per NICHD).
- Developmental milestone
- A skill or behavior that most children (75% or more, per the CDC) can do by a given age, used as a general guide rather than a deadline.
- Gross motor skills
- Large-movement abilities involving the body's big muscles, such as rolling over, sitting, crawling, pulling up, and walking.
- Sensorimotor stage
- Piaget's first stage of cognitive development, from birth to about age two, when infants learn about the world through their senses and actions.
- Object permanence
- The understanding that an object continues to exist even when it is out of sight, which typically develops between about 5 and 8 months of age.
- Babbling
- Repetitive consonant-vowel sound play, such as 'mamamama' or 'bababababa,' typically heard around 6 to 9 months before real words appear.
- First words
- The first meaningful single words, such as 'mama' or 'dada' used for a specific person, which typically appear around the first birthday.
- Reflex
- An automatic, unlearned physical response present at birth, such as the startle (Moro) reflex or the grasp reflex of a newborn's hand.
Sources & references
- CDC Learn the Signs. Act Early. — Important Milestones (index and age-specific milestone pages) — U.S. Centers for Disease Control and Prevention (CDC)
- About Infant Care and Infant Health — What Is Infancy? — Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD)
- OpenStax Psychology 2e, Section 9.1: What Is Lifespan Development? — OpenStax, Rice University
- OpenStax Psychology 2e, Section 9.2: Lifespan Theories — OpenStax, Rice University
- Developmental Milestones Record — 9 Months (Medical Encyclopedia) — MedlinePlus / U.S. National Library of Medicine (NIH)
- Infant — Newborn Development (Medical Encyclopedia) — MedlinePlus / U.S. National Library of Medicine (NIH)
EliExplains lessons are original prose written from the open, credible references above. See Copyright & Licensing.
Researched 2026-08-22
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