Biosocial Development: The First Two Years

Body Changes and Growth During Infancy

• Body Size and Physical Growth:

  • Weight: An infant's birthweight typically doubles by month $4$ and triples by the age of $1$ year.
  • Height: Height generally increases by approximately $10 \text{ in}$ by the first birthday.
  • Variation: Growth varies substantially based on genetics, nutrition, and initial birthweight. Infants may experience "catch-up growth" to reach expected developmental milestones.
  • Percentiles: Growth progress is often expressed in percentiles. It is critical to compare a child's current percentile with their previous measurements to ensure a healthy growth trajectory.

• Contributors to Weight Gain and Health:

  • Multiple factors affect weight, including allergies, the status of the microbiome, and micronutrient deficiencies.
  • Health is also impacted by diseases, sleep problems, and various causes contributing to obesity.

Sleep Patterns and Caregiving Strategies

• Sleep Requirements:

  • Throughout the lifespan, health and physical growth are correlated with regular and ample sleep.
  • Newborns typically sleep between $15 \text{ to } 17 \text{ hours}$ within a $24 \text{ hour}$ period.
  • REM (Rapid Eye Movement) Sleep: Approximately half of a newborn's sleep is REM sleep, characterized by flickering eyes behind closed lids, dreaming, and rapid brain waves.
  • Developmental Changes: During the first year, babies become more alert, resulting in less overall sleep and dreaming. Sleep gradually becomes longer and deeper as the child ages.

• Sleep Hygiene and the Five "S" Responses:

  • To assist the body clock, infants should be exposed to daylight during the day and darkness and silence at night. The $24 \text{ hour}$ body clock usually begins to develop around the third month.
  • The Five "S" Responses to crying: Swaddle, suck, sway, shush, and side position.
  • Routines: Infants are rapid learners; therefore, established routines are foundational for emotional and cognitive development.

• Sleeping Environments and Cultural Values:

  • Co-sleeping: A general term used when caregivers and children sleep in the same room. This is a common practice in Asia, Africa, and Latin America.
  • Bed-sharing: A specific form of co-sleeping where people sleep in the same bed. While North Americans often regard this as harmful, many other cultures view sleeping in separate rooms (common in North America) as emotional abuse.
  • Safety: Co-sleeping is considered safer when the baby is on a hard surface with no blankets. Bed-sharing poses a suffocation risk if soft bedding, pillows, or blankets are present or if the parent moves.

Sudden Infant Death Syndrome (SIDS) and the Triple-Risk Model

• Research by Susan Beal:

  • Investigated over $500 \text{ SIDS}$ cases in South Australia. She identified that most SIDS victims slept on their stomachs, whereas Chinese infants, who typically slept on their backs, had lower risks.
  • Her hypothesis confirmed that back-sleeping significantly reduced SIDS. In her test group, zero back-sleeping babies died.
  • Follow-up research in the Netherlands saw SIDS rates drop by $40\%$ in one year.
  • The "Back to Sleep" campaign in the $1990\text{s}$ reduced the U.S. SIDS rate by $50\%$ by $1996$.

• Risk Factors for SIDS:

  • Proven risks include stomach-sleeping, low birthweight, exposure to cigarette smoke, being male, winter months, respiratory infections, soft bedding, and poor prenatal/postnatal care.

• The Triple-Risk Model:

  1. Underlying Vulnerability: Undetected health issues.
  2. Specific Timing: Infants under $6 \text{ months}$ of age.
  3. External Stressor: Factors such as heat or cigarette smoke.

Brain Development: Exuberance and Pruning

• Brain Growth:

  • At birth, the brain is approximately $25\%$ of its adult weight. By age $2$, it reaches $75\%$ of its adult weight.
  • The volume of the cortex triples from birth to age $2$.
  • Head-sparing: A biological mechanism that protects the brain during malnutrition. The brain is the last part of the body to be damaged by lack of nutrients.

• Neuroscience Vocabulary:

  • Neurons: Nerve cells in the central nervous system.
  • Axons: Fibers that extend from neurons and transmit electrochemical impulses to other neurons.
  • Dendrites: Fibers that receive electrochemical impulses from other neurons.
  • Synapses: The intersection between the axon of one neuron and the dendrites of another.
  • Neurotransmitters: Chemicals that carry information across the synaptic gap.
  • Myelin/Myelination: The process of coating axons with an insulating fatty layer that speeds up transmission.

• Brain Structures:

  • Divisions: Hindbrain, midbrain, and forebrain.
  • Prefrontal Cortex: The area for anticipation, planning, and impulse control.
  • Limbic System: Includes the Amygdala, Hippocampus, and Hypothalamus (which releases cortisol).
  • Pituitary: A gland that responds to signals from the hypothalamus by secreting hormones.

• Transient Exuberance and Pruning:

  • Transient Exuberance: A temporary but vast increase in the number of dendrites during the first two years.
  • Pruning: The process where unused synapses atrophy and die to maximize brain efficiency. This occurs primarily in the first two years and again during adolescence, and it represents a specialization of the brain at the expense of plasticity.

Experience and Stimulation

• Types of Experience:

  • Experience-expectant: Basic common experiences that the brain requires to develop typically.
  • Experience-dependent: Variable experiences that may or may not occur, causing idiosyncratic brain development.

• Impact of Stimulation:

  • Sights, sounds, and social interactions connect neurons and grow dendrites. Research shows babies respond to language long before they speak.
  • Lack of Stimulation: Severe deprivation can stunt the brain (e.g., the Davenport, Iowa study).
  • Stress: Overabundance of cortisol early in life leads to lifelong unhealthy stress responses.
  • Harm: Shaking a baby can result in Abusive Head Trauma.

Sensation, Perception, and Motor Skills

• Sensory Development:

  • Sensation: The response of a sensory system (eyes, ears, tongue, etc.) to a stimulus.
  • Perception: Being conscious of a sensation; the brain processing the stimulus.
  • Hearing: Develops during pregnancy; newborns have sound-triggered reflexes. Singing reduces infant distress.
  • Seeing: Immature at birth. Focus is limited to between $4 \text{ and } 30 \text{ inches}$. Binocular vision develops by the second or third month. By $12 \text{ months}$, infants prefer looking at people over other objects.
  • Tasting and Smelling: Adapt to social context and the parent's diet (via amniotic fluid and breast milk). Introducing vegetables early helps teach healthy eating. Babies recognize the parent's scent, which aids in breast-feeding initiation.
  • Touch and Pain: Gentle touch (wrapping, massaging) soothes infants. While it was once believed newborns did not feel pain, neurological research has disproven this.

• Motor Skills:

  • Gross Motor Skills: Movements involving large muscles (walking, jumping). Development follows Cephalocaudal (head-down) and Proximodistal (center-out) patterns.
    • Milestones (50% Completion):
      • Holds head steady: $2 \text{ months}$
      • Rolls over: $5.7 \text{ months}$
      • Sits unsupported: $6.5 \text{ months}$
      • Pulls to stand: $8.5 \text{ months}$
      • Walks alone: $13 \text{ months}$
      • Kicks a ball: $14 \text{ months}$
  • Fine Motor Skills: Movements involving small muscles (fingers, tongue, toes).
    • Toward the end of the first year, infants master the pincer movement and self-feeding.
    • Milestones (50% Completion):
      • Opens hands: $2 \text{ months}$
      • Holds a toy: $4 \text{ months}$
      • Grasps with thumb/finger: $9.4 \text{ months}$
      • Scribbles: $13.5 \text{ months}$

Public Health, Nutrition, and Immunization

• Survival Statistics:

  • In $1950$, $1 \text{ in } 6$ infants died before age $1$. By $2020$, the rate dropped to $1 \text{ in } 40$.
  • Improvements are credited to clean water, nourishing food, and immunizations.

• Nutrition:

  • Breast milk is sterile, nutrient-rich, and easily digestible. It contains protective antibodies.
  • Colostrum: High-calorie fluid secreted early in lactation; crucial for preterm infants.
  • Benefits of breastfeeding: Lower rates of childhood asthma, allergies, and adult obesity/diabetes.

• Malnutrition:

  • Protein-calorie Malnutrition: Insufficient food intake leading to weight loss and disease.
  • Stunting: Chronically malnourished children remaining short for their age.
  • Wasting: Children being severely underweight for their age.
  • Specific Diseases: Marasmus (wasting) and Kwashiorkor (swelling/protein lack).

• Immunization:

  • Protects against measles, polio, mumps, and more. Smallpox was eradicated by $1980$.
  • Herd Immunity: Requires a population immunity rate usually above $90\%$ to stop disease transmission.
  • Evidence confirms immunizations do not cause disabilities.