This period consists of the germinal (first 2 weeks), embryonic (weeks 3–8), and fetal (weeks 9–40) stages. The fertilized egg (zygote) undergoes rapid cell division and differentiation. Major organs and body structures begin to form, and the fetus becomes responsive to environmental influences like maternal nutrition, toxins, and stress. Research focuses on teratogens (e.g., drugs, malnutrition) and their long-term effects on development^6,7.

Infants experience rapid physical and neural growth. Birth weight typically triples by age two, and the brain reaches around 80–90% of its adult size⁸. Motor milestones include lifting the head, crawling, and walking. Cognitive development follows Piaget’s sensorimotor stage, where infants learn object permanence and basic problem-solving. Socially, this is the stage of attachment formation and Erikson’s trust vs. mistrust.

Physical growth slows, but motor skills and independence improve (running, dressing, drawing). Language expands rapidly, and children enter Piaget’s preoperational stage, characterized by symbolic thought but limited logic. They begin developing self-concept and initiative. Focus is often on creativity, literacy, and social play⁹.

Steady physical growth continues. Cognitively, children enter Piaget’s concrete operational stage and grasp logical rules, conservation, and classification . Social development includes forming peer relationships and engaging in teamwork. Erikson’s industry vs. inferiority stage emphasizes building competence through school and social experiences¹⁰.

Adolescence begins with puberty and includes major physical, cognitive, and emotional transitions. Teens typically reach Piaget’s formal operational stage allowing for abstract reasoning. Erikson’s identity vs. role confusion stage highlights the search for values, beliefs, and identity. Emotional regulation fluctuates due to hormonal shifts and peer pressures¹¹.

Physical peak is reached, though fertility and metabolism begin to decline. Cognitive capacity stabilizes, and some propose the development of postformal thought. Erikson’s stage is intimacy vs. isolation, as individuals pursue romantic partnerships, careers, and parenthood. Life achievements such as higher education and employment are emphasized¹².

Signs of aging appear, and health issues like menopause may emerge. Cognitive abilities are often maintained with increased expertise and practical intelligence. Erikson’s stage is generativity vs. stagnation, involving the nurturing of others and societal contributions. Midlife often involves reevaluation of goals and taking on new social roles like caregiving for parents¹³.

This stage varies widely between individuals. Physical decline may occur, including sensory loss and chronic illness. Cognitive aging includes some memory reduction, though crystallized intelligence often remains stable. Erikson’s stage is integrity vs. despair as individuals reflect on life. Research explores dementia, successful aging, and emotional well-being in this stage¹⁴.

Development is cumulative — earlier experiences like secure attachment in infancy can shape outcomes in adolescence and adulthood. Psychologists study both normative changes (common to most individuals) and individual differences at each stage of life^6,15.