Prenatal Development Stages

Why This Matters

Prenatal development is one of the most heavily tested topics in developmental psychology because it illustrates foundational concepts you'll see throughout the course: critical periods, nature vs. nurture interactions, and biological maturation sequences. When the AP exam asks about teratogens, sensitive periods, or the origins of developmental disorders, it's drawing directly from your understanding of what happens before birth. These nine months establish the template for how psychologists think about timing, vulnerability, and environmental influence across the entire lifespan.

You're being tested on more than just "what develops when"—you need to understand why certain periods matter more than others, how environmental factors interact with biological programming, and what makes some developmental processes irreversible. Don't just memorize that the embryonic stage is weeks 2-8; know that this is when organogenesis makes the developing organism maximally vulnerable to teratogens. Each stage and process illustrates a broader principle that will appear in multiple-choice questions and FRQs alike.

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The Three Major Stages

Prenatal development unfolds in a predictable sequence of three stages, each defined by what structures are forming and how vulnerable the organism is to disruption. Understanding these stages means understanding developmental timing.

Germinal Stage (Conception to 2 Weeks)

• Fertilization creates a zygote—a single cell containing the complete genetic blueprint from both parents, marking the true beginning of development
• Rapid cell division (mitosis) transforms the zygote into a blastocyst as it travels through the fallopian tube toward the uterus
• Implantation occurs around days 6-10 when the blastocyst attaches to the uterine wall, establishing the pregnancy and triggering hormonal changes

Embryonic Stage (2 to 8 Weeks)

• Organogenesis occurs—all major organs and body structures begin forming from three germ layers, making this the most critical period for structural development
• Maximum teratogen vulnerability exists because rapidly differentiating cells are highly sensitive to environmental disruption
• Support structures develop including the placenta and umbilical cord, creating the maternal-fetal connection that sustains development

Fetal Stage (8 Weeks to Birth)

• Growth and refinement dominate as organs mature, body systems become functional, and the fetus increases dramatically in size
• Behavioral development emerges—movements begin around 8-10 weeks, reflexes like sucking and grasping appear, and responsiveness to external stimuli increases
• Viability becomes possible around 24 weeks when lungs and other vital organs can support life outside the womb

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Key Cellular Processes

These biological mechanisms explain how a single cell becomes a complex organism. The AP exam expects you to connect these processes to broader concepts like differentiation and critical periods.

Zygote Formation

• Genetic combination occurs when sperm and egg fuse, creating a cell with 46 chromosomes—23 from each parent
• Totipotent cells mean the early zygote can develop into any cell type, representing maximum developmental potential
• Cell division begins immediately, launching the developmental cascade that continues for nine months

Gastrulation

• Three germ layers form—ectoderm (skin, nervous system), mesoderm (muscles, bones, circulatory system), and endoderm (internal organs, digestive system)
• Cellular differentiation begins as cells commit to specific developmental pathways, losing their totipotent flexibility
• Body plan establishment occurs as this reorganization sets the foundation for all subsequent organ development

Neurulation

• Neural tube formation occurs around weeks 3-4, creating the structure that becomes the brain and spinal cord
• Folic acid is critical during this period—deficiency can cause neural tube defects like spina bifida
• CNS development continues throughout pregnancy but this early foundation determines basic neural architecture

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Environmental Influences and Vulnerability

This is where nature meets nurture in the most literal sense. The AP exam frequently tests your understanding of how, when, and why environmental factors affect prenatal development.

Teratogens and Their Effects

• Teratogens are environmental agents that cause developmental malformations—including alcohol, certain medications, infections (rubella, Zika), and radiation
• Timing determines damage type—exposure during organogenesis causes structural defects; later exposure affects growth and function
• Dose-response relationships mean that amount and duration of exposure matter, though some teratogens (like alcohol) have no known safe level

Critical Periods of Development

• Windows of maximum vulnerability exist when specific structures are actively forming and most susceptible to disruption
• Irreversibility characterizes critical period damage—once the window closes, the developmental opportunity is lost
• Different structures, different timing—the heart's critical period is weeks 3-6, while the brain remains vulnerable throughout pregnancy

Maternal Nutrition and Its Impact

• Folic acid prevents neural tube defects and is most critical in the first weeks, often before pregnancy is confirmed
• Iron supports oxygen delivery to developing tissues; deficiency linked to low birth weight and cognitive effects
• Malnutrition effects vary by timing—early deficiency affects organ formation, later deficiency affects growth and brain development

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Developmental Milestones and Assessment

These concepts connect prenatal development to real-world applications in medicine and psychology—a favorite AP exam angle.

Fetal Movements and Reflexes

• Quickening (mother feeling movement) typically occurs around 16-20 weeks, though movement begins much earlier
• Primitive reflexes develop in utero—sucking, grasping, and startle responses that will be tested after birth
• Movement patterns indicate health—changes in fetal activity can signal distress or developmental problems

Viability

• 24 weeks marks general viability—the point at which survival outside the womb becomes possible with medical intervention
• Lung development is the key factor—surfactant production enables breathing, making this the limiting organ system
• Medical advances continue shifting this threshold, raising ethical questions about intervention and outcomes

Prenatal Testing and Screening

• Ultrasound visualizes development and can detect structural abnormalities, growth patterns, and multiple pregnancies
• Genetic screening (blood tests, amniocentesis, CVS) identifies chromosomal abnormalities like Down syndrome
• Ethical considerations arise around testing decisions, information disclosure, and pregnancy choices

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Quick Reference Table

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Self-Check Questions

1. Which two processes both involve cellular reorganization during the first month, and how do they differ in what structures they create?

2. If a pregnant person is exposed to a teratogen at week 5 versus week 30, how would you predict the effects would differ, and why?

3. Compare and contrast the embryonic and fetal stages in terms of what is developing and the type of vulnerability present in each.

4. A student claims that "the fetal stage is the most important because it's the longest." Using your knowledge of critical periods, explain why this reasoning is flawed.

5. An FRQ asks you to explain how prenatal development illustrates the interaction between biological maturation and environmental factors. Which three examples from this guide would best support your answer, and why?