Lecture: Infancy and Toddlerhood: Physical Development
Lecture script · about 15–20 minutes spoken · Week 5
Opening
Picture three people in three different places. A nurse at a well-baby visit is listening to a worried parent. A teacher in a childcare center's infant room is settling a baby down for a nap. And a family member is standing in a store, trying to pick a crib. Each of them is asking a version of the same question: what is typical for a baby's body in the first two years, and what do the health experts actually say?
That's what this lecture is about. In Week 1 you met the three domains and the idea that nature and nurture work together. In Week 2 you met the theories, including the ethological view, which asks what a behavior does for survival. In Week 3 you learned to keep what a child did apart from what it might mean. And in Week 4 you followed development from conception to birth and read the running case's first record, R1, with its Apgar scores.
In this lecture we pick up that same newborn and follow the body. I'll take you through six ideas in order. First, how the brain grows, and then the reflexes a baby is born with. Next come the two kinds of motor skills and how to read a milestone fairly. Last, what the issuing bodies say about safe sleep, and what one health body recommends about feeding. By the end, I want you to be able to read a baby's movements the way a careful observer would, as a range and never as a deadline.
1. The brain builds paths and then trims them
Let's start with a picture you've probably seen. Think of a new park where the walking paths haven't been paved. At first, people cut across the grass every which way. A few months later, the paths people use every day are worn smooth, and the ones nobody uses have grown back over. A baby's brain works a lot like that park.
Here are the words for it. 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. In infancy, the brain forms synapses in great numbers. Then it does something that surprises a lot of people: it trims them. Synaptic pruning is the brain's removal of connections between neurons that go unused, often summed up as "use it or lose it."
Two more terms finish the picture. Myelination is the wrapping of nerve fibers in a fatty coating called myelin, which speeds signals through the brain and body. Think of it as paving the paths that stay. And plasticity is the brain's ability to be shaped and reorganized by experience, which is especially high in infancy.
So here's how it fits together. Genes set the broad schedule for growth. Experience, like being talked to, held, fed, and given room to move, helps decide which connections stay. That's nature and nurture working together, the idea you met in Week 1. Neither one does the job alone.
Pause and try this. A friend reads that a baby's brain "loses connections" in the first years and worries that something is going wrong. What would you tell your friend?
Take a moment and say your reply out loud before I go on.
Here's the answer. Pruning isn't damage, because it 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 can grow.
Hold on to that gardener. Pruning is part of healthy growth, and you'll see the same "keep what you use" idea again when we get to motor skills.
2. Babies arrive with reflexes nobody taught them
Here's something you can see at any hospital nursery. Touch a newborn's cheek, and the baby turns toward your finger and opens their mouth. Nobody taught that. It's built in, the same way your knee jerks when a doctor taps it with a little hammer.
The term is reflex: an automatic, unlearned response to a particular touch, sound, or movement. This course names six newborn reflexes, and I'll walk you through each one in words.
The rooting reflex is that turn toward a touch on the cheek, with the mouth opening. The sucking reflex is sucking when something touches the roof of the mouth. The Moro reflex is a startle in which the arms fling out and then come back in. The palmar grasp reflex is the fingers closing around an object pressed into the palm, like an adult's finger. The stepping reflex is stepping motions when a baby is held upright with the feet on a flat surface, long before the baby can stand. And the Babinski reflex is the toes fanning 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.
Now let's read a real record from the running case. Remember, the running case is illustrative. It was made up for this course and describes no real child. Record R1 was written in a hospital birthing room. It says the newborn turns toward a touch on the cheek and opens the mouth. It also carries the Apgar scores you read in Week 4: 7 at 1 minute and 9 at 5 minutes. And a score of 7, 8, or 9 is normal.
We read it in the course's order. What the child did: turned toward the touch and opened the mouth. What it shows: the rooting reflex. 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.
Pause and try this. A classmate says the newborn in R1 must have learned to root in the first hours after birth, because it looks so purposeful. Is that right?
Think about what the word "reflex" means before you decide.
Here's the answer. It isn't right, because a reflex is unlearned, present at birth without any practice, and that's exactly why most reflexes fade as learned, voluntary movement takes over.
3. Big muscles and small muscles
Let me take you somewhere else now: a library's children's room. A toddler is climbing three carpeted steps up to the reading nook, one hand on the rail. On the top step, an older sibling is turning the thick pages of a board book. Both kids are moving. But they're using very different muscles.
That difference has a name. 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. Climbing the steps is gross motor. Turning a page is fine motor.
Here's a picture from a family meal. A caregiver notices four things a toddler does. The toddler walks from the kitchen to the table, which is gross motor. The toddler picks up a single pea between thumb and finger, which is fine motor. The toddler climbs onto a booster seat, gross motor again. And the toddler turns a cup to find the handle and lifts it, which is fine motor, because the small muscles of the hand do the precise work. Two of each.
There's a trap here worth naming. A lot of people think "motor development" means only the big milestones, like walking, because those are the ones families talk about at holidays. But fine motor skills grow at the same time, and they matter as much for feeding, for play, and later for drawing and writing.
Pause and try this. At that same family meal, the toddler sits up in a high chair without leaning on anything. Is sitting up a gross motor skill or a fine motor skill?
Ask yourself which muscles are doing the main work.
Here's the answer. It's gross motor, because sitting depends on the large muscles of the trunk and back, not the small muscles of the hands.
4. A milestone is a window, not a deadline
Let's go back to that well-baby visit from the opening. Picture a parent holding a 10-month-old on one knee and asking, "Shouldn't she be walking by now?" The parent wants one number. The research gives something better than one number. It gives a range.
In 2006, the WHO Multicentre Growth Reference Study Group reported its Motor Development Study. For six gross motor milestones, it gave what's called a window of achievement: the age range, in months, from the 1st to the 99th percentile of the age at which children in the study reached a milestone. The 1st percentile is the age by which 1 percent of the children had reached it. The 99th percentile is the age by which 99 percent had.
I'll give you three of the six windows, the ones our records use. Sitting without support runs from 3.8 to 9.2 months. Hands-and-knees crawling runs from 5.2 to 13.5 months. And walking alone runs from 8.2 to 17.6 months. Put plainly, most children walk alone somewhere between about 8 and 18 months.
Now the running case. 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. Nine months falls inside the sitting window and inside the crawling window. Record R3 was written at 14 months, at a childcare center's morning drop-off. The toddler walks alone, and 14 months falls inside the walking-alone window.
Here's a little arithmetic that makes the point. To find a window's width, you subtract the early end from the late end. For sitting, 9.2 minus 3.8 leaves 5.4 months. For crawling, 13.5 minus 5.2 leaves 8.3 months. For walking alone, 17.6 minus 8.2 leaves 9.4 months. That walking window is 4.0 months wider than the sitting window, so the later milestone has the wider spread of typical ages.
So what about the parent at the clinic? A 10-month-old who isn't walking yet is inside that range, and so is a 16-month-old who is only now starting. And here's the line the course always draws: a child outside a window is a reason for a family to talk with a health professional, never a conclusion we draw.
One more trap. Many people think every baby has to crawl before walking. Some children skip hands-and-knees crawling and go straight to pulling up, standing, and walking. Crawling is common, not required.
Pause and try this. Using the walking-alone window of 8.2 to 17.6 months, which is the fairer sentence: "Children walk at 12 months," or "Most children walk alone somewhere between about 8 and 18 months"?
Read both sentences to yourself once more.
Here's the answer. The second one is fairer, because the WHO window is a wide range and the first sentence turns one age into a deadline every child is supposed to meet.
5. Safe sleep, in the issuing body's own words
Now picture a parent in a store, choosing a crib for a first baby. Every display crib is dressed up with thick bumpers, soft quilts, and a stuffed bear. It looks cozy. The parent wants to know what the health experts say.
Here's how this course handles a question like that. We quote the issuing body, we name the body and the year, and we never soften the guidance or turn it into advice for one baby. 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. In this course, safe sleep means exactly that guidance.
There's history behind it. The NICHD launched the Back to Sleep campaign in 1994. In 2012, it expanded the campaign and renamed it Safe to Sleep. That's 18 years from the first campaign to its expansion.
Notice something about that store display. It doesn't match the guidance, and that's fine for a lecture to point out. When you write a report, you quote the health body, not the store.
Pause and try this. A family says, "Our baby sleeps better on the stomach, so stomach sleeping must be safe for this one." What does the 2022 policy statement say?
Think about whether "sleeps better" changes the guidance.
Here's the answer. It says to place babies on their backs for every sleep until age 1, so "sleeps better" doesn't change the guidance, and any question about one baby belongs with that family's health professional.
6. Feeding, and the whole week in one sentence
Feeding is the first physical routine a family builds with a baby, and families do it in many ways. Some breastfeed. Some use formula. Many combine the two.
The World Health Organization recommends exclusive breastfeeding for the first 6 months, with breastfeeding continuing up to 2 years of age or beyond. Exclusive breastfeeding means feeding an infant only breast milk, with no other food or drink. This course states that as the World Health Organization's recommendation, and that's all it is here. It isn't a ranking of families. A family who feeds a baby another way hasn't failed at anything, and this course never tells a family what one baby needs.
Now let's put the week together, using our three records. What the child did: at birth, R1 turns toward a touch on the cheek. At 9 months, R2 sits without support and crawls. At 14 months, R3 walks alone. What it shows: the rooting reflex, then sitting and crawling inside their windows, then walking alone inside its window. Which theory explains it: nature and nurture as interaction, because the reflex needs no practice, while sitting, crawling, and walking need a maturing body and brain and the chance to practice. What a second theory adds: the ethological view, which reminds you that rooting helps a newborn feed.
Pause and try this. Say the whole reading of R1, R2, and R3 in one plain sentence that names the behaviors and the evidence.
Give it a try before you hear my version.
Here's the answer. 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 one sentence is the habit this course is building: behavior, concept, and evidence, in words a family could follow.
Closing
Let's recap in the order we went. The infant brain forms synapses in great numbers, prunes the ones that go unused, and speeds signals with myelination, and that pruning is growth, not damage. Newborns arrive with six reflexes, like rooting and the Moro startle, and most fade in the first months. Gross motor skills use the big muscles, and fine motor skills use the small ones, especially in the hands.
A milestone is a window, not a deadline: walking alone runs from about 8.2 to 17.6 months. Safe sleep, in the American Academy of Pediatrics' 2022 words, means on the back for every sleep until age 1, on a firm, flat surface. And the World Health Organization recommends exclusive breastfeeding for the first 6 months, which this course states as that body's recommendation, ranking no family.
Next in this module, you'll find the slide deck, the Readings and Resources page, the lecture tutorial, the practice set, and the quiz. And in Week 6, we stay with these same infants and turn to their thinking and their first words, through Piaget's sensorimotor stage.
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