Week 5 · Chapter

Chapter 5 — Infancy and Toddlerhood: Physical Development

Child Development Free sample edition

The module's primary reading · ~35–45 minutes · Objective 4 (analyze physical, cognitive, and language development in infancy and toddlerhood: brain development, reflexes, motor milestones as windows of achievement, safe sleep, the sensorimotor stage, object permanence, and early language) · Read it first this week; the lecture, the slides, the tutorial, the quiz, and the workshop all build on it.

In plain language

This chapter is about how babies' bodies and brains grow in the first two years. The brain builds connections fast, keeps the ones a child uses, and trims the rest, which is called synaptic pruning (removing unused links between nerve cells). Newborns arrive with reflexes (automatic movements nobody taught them), and over the next months they learn to sit, crawl, stand, and walk. You will read each new skill against a window of achievement (the range of ages in which most children in a large study reached it), not against one magic birthday. You will also learn what two health bodies say about safe sleep and infant feeding, stated their way and never as advice for one baby.

Why you already care about this

Picture a well-baby visit at a busy pediatric clinic. A parent holds a 10-month-old on one knee and asks the nurse, "Shouldn't she be walking by now?" Across the waiting room, a grandparent watches a toddler grip the edge of a chair and pull up to stand. Both adults are doing what you will do all term: watching a child and wondering what is typical.

Here is the honest reply. Most children walk alone somewhere between about 8 and 18 months. A 10-month-old who is not yet walking is inside that range, and so is a 16-month-old who is only now starting. The World Health Organization's Motor Development Study, published by the WHO Multicentre Growth Reference Study Group in 2006, gives you those windows for six big motor milestones. The American Academy of Pediatrics' 2022 safe sleep policy statement gives you the sleep guidance. This chapter teaches you to read both the way the issuing bodies wrote them.

By the end of this chapter you will be able to:

describe how the infant brain grows through synapses, pruning, myelination, and plasticity;
name the six newborn reflexes, say what each looks like, and explain that most fade in the first months;
sort a movement into gross motor or fine motor;
read a motor milestone against the WHO windows of achievement and work a window's width in months;
state safe sleep exactly as the American Academy of Pediatrics' 2022 policy statement states it, with the NICHD campaign years;
state the World Health Organization's breastfeeding recommendation as that body's recommendation, with respect for every family.

Carry this line with you: a milestone is a window, not a deadline.

Where we are

Already covered: Week 1 gave you the three domains, the periods, and nature and nurture as interaction. Week 2 gave you the theories, including the ethological view and sensitive periods. Week 3 taught you to observe and to keep what a child did apart from what it might mean. Week 4 followed development from conception to birth and taught the Apgar score.
This week: the physical domain in the first two years — the brain, the newborn reflexes, gross and fine motor skills read against the WHO windows, safe sleep, and infant feeding.
How they connect: Week 4 ended at birth with record R1's Apgar scores; this week picks up the same newborn and follows the body through the first two years.
Where we're going next: Week 6 stays with the same infants and turns to the cognitive domain, where Piaget's sensorimotor stage, object permanence, and first words take center stage.

Three earlier ideas carry this week. From Week 1, nature and nurture work together; genes and experience never act alone. From Week 2, the ethological view studies behavior in its evolutionary context, asking what a behavior does for survival. From Week 3, an observation record keeps the behavior in one column and the interpretation in another. You will use all three on the running case, the illustrative, unnamed child whose records follow them from birth to 15.

1. Grow the brain: synapses, pruning, myelination, and plasticity

Think of a new neighborhood where the walking paths are not paved yet. People cut across the grass in many directions at first. After a few months, the paths people use every day are worn smooth, and the ones nobody uses grow over. An infant's brain works a little like that.

Here are the course's terms, set off so you can find them:

A neuron is a nerve cell that receives and sends signals.
A synapse is the tiny gap where one neuron passes a signal to another.
Synaptic pruning is the brain's removal of connections that go unused, often summed up as "use it or lose it".
Myelination is the wrapping of nerve fibers in a fatty coating that speeds signals.
Plasticity is the brain's ability to be shaped and reorganized by experience.

In infancy the brain forms synapses in great numbers and then prunes the ones that go unused. Myelination speeds signals along the paths that remain. Plasticity is high, so the experiences a baby has — being talked to, held, fed, and given room to move — help decide which connections stay.

This is nature and nurture working together. Genes set the broad schedule for growth. Experience decides which connections are kept. Neither one does the job alone.

Misconception check. Students often think pruning means the brain is losing something, as if a baby were being damaged. It is tempting, because "losing connections" sounds like harm. The cure: pruning strengthens the connections a child uses by clearing away the ones that go unused, the way a gardener trims a plant so the strong stems grow. Pruning is a normal part of growth, not damage.

Pause and think. Choose one option and answer it in two or three sentences:

(a) Think of a skill you practiced until it felt automatic, such as typing or a sport. How does "use it or lose it" fit your experience?
(b) Why might a brain that keeps every connection work less well than one that prunes? Explain with the path analogy.

2. Spot the newborn reflexes: six automatic responses

Touch a newborn's cheek, and the baby turns toward your finger. Nobody taught that. It is built in, the way your knee jerks when a doctor taps it.

A reflex is an automatic, unlearned response to a particular touch, sound, or movement.

The course names six newborn reflexes:

The rooting reflex: the baby turns toward a touch on the cheek and opens the mouth.
The sucking reflex: the baby sucks when something touches the roof of the mouth.
The Moro reflex: a startle in which the arms fling out and then come back in.
The palmar grasp reflex: the fingers close around an object pressed into the palm.
The stepping reflex: the baby makes stepping motions when held upright with the feet on a flat surface.
The Babinski reflex: the toes fan out when the sole of the foot is stroked.

Most of these reflexes fade in the first months, as the brain matures and the baby gains control of movement on purpose.

Worked example — reading record R1. The running case (Dataset A) is illustrative, constructed for this course, and describes no real child. Record R1 was written in a hospital birthing room.

Record R1 of the running case (illustrative), with behavior and interpretation in separate columns
What the newborn did (behavior only)	What it might show (interpretation)
Turns toward a touch on the cheek and opens the mouth	the rooting reflex
Apgar scores of 7 at 1 minute and 9 at 5 minutes	the newborn's condition at birth, read in Week 4

Walk it in the course's order. What the child did: the newborn turns toward a touch on the cheek and opens the mouth. What it shows: the rooting reflex, an automatic, unlearned response that most children lose in the first months. Which theory explains it: the ethological view asks what an inborn behavior does for survival, and rooting helps a newborn find the breast or bottle to feed. What a second theory adds: nature and nurture as interaction reminds you that the reflex is present at birth without practice, while the feeding it starts soon becomes a skill shaped by experience.

In words: R1's turn toward the touch is the rooting reflex, an unlearned response that helps a newborn feed. The same record's Apgar totals, 7 at 1 minute and 9 at 5 minutes, were read in Week 4 as a check on the newborn's condition, and a score of 7, 8, or 9 is normal.

Misconception check. Students often think a newborn learned to root or grasp in the first hours of life, because the movements look so purposeful. The cure: a reflex is unlearned. It is present at birth without any practice, and that is exactly why most reflexes fade as learned, voluntary movement takes over.

3. Tell gross motor from fine motor: big muscles versus small

Watch a toddler at a library's children's room (an illustrative scene). The toddler climbs the three carpeted steps to the reading nook, one hand on the rail. On the top step, an older sibling turns the thick pages of a board book. Both are moving, but they are using very different muscles.

Gross motor skills are movements that use the large muscles of the body, such as sitting, crawling, standing, and walking.
Fine motor skills are movements that use the small muscles, especially of the hands and fingers, such as grasping a spoon or picking up a small piece of food.

Worked example — sorting four movements. At an illustrative family meal, a caregiver writes down four things a toddler does. Sort each one.

Walks from the kitchen to the table: large muscles of the legs and trunk, so gross motor.
Picks up a single pea between thumb and finger: small muscles of the hand, so fine motor.
Climbs onto a booster seat: large muscles, so gross motor.
Turns a cup to find the handle and lifts it: small muscles of the hand and fingers do the precise work, so fine motor.

In words: two of the four movements are gross motor and two are fine motor, and the test is which muscles do the main work.

Misconception check. Students often think "motor development" means only the big milestones, such as walking, because those are the ones families talk about. The cure: fine motor skills grow at the same time and matter as much for feeding, play, and later drawing and writing. A full record of motor development notes both kinds.

The WHO windows in the next section cover six gross motor milestones only. That is a limitation worth naming: the study did not set windows for fine motor skills, so this course gives no fine motor ages.

4. Read the windows of achievement: ranges, not deadlines

Back at the clinic, the parent wants one number: the age by which a child "should" walk. The research gives something more useful than one number. It gives a range.

The WHO Multicentre Growth Reference Study Group (2006) followed children in its Motor Development Study and reported, for six gross motor milestones, a window from the 1st to the 99th percentile.

A window of achievement is the age range, in months, from the 1st to the 99th percentile of the age at which children in the WHO study reached a milestone.
The 1st percentile is the age by which 1 percent of the children in the study had reached the milestone; the 99th percentile is the age by which 99 percent had.

Here are the six windows, as the study reported them.

WHO Motor Development Study (2006): windows of achievement for six gross motor milestones, in months, 1st to 99th percentile
Milestone	Window (months)
Sitting without support	3.8 to 9.2
Standing with assistance	4.8 to 11.4
Hands-and-knees crawling	5.2 to 13.5
Walking with assistance	5.9 to 13.7
Standing alone	6.9 to 16.9
Walking alone	8.2 to 17.6

Worked example — reading R2 and R3 against the windows. Two more records from the running case (illustrative) carry the motor milestones.

Record R2 was written at 9 months, in the family home with a parent present: the infant sits without support and crawls on hands and knees. Sitting without support has a window of 3.8 to 9.2 months, and 9 months falls inside it. Hands-and-knees crawling has a window of 5.2 to 13.5 months, and 9 months falls inside it.
Record R3 was written at 14 months, at a childcare center's morning drop-off: the toddler walks alone. Walking alone has a window of 8.2 to 17.6 months, and 14 months falls inside it.

Now work each window's width. You subtract the early end from the late end.

Width of the sitting window: 9.2 − 3.8 = 5.4 months.
Width of the crawling window: 13.5 − 5.2 = 8.3 months.
Width of the walking-alone window: 17.6 − 8.2 = 9.4 months.
Comparing the widths: the walking-alone window is wider than the sitting window by 9.4 − 5.4 = 4.0 months.

In words: R2's sitting and crawling at 9 months and R3's walking alone at 14 months each fall inside the WHO windows, and the walking-alone window spans 9.4 months, so the later milestone shows the wider spread of typical ages. Most children walk alone somewhere between about 8 and 18 months.

In practice (your tools). In a free spreadsheet, type each milestone in column A, the early end in column B, and the late end in column C. In cell D2, type =C2-B2 and fill it down; column D then holds each window's width in months. Write the result out in words beside the table, the way the worked example does.

Misconception check — the deadline. Students often think a milestone age is a deadline, as in "children walk at 12 months". It is tempting, because one number is easier to remember than a range. The cure: the WHO windows are wide, and walking alone runs from about 8.2 to 17.6 months. A child outside a window is a reason for a family to talk with a health professional, never a conclusion this course draws.

Misconception check — the crawling step. Students also often think every baby must crawl before walking. The cure: some children skip hands-and-knees crawling and go straight to pulling up, standing, and walking. Crawling is common, not required.

One caveat keeps the windows honest. They describe the children in one large study, read as groups; they say nothing about whether any one real child is ahead or behind, and you never use them to judge a child.

5. State safe sleep as the issuing bodies state it

Picture a parent in a store, choosing a crib for a first baby (an illustrative scene). The store shows cribs with thick bumpers, soft quilts, and a stuffed bear in every display. The parent wants to know what the health experts say.

The American Academy of Pediatrics' 2022 safe sleep policy statement says to place babies on their backs for every sleep, for all naps and at night, until age 1, on a firm, flat, non-inclined surface.

Safe sleep, in this course, means exactly that guidance: on the back for every sleep, for all naps and at night, until age 1, on a firm, flat, non-inclined surface, as the American Academy of Pediatrics' 2022 policy statement states it.

A federal campaign has carried this message for decades. The NICHD launched the Back to Sleep campaign in 1994 and expanded and renamed it the Safe to Sleep campaign in 2012. That is 2012 − 1994 = 18 years from the first campaign to its expansion.

Notice how the course handles this guidance. It quotes or closely paraphrases the issuing body, names the body and the year, and never softens or extends it. It also never turns the guidance into advice for one baby. Questions about one baby's sleep belong with that family's health professional.

Misconception check. Students often hear a family say, "Our baby sleeps better on the stomach, so stomach sleeping must be safe for this one." It is tempting, because a baby who sleeps longer seems more comfortable. The cure: the American Academy of Pediatrics' 2022 policy statement says to place babies on their backs for every sleep until age 1. "Sleeps better" does not change the guidance.

Pause and think. Choose one option and answer it in two or three sentences:

(a) The crib display in the store does not match the guidance. Why might a store's display and a health body's guidance differ, and which one would you quote in a report?
(b) Bronfenbrenner's ecological model from Week 1 places a national campaign in the macrosystem and the crib in the microsystem. How might a campaign reach a family's bedroom?

6. Feed infants with respect, and read the records together

Feeding is the first physical routine a family builds with a baby, and families do it in many ways. Some breastfeed, some use formula, and many combine the two. The course states one issuing body's recommendation and respects every family.

The World Health Organization recommends exclusive breastfeeding for the first 6 months, with breastfeeding continuing up to 2 years of age or beyond. This course states it as that body's recommendation.

Exclusive breastfeeding is feeding an infant only breast milk, with no other food or drink; the World Health Organization recommends it for the first 6 months of life.

Families feed infants in other ways for many reasons, including health, work, adoption, and personal choice. The course describes those families without ranking them, and it never tells a family what one baby needs.

Misconception check. Students often read a health body's recommendation as a ranking of families, as if a family who uses formula has failed. The cure: a recommendation describes what one body advises for populations; it is not a judgment of any family, and this course never ranks families by how they feed a baby.

Worked example — reading R1, R2, and R3 together. Put the week's records side by side and walk the course's order once more.

What the child did: at birth (R1), turns toward a touch on the cheek; at 9 months (R2), sits without support and crawls on hands and knees; at 14 months (R3), walks alone.
What it shows: the rooting reflex in R1; sitting and crawling inside their WHO windows in R2; walking alone inside its window in R3.
Which theory explains it: nature and nurture as interaction. The reflex in R1 is present at birth without practice, while the sitting, crawling, and walking in R2 and R3 need both a maturing body and brain and the chance to practice.
What a second theory adds: the ethological view asks what an inborn behavior does for survival, and rooting helps a newborn find the breast or bottle to feed.

The reading in one plain sentence: R1's turn toward the touch is the rooting reflex, an unlearned response, and R2's sitting and crawling at 9 months and R3's walking alone at 14 months each fall inside the WHO windows, which are wide ranges, not deadlines.

That sentence is the habit this course grades: name the behavior, name the concept, and say the evidence, in words a family could follow.

Key terms

Neuron — A nerve cell that receives and sends signals; neurons connect to one another to carry every movement, sensation, and thought. (first used: 1. Grow the brain: synapses, pruning, myelination, and plasticity)
Synapse — The tiny gap where one neuron passes a signal to another; in infancy the brain forms synapses in great numbers. (first used: 1. Grow the brain: synapses, pruning, myelination, and plasticity)
Synaptic pruning — The brain's removal of connections between neurons that go unused, often summed up as "use it or lose it"; it strengthens the connections a child uses and is a normal part of growth. (first used: 1. Grow the brain: synapses, pruning, myelination, and plasticity)
Myelination — The wrapping of nerve fibers in a fatty coating called myelin, which speeds signals through the brain and body. (first used: 1. Grow the brain: synapses, pruning, myelination, and plasticity)
Plasticity — The brain's ability to be shaped and reorganized by experience, which is especially high in infancy. (first used: 1. Grow the brain: synapses, pruning, myelination, and plasticity)
Reflex — An automatic, unlearned response to a particular touch, sound, or movement; most newborn reflexes fade in the first months. (first used: 2. Spot the newborn reflexes: six automatic responses)
Rooting reflex — A newborn's automatic turn toward a touch on the cheek, with the mouth opening. (first used: 2. Spot the newborn reflexes: six automatic responses)
Moro reflex — A newborn's startle response in which the arms fling out and then come back in. (first used: 2. Spot the newborn reflexes: six automatic responses)
Gross motor skills — Movements that use the large muscles of the body, such as sitting, crawling, standing, and walking. (first used: 3. Tell gross motor from fine motor: big muscles versus small)
Fine motor skills — Movements that use the small muscles, especially of the hands and fingers, such as grasping a spoon or picking up a small piece of food. (first used: 3. Tell gross motor from fine motor: big muscles versus small)
Window of achievement — The age range, in months, from the 1st to the 99th percentile of the age at which children in the WHO Motor Development Study (2006) reached a gross motor milestone. (first used: 4. Read the windows of achievement: ranges, not deadlines)
Safe sleep — The American Academy of Pediatrics' 2022 guidance to place babies on their backs for every sleep, for all naps and at night, until age 1, on a firm, flat, non-inclined surface. (first used: 5. State safe sleep as the issuing bodies state it)
Exclusive breastfeeding — Feeding an infant only breast milk, with no other food or drink, which the World Health Organization recommends for the first 6 months of life. (first used: 6. Feed infants with respect, and read the records together)

Summary

The infant brain forms synapses in great numbers, prunes the unused ones, and speeds signals through myelination.
Pruning is normal growth, not damage; plasticity lets experience shape the brain.
Newborns have six reflexes in this course — rooting, sucking, Moro, palmar grasp, stepping, Babinski — and most fade in the first months.
Gross motor skills use large muscles; fine motor skills use small muscles, especially of the hands.
The WHO windows (2006) are wide ranges: walking alone runs from about 8.2 to 17.6 months.
Safe sleep, as the American Academy of Pediatrics' 2022 policy statement states it: on the back for every sleep until age 1, on a firm, flat surface.
The World Health Organization recommends exclusive breastfeeding for the first 6 months; the course ranks no family by how it feeds a baby.

Check your understanding

Work these before looking at the answers — they are ungraded, and the answers (with reasoning) are at the bottom of the page.

At an illustrative pediatric checkup, a nurse strokes the sole of a newborn's foot, and the toes fan out. Then a soft clap makes the baby fling both arms out and bring them back in. Name the two reflexes.
A caregiver at an illustrative infant room writes four notes: (1) pulls up to stand at a low shelf; (2) passes a rattle from one hand to the other; (3) rolls from back to tummy; (4) picks up a crumb with thumb and finger. Sort each into gross motor or fine motor, and count each kind.
In an illustrative family, one cousin first sat without support at 5 months and another at 8 months. How many months apart are they, and what is the fair way to describe the gap?
A classmate writes, "Babies have to crawl before they can walk, so a baby who skips crawling is behind." Name the misconception and rewrite the sentence.

Answers (worked reasoning — read after attempting)

The toes fanning out when the sole is stroked is the Babinski reflex. The arms flinging out and coming back in after a sudden sound is the Moro reflex. Both are automatic and unlearned, which is why the nurse can see them in a newborn who has had no practice.
Notes (1) and (3) use the large muscles of the legs and trunk, so they are gross motor. Notes (2) and (4) use the small muscles of the hands and fingers, so they are fine motor. That makes 2 gross motor and 2 fine motor, and 2 + 2 = 4 notes in all.
Subtract the earlier age from the later one: 8 − 5 = 3 months. The fair description is that both cousins sat without support at ages inside the wide range of typical ages; a 3-month gap between two children says nothing about which one is ahead in a way that matters. The trap here is reading one child's age as a deadline for the other.
The sentence reads crawling as a required step before walking. Some children skip hands-and-knees crawling and go straight to pulling up, standing, and walking. A fixed sentence: "Many babies crawl before they walk, but some skip crawling, and skipping it is one typical path to walking."

Next up in this module: the Readings & Resources page, the lecture tutorial, the practice set, the quiz, and the discussion — and next week, Piaget's sensorimotor stage gives these same infants' thinking its first model.

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