Biosocial Development During the First Two Years

Physical Growth and Body Changes in Infancy

• Body Size Measurements

  • Weight: An infant's birthweight typically reaches a doubling point by month $4$ and proceeds to triple by the end of the first year ($12$ months).
  • Height: Generally, height increases by approximately $10$ inches during the first year of life.
  • Variation: There is significant variation in these averages, influenced by genetics, nutrition, and initial birthweight.
  • Catch-up Growth: Children may experience accelerated growth to reach their genetically determined path if they were previously slowed by environmental factors.
  • Percentiles: Growth progress is measured using percentiles, which compare a child to others of the same age. It is vital to track a child's progress relative to their own previous percentile rankings.

• Factors Influencing Weight and Growth

  • Growth and weight gain are affected by several variables including:
    • Allergies
    • The microbiome
    • Micronutrient deficiencies
    • Presence of diseases
    • Sleep disturbances
    • Obesity, which is a multifactorial condition with various causes.

Sleep Patterns and Caregiving Practices

• Sleep Requirements and Characteristics

  • Proper health and growth are closely tied to regular and sufficient sleep throughout the life span.
  • Newborn Sleep Duration: Newborns typically sleep between $15$ and $17$ hours within a $24$-hour period.
  • REM (Rapid Eye Movement) Sleep: Roughly half of a newborn's sleep time is REM sleep.
    • Definition: A stage of sleep involving flickering eyes, dreaming, and high-frequency brain wave activity.
  • Developmental Shift: During the first year, infants spend more time alert, while total sleep and dreaming time decrease. Over the subsequent years, sleep cycles become longer and deeper.

• Establishing Sleep Routines

  • Circadian Rhythm: Infants begin developing a $24$-hour body clock around the third month ($3$). Parents can assist by providing exposure to daylight during the day and ensuring darkness and silence at night.
  • Soothing Techniques (The Five "S" Responses): Research identifies five methods to soothe a crying newborn:
    • Swaddle
    • Suck
    • Sway
    • Shush
    • Side position
  • Importance of Routines: Because infants are rapid learners, stable routines help establish healthy sleep patterns, which serve as the basis for later emotional and cognitive growth.

• Sleeping Arrangements

  • Cultural Variations: Cultural values deeply influence where a baby sleeps.
    • North America: Infants often sleep in cribs in separate rooms.
    • Asia, Africa, and Latin America: Infants often sleep in the same room or bed as parents.
  • Co-sleeping: Caregivers and children sleeping in the same room.
  • Bed-sharing: Caregivers and children sleeping in the same bed.
  • Safety Considerations:
    • Safe co-sleeping involves a hard surface without blankets.
    • Bed-sharing risks include suffocation from soft bedding, pillows, or blankets if a parent moves or a baby rolls.

Sudden Infant Death Syndrome (SIDS) Research and Risk Factors

• Susan Beal's Investigation

  • Beal studied over $500$ SIDS cases in South Australia, noting that most SIDS infants slept on their stomachs.
  • She observed that Chinese infants, who historically slept on their backs, had lower rates of SIDS.
  • Hypothesis: Back-sleeping reduces SIDS risk. This was confirmed when no back-sleeping babies in her test group died from SIDS.
  • Global Impact: Dutch doctors followed this research, leading to a $40\%$ reduction in SIDS in the Netherlands in one year.
  • "Back to Sleep" Campaign: Launched in the $1990\text{s}$, this initiative reduced U.S. SIDS rates by $50\%$ by $1996$.

• The Triple-Risk Model of SIDS

  • SIDS is now understood through three intersecting factors:
    1. Underlying Vulnerability: Undetected health complications.
    2. Specific Timing: The critical age period under $6$ months.
    3. External Stressor: Factors such as cigarette smoke, excessive heat, or stomach-sleeping.
  • Additional Risk Factors: Low birthweight, being male, winter season, respiratory infections, soft bedding, and inadequate prenatal or postnatal care.

Brain Development, Anatomy, and Connectivity

• Brain Growth

  • At birth, the brain is approximately $25\%$ of its adult weight; it reaches $75\%$ of its adult weight by age $2$.
  • The volume of the cortex triples between birth and age $2$.
  • Head-sparing: A biological mechanism that prioritizes brain growth when malnutrition affects the rest of the body. The brain is the final organ to be compromised by nutritional deficiency.

• Neuroscience Vocabulary

  • Neurons: Basic nerve cells in the central nervous system.
  • Axons: Fibers that extend from neurons to transmit impulses to other cells.
  • Dendrites: Fiber extensions that receive impulses from other neurons.
  • Synapses: The intersection/gap between the axon of one neuron and the dendrites of another.
  • Neurotransmitters: Chemicals that carry information across the synaptic gap.
  • Myelin: A fatty coating on axons that speeds up signal transmission (myelination).

• Brain Structures

  • Divisions: Hindbrain, midbrain, and forebrain.
  • Prefrontal Cortex: Involved in planning, impulse control, and anticipation.
  • Limbic System: Responsible for emotions and memory.
    • Amygdala: Processes emotions, especially fear.
    • Hippocampus: Central to memory processing.
    • Hypothalamus: Coordinates with the limbic system; releases the stress hormone cortisol.
  • Pituitary: Gland that responds to signals from the hypothalamus to release hormones.

Experience and Brain Maturation

• Exuberance and Pruning

  • Transient Exuberance: A massive but temporary increase in dendrites occurring in the first two years.
  • Pruning: The process where unused synapses atrophy and die to increase brain efficiency. This occurs primarily in the first two years and again during adolescence.

• Role of Experience

  • Experience-expectant: Essential, common experiences required for normal brain development (e.g., seeing objects, hearing language).
  • Experience-dependent: Variable experiences that may or may not occur, causing individual differences in brain development.
  • Lack of Stimulation: Severe deprivation can stunt brain growth, as seen in the Davenport, Iowa study.
  • Stress and Trauma: Excessive cortisol early in life can lead to lifelong abnormal stress responses. Shaking an infant can result in abusive head trauma (shaken baby syndrome).

Sensory Development: Perception and Sensation

• Definitions

  • Sensation: The response of sensory organs (eyes, ears, tongue, nose, skin) to a stimulus.
  • Perception: The brain's conscious awareness and interpretation of a sensation.

• Hearing

  • Hearing develops during pregnancy and is the most advanced sense at birth.
  • Hospital screenings in North America and Europe identify deafness early to allow for remediation.
  • Singing is effective at reducing infant distress.

• Vision

  • Vision is the least mature sense at birth. Newborns focus on objects between $4$ and $30$ inches away.
  • Binocular Vision: The ability to focus both eyes on a single object, appearing around the second or third month.
  • By $12$ months, infants prefer looking at people over other objects.

• Taste and Smell

  • These senses adapt quickly to the cultural social context.
  • Infants prefer the flavors of their parents' diet, encountered via amniotic fluid, breast milk, and family smells.
  • Newborns recognize their parents' scent, which aids in initiating breastfeeding.

• Touch and Pain

  • Touch (wrapping, cradling, massaging) is highly effective for soothing.
  • Neurological research has disproven the old belief that newborns do not feel pain; pain and temperature are biologically linked to the sense of touch.

Motor Skill Development

• Motor Skill Principles

  • Gross Motor Skills: Movements involving large muscle groups (e.g., walking, jumping).
    • Directionality: Development follows cephalocaudal (head-to-toe) and proximodistal (center-outward) patterns.
  • Fine Motor Skills: Small body movements, particularly of hands and fingers (e.g., writing, using the pincer grasp, self-feeding).

• Milestones ($50\%$ Completion Rate)

  • Gross Motor Milestones:
    • Holds head steady: $2$ months (Screening at $4$)
    • Rolls over: $5.7$ months (Screening at $6$)
    • Sits unsupported: $6.5$ months (Screening at $9$)
    • Pulls to stand: $8.5$ months (Screening at $12$)
    • Walks holding on: $9.7$ months (Screening at $12$)
    • Walks alone: $13$ months (Screening at $18$)
    • Kicks a ball: $14$ months (Screening at $24$)
  • Fine Motor Milestones:
    • Opens hands: $2$ months (Screening at $2$)
    • Holds a toy: $4$ months (Screening at $4$)
    • Thumb and finger grasp: $9.4$ months (Screening at $12$)
    • Scribbles: $13.5$ months (Screening at $18$)

Infant Health, Nutrition, and Survival

• Public Health and Survival Rates

  • In $1950$, $1$ in $6$ infants died before age $1$. By $2020$, the rate dropped to $1$ in $40$.
  • Key contributors: Clean water, better nutrition, and immunizations.

• Nutrition

  • Breastfeeding: Considered the best nutritional source.
    • Colostrum: High-calorie fluid secreted right after birth; contains lifesaving antibodies.
    • Benefits: Reduces risks of allergies, asthma, childhood obesity, and adult diabetes. It is sterile and nutrient-dense.

• Malnutrition Types

  • Protein-calorie malnutrition: Insufficient food intake leading to illness or death.
  • Stunting: Being too short for one's age due to chronic malnutrition.
  • Wasting: Being severely underweight for one's age.
  • Diseases: Marasmus (severe wasting) and Kwashiorkor (protein deficiency causing edema/swelling).

Immunization and Public Health

• Impact of Immunization

  • Immunizations have significantly extended the human life span and reduced diseases like measles, polio, and whooping cough.
  • They prevent long-term complications such as meningitis, blindness, and sterility.
  • Smallpox: Eradicated globally as of $1980$.
  • Polio: Effectively eliminated in the Americas.

• Herd Immunity

  • Definition: The level of immunity in a population needed to halt disease transmission.
  • Threshold: Usually requires a vaccination rate above $90\%$.
  • Scientific Evidence: Longitudinal and experimental studies confirm that immunizations do not cause disabilities.