Notes on The First Two Years: The Developing Body & Brain

  • Body Growth

    • Average weight at birth: 7\ \text{lb} \;(3.2\ \text{kg})
    • Average weight at 24 months: 28\ \text{lb} \;(13\ \text{kg})
    • Average length at birth: 20\ \text{inches}
    • Average length at 24 months: 34\ \text{inches}
    • These values are norms (average measurements); note that failure to thrive indicates deviation from normative growth curves.

  • Averages and Individuals

    • Weight of Girls and Boys from birth to 24 months presented in a percentile-style chart (90th, 50th, 10th).
    • At birth: both sexes begin around the same weight (roughly the normative birth weight of 3.2\ \text{kg} / 7\ \text{lb}).
    • At 24 months: weight ranges across percentiles with 90th, 50th, and 10th percentiles visible.
    • For context, typical end-of-two-year weight is around 13\ \text{kg} (≈ 28\ \text{lb}) at the 50th percentile, with higher values up to ~16\ \text{kg} (≈ 35\ \text{lb}) at the 90th percentile and lower values around ~8-9\ \text{kg} (≈ 18-20\ \text{lb}) at the 10th percentile.
    • Gender differences exist but are moderate in early infancy; growth charts plot separate curves for girls and boys.

  • Physical Development: Sleep

    • Sleep varies due to biology, caregiving, and culture.
    • Newborns sleep about 15\text{--}17\text{ hours} per day.
    • By 12 months, the norm is 12\text{--}13\text{ hours} per day.
    • Sleep can be disrupted by pain (e.g., colic, hunger).

  • Where should babies sleep?

    • In the U.S. middle class, infants often sleep separated from parents.
    • Decisions to co-sleep or bed-share are linked to culture, infant age, mother’s education, depressive state, and father involvement.
    • Infants in Asian, African, and Latin American cultures more commonly co-sleep or bed-share.
    • SIDS (sudden infant death syndrome) considerations influence sleep arrangements and safety practices.

  • Who is Co-Sleeping?

    • The disparity in co-sleeping between Asian and non-Asian populations may be partly explained by how Western parents use gadgets and objects (monitors, night-lights, pacifiers, cuddle cloths, sound machines) to approximate the proximity of a co-sleeping arrangement.

  • Brain Growth

    • Prenatal and postnatal brain growth (measured via head circumference) is crucial for later cognition.
    • Head-sparing: a biological mechanism that protects brain development when nutritional intake is insufficient for body growth.
    • The brain is the last part of the body to be damaged by malnutrition, relative to other tissues.

  • Brain Changes

    • Exuberance and pruning: early dendritic growth is followed by selective pruning.
    • Brain development depends on both genes/maturation and experience.
    • Early dendrite growth is called transient exuberance.
    • Unused dendrites wither (pruning) to create space between neurons, enabling more synapses and more complex thinking (sculpting).
    • Experience-expectant vs. experience-dependent development:
    • Experience-expectant: brain expects certain basic experiences (e.g., visual input) to develop normally.
    • Experience-dependent: development that depends on individual experiences and environment.

  • The Biology of the Brain

    • Key cellular components:
    • Dendrites: receive messages from other cells
    • Terminal branches of axon: form junctions with other cells
    • Axon: passes messages away from the cell body to other neurons, muscles, or glands
    • Cell body: the cell’s life-support center
    • Neural impulse (action potential): electrical signal traveling down the axon
    • Myelin sheath: covers the axon of some neurons and speeds neural impulses
    • Diagram labels (for reference):
    • Myelin sheath
    • Axon
    • Dendrites
    • Cell body
    • Axon terminals
    • Synapse and synaptic gap

  • How Neurons Communicate

    • Sending neuron process: 1) Electrical impulses (action potentials) travel down an axon to a synapse. 2) When an action potential reaches the axon terminal, it stimulates neurotransmitter release.
      • Neurotransmitters cross the synaptic gap and bind to receptor sites on the receiving neuron.
      • This binding allows electrically charged atoms to enter the receiving neuron, exciting or inhibiting a new action potential.
        3) Reuptake or breakdown:
      • Excess neurotransmitters are reabsorbed (reuptake), drift away, or are degraded by enzymes.
    • Key terms: synapse, receptor sites, neurotransmitters, action potential, reuptake.

  • Harming the Infant Body & Brain

    • Necessary stimulation: babies need stimulation; severe lack stunts brain development.
    • Stress and the brain: both under-stimulation and inappropriate overstimulation can have adverse effects.
    • Shaken baby syndrome (abusive head trauma) is a critical risk associated with head injuries.

  • The Developing Senses

    • Hearing
    • Develops during the last trimester of pregnancy; among the most advanced newborn senses.
    • Speech perception is evident by about four months after birth.
    • Seeing
    • Least mature sense at birth.
    • Newborns focus best between about 4\;\text{inches} and 30\;\text{inches} from the face.
    • Binocular vision typically develops between 2\text{--}4\text{ months}.
    • Experience and maturation of the visual cortex improve shape recognition, visual scanning, and detail detection.
    • Tasting and smelling
    • Smell and taste are functional at birth and rapidly adapt to the social world.
    • Foods and flavors from one’s culture can aid survival.
    • Adaptation occurs for both senses as infants grow.
    • Touch
    • Sense of touch is acute in infants.
    • All newborns respond to gentle holding; over time, babies may show preferences for certain touches.
    • Some touches are experience-expectant for normal growth.
    • Pain and temperature are linked to touch; pain is probably less intense than in adults but not absent.

  • Motor Skills

    • Gross motor skills
    • All basic gross motor skills develop across the first two years.
    • Development follows cephalocaudal (head-down) and proximodistal (center-out) patterns.
    • Emerging Gross Motor Skills: Age Norms (50% mastery vs. 90% mastery)
    • Sits unsupported: 50% by about 6\ \text{months}, 90% by about 7.5\ \text{months}
    • Stands, holding on: 50% by about 7.4\ \text{months}, 90% by about 9.4\ \text{months}
    • Crawls (creeps): 50% by about 8\ \text{months}, 90% by about 10\ \text{months}
    • Stands, not holding on: 50% by about 10.8\ \text{months}, 90% by about 13.4\ \text{months}
    • Walks well: 50% by about 12\ \text{months}, 90% by about 14.4\ \text{months}
    • Walks backward: 50% by about 15\ \text{months}, 90% by about 17\ \text{months}
    • Runs: 50% by about 18\ \text{months}, 90% by about 20\ \text{months}
    • Jumps: 50% by about 26\ \text{months}, 90% by about 29\ \text{months}
    • Emerging Fine Motor Skills
    • Fine motor skills involve smaller body movements, especially of the hands and fingers (e.g., drawing, picking up a coin).
    • Shaped by culture and opportunity.

  • Emerging Fine Motor Skills: Age Norms (50% vs. 90% mastery)

    • Grasps a rattle when placed in hand: 50% at 3\ \text{months}, 90% at 4\ \text{months}
    • Reaches to hold an object: 50% at 4.5\ \text{months}, 90% at 6\ \text{months}
    • Thumb and finger grasp: 50% at 8\ \text{months}, 90% at 10\ \text{months}
    • Stacks two blocks: 50% at 15\ \text{months}, 90% at 21\ \text{months}
    • Imitates a vertical line (drawing): 50% at 30\ \text{months}, 90% at 39\ \text{months}

  • Walking

    • Aeonically discussed in media (e.g., a Netflix documentary on First Steps) as part of researching developing motor skills.
    • Common observation: babies seem to have an innate ability to walk, but their strength must catch up with their mass.
    • Motivation is important for walking development; practice and environment matter for progress.

  • Everyday Note: Toddlers and Energy

    • A popular takeaway question: Why are parents of toddlers often tired?
    • Insightful statistic (Adolph & Franchak, 2017): In 1 hour of free play, a toddler may
    • Take about 2,400\ text{ steps},
    • Travel the length of about 8\ \text{U.S. football fields},
    • Fall about 17\text{ times}.

  • Summary of Key Concepts (Recap)

    • Growth in infancy follows normative curves but individual variation is substantial.
    • The brain exhibits rapid early growth, followed by pruning guided by experience.
    • Sensory systems develop in a staged manner, with early maturity in hearing and later maturation in vision.
    • Motor development proceeds from gross to fine control, guided by both biology and environment.
    • Sleep patterns are culturally and biologically influenced and are important for development.
    • Caregiving contexts (sleep arrangements, stimulation, stress) can influence developmental trajectories.

  • Formulas and Notation to Remember

    • Weight conversions: 1\ \text{lb} \approx 0.453592\ \text{kg}; thus 7\ \text{lb} \approx 3.175\ \text{kg}, etc.
    • Temporal milestones are approximate medians (50th percentile) and can vary across populations.

  • Quick Reference Milestones (rough guide, not exhaustive)

    • Sleep: newborn 15–17 hours/day; by 1 year 12–13 hours/day.
    • Gross motor: sit ~6 months, crawl ~8 months, stand ~9–11 months, walk ~12 months (range variances apply).
    • Fine motor: rattle grasp ~3–4 months; pincer-like grasp later (10 months+); stack blocks ~15–21 months; imitate simple lines ~30–39 months.
    • Senses: hearing well developed early; binocular vision ~2–4 months; smell/taste functional at birth.

  • Note on Citations

    • Example citation from the material: Adolph & Franchak (2017) referenced for toddler activity data.