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Human Development: All Stages and Key Activities

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1. Prenatal Stage (Conception to Birth)

a) Germinal Period (0-2 weeks)

• Fertilization occurs; the zygote forms
• Rapid cell division (cleavage) produces a morula, then a blastocyst
• Implantation into the uterine wall occurs around day 6-10
• The placenta begins to develop

b) Embryonic Period (Weeks 2-8)

• The three germ layers form: ectoderm, mesoderm, endoderm
• Neural plate folds into the neural tube (future brain and spinal cord); the ectoderm immediately superficial becomes the neural crest, giving rise to the peripheral nervous system
• All major organ systems begin forming (organogenesis)
• The heart starts beating around week 4
• Limb buds, face, and sensory organs emerge
• This is the most sensitive period to teratogens

c) Fetal Period (Week 9 to Birth)

• Rapid growth and maturation of all systems
• At peak neuronal proliferation in the mid-second trimester, 250,000 neurons are born each minute - Kaplan & Sadock's Synopsis of Psychiatry
• Neurons migrate to their adult cortical positions guided by glial fibers; this occupies most of the first 6 months of gestation
• Synaptogenesis begins furiously in the second trimester
• By the third trimester, the fetus can hear, respond to light, and perform breathing movements
• Myelination begins prenatally

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2. Neonatal Period / Infancy (Birth to ~2 years)

Physical Development

• At birth, brain mass is only 26% of adult brain mass; it reaches 55% by 1 year
• Fontanels and sutures allow brain growth; they close allowing only 20% additional growth beyond age 2 - Guyton & Hall Medical Physiology
• Behavioral milestones (from chart below):

Cognitive Development (Piaget: Sensorimotor Stage, Birth-2 years)

• Infants learn through sensory observation and motor exploration
• Born with reflexes (sucking); early schemas are built on these
• Key achievement: object permanence - understanding that objects exist even when out of sight
• At ~18 months: symbolization begins - creating mental images of objects and using words to represent them
• Synaptogenesis peaks in the first 2 years: 30 million synapses form each second at its peak

Social/Emotional Development

• Attachment to primary caregiver forms (Bowlby's attachment theory)
• Stranger anxiety appears around 8 months
• Separation anxiety reflects lack of fully consolidated object permanence

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3. Early Childhood / Toddler & Preschool (2-7 years)

Physical Development

• Continued rapid growth; brain reaches near-adult proportions by end of year 2
• Motor skills refine: running, jumping, drawing, dressing
• Myelination continues, mostly complete by early childhood (though full extent not until the 3rd decade)

Cognitive Development (Piaget: Preoperational Stage, 2-7 years)

• Symbols and language used much more extensively
• Egocentrism: child cannot take another person's perspective (e.g., Piaget's three-mountain task)
• Animism: inanimate objects are thought to be alive
• Centration: focus on only one aspect of a situation at a time
• No conservation: a child believes a tall, thin glass holds more water than a short, wide one with the same volume
• Thinking is intuitive, not logical; events are not yet linked by cause and effect
• Fantasy and magical thinking are prominent
• Language develops rapidly: vocabulary expands from ~50 words at age 2 to thousands by age 5

Social/Emotional Development (Erikson: Initiative vs. Guilt, ages 3-5)

• Play becomes central to learning and social interaction
• Parallel play gives way to cooperative play
• Gender identity begins to form
• Moral development begins: distinguishing right from wrong (Kohlberg's preconventional level)
• Freud's phallic stage (ages 3-6): Oedipal/Electra complex; identification with same-sex parent

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4. Middle Childhood / School Age (7-11 years)

Physical Development

• Steady, slow physical growth
• Fine and gross motor skills become well-coordinated
• Girls begin puberty-related changes earlier (by end of this stage)

Cognitive Development (Piaget: Concrete Operations, 7-11 years)

• Children begin to think logically about concrete objects
• Conservation is achieved (substance at 6-7 years; number, length, liquid at 7-8; area at 8-10)
• Reversibility: understanding that actions can be reversed (ice → water → ice)
• Seriation: ordering objects along a dimension (size, weight)
• Classification: grouping objects into categories and subcategories
• Inductive reasoning develops; deductive reasoning is still limited
• Focus shifts to academic learning, rule-based games

Social/Emotional Development (Erikson: Industry vs. Inferiority)

• Industry: children master skills valued by society (reading, writing, mathematics)
• Peer relationships become very important
• Self-concept and self-esteem develop
• Kohlberg's conventional morality: rules are followed to gain approval and maintain social order

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5. Adolescence (11/12 to 18/20 years)

Physical Development

• Puberty: triggered by hypothalamic-pituitary-gonadal axis activation
  • Girls (11-13 years): estrogens cause rapid height growth, but early epiphyseal closure at ~14-16 years ends growth
  • Boys (13-17 years): testosterone causes height increase, later epiphyseal fusion, resulting in greater final height - Guyton & Hall
• Secondary sexual characteristics develop (breast development, pubic hair, voice changes, etc.)
• Brain undergoes significant synaptic pruning - prefrontal cortex matures last

Cognitive Development (Piaget: Formal Operations, 11 years onward)

• Abstract thinking: ability to reason about hypothetical situations, symbols, and possibilities
• Hypothetico-deductive reasoning: form a hypothesis and test it systematically
• Deductive reasoning: moves from general principles to specific conclusions
• Can think about permutations, combinations, and probabilities
• Interest in philosophy, religion, ethics, and politics
• Language use becomes complex and grammatically formal
• Note: not all adolescents reach full formal operations - some remain in concrete operational mode throughout life - Kaplan & Sadock's Synopsis of Psychiatry

Social/Emotional Development (Erikson: Identity vs. Role Confusion)

• Central task: forming a coherent sense of personal identity
• Peer influence at its peak; risk-taking behaviors increase
• Independence from parents increases
• May experience self-consciousness due to new capacity for meta-cognition (thinking about thinking)
• Adolescent egocentrism: "imaginary audience" and "personal fable"

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6. Early Adulthood (20-40 years)

Physical Development

• Peak physical fitness, strength, and reproductive capacity
• Myelination reaches full extent in the late third decade
• Sensory acuity begins to decline slightly (late 20s)

Cognitive Development

• Formal operational thinking fully consolidated
• Post-formal thought: ability to deal with ambiguity, contradictions, and real-world complexity
• Career and professional skill development
• Problem-solving becomes more pragmatic

Social/Emotional Development (Erikson: Intimacy vs. Isolation)

• Establishing intimate romantic relationships
• Starting families; parenthood
• Building careers and social networks
• Developing adult identity separate from family of origin

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7. Middle Adulthood (40-65 years)

Physical Development

• Gradual decline in physical performance
• Menopause in women (~age 50); hormonal changes in men (andropause)
• Increased risk of chronic diseases (cardiovascular, metabolic)
• Vision and hearing begin to decline more noticeably

Cognitive Development

• Crystallized intelligence (accumulated knowledge, vocabulary) continues to increase
• Fluid intelligence (speed, working memory) begins a slow decline
• Peak career expertise and judgment

Social/Emotional Development (Erikson: Generativity vs. Stagnation)

• Generativity: concern for guiding and nurturing the next generation
• Parenting, mentoring, community contribution
• "Midlife transition": reevaluation of life choices and goals
• Sandwich generation: caring for both children and aging parents

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8. Late Adulthood / Old Age (65 years onward)

Physical Development

• Progressive decline in most organ systems (sarcopenia, osteoporosis, reduced cardiac output)
• Immune senescence; increased vulnerability to infection and cancer
• Neuronal loss; brain volume decreases
• Sleep architecture changes; slower reaction times

Cognitive Development

• Working memory and processing speed decline
• Long-term memory and procedural memory relatively preserved
• Wisdom and perspective-taking often increase
• Risk of dementia rises with age (Alzheimer's disease most common)

Social/Emotional Development (Erikson: Ego Integrity vs. Despair)

• Life review: reflecting on the meaning and worth of one's life
• Integrity: sense of satisfaction and acceptance of one's life as it was lived
• Despair: regret, bitterness if life review is negative
• Coping with loss: retirement, death of peers/spouse, declining independence
• Many older adults show resilience and high life satisfaction (paradox of aging)

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Summary Table

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• Piaget - Cognitive stages (Sensorimotor → Preoperational → Concrete Operations → Formal Operations)
• Erikson - Psychosocial stages (Trust → Autonomy → Initiative → Industry → Identity → Intimacy → Generativity → Integrity)
• Kohlberg - Moral development (Preconventional → Conventional → Postconventional)
• Bowlby - Attachment theory