Prenatal development spans three periods: the germinal period (conception to ~2 weeks, implantation), the embryonic period (weeks 2–8, major organ formation), and the fetal period (week 9 to birth, growth and refinement). The embryonic period is the most vulnerable to teratogens — substances or conditions (alcohol, certain medications, infections) that disrupt normal development. The developing organism is not simply a passive recipient; gene–environment interactions begin at fertilization, with maternal nutrition, stress hormones, and environmental exposures all shaping fetal trajectories.
Map each prenatal period to what is developing and what the key vulnerabilities are. Case studies of fetal alcohol spectrum disorder or rubella exposure illustrate teratogen timing effects vividly.
You already know from developmental psychology that development is not a simple unfolding of a genetic program but a continuous interplay between genes and environment. And from cell differentiation, you know that a single fertilized cell gives rise to hundreds of specialized cell types through regulated gene expression. Prenatal development is where this interplay plays out at its most dramatic: in nine months, a single cell becomes a fully formed organism with a beating heart, a functioning nervous system, and the capacity for behavior.
The three periods carve up development by what is happening biologically. The germinal period (conception to ~2 weeks) is defined by rapid cell division and implantation in the uterine wall — the embryo is a cluster of undifferentiated cells. The embryonic period (weeks 2–8) is when the body's architecture is established: the three germ layers differentiate into tissues, and the major organ systems — heart, brain, limbs, face — take shape through organogenesis. The fetal period (week 9 to birth) is defined by growth, refinement, and preparation for independent life: the fetus gains weight, reflexes become coordinated, and the brain undergoes extraordinary expansion. The division matters because vulnerability differs by period: the embryonic period is when structures are being built, making it the highest-risk window for teratogens to cause structural abnormalities.
Teratogen timing follows the logic of critical periods: each organ system has a window of maximum vulnerability during which it is actively forming. The heart is most vulnerable during weeks 3–8; limbs during weeks 4–8; the brain and nervous system throughout (with peak vulnerability in the embryonic period, continuing into the fetal period). A single alcohol exposure during peak neural migration can disrupt cortical organization in ways no later intervention can repair. This is why fetal alcohol spectrum disorder is irreversible — not because alcohol is uniquely toxic to the nervous system, but because it acts during a window of development that cannot be revisited.
The placental barrier is commonly misunderstood as a protective wall. In reality it is a selective filter. It blocks some large molecules and certain pathogens, but many drugs (including alcohol and nicotine), viruses (rubella, HIV), and stress hormones cross it readily. This means the fetal environment is directly shaped by maternal behavior, health, and stress. Maternal cortisol levels, for example, influence fetal HPA axis calibration, with effects on stress reactivity that persist into childhood. Gene–environment interaction begins at fertilization: the same genetic predisposition for a disorder may or may not express depending on prenatal exposures — nicotine exposure, nutritional deficits, maternal infection each alter the gene expression landscape of the developing organism.
The lesson that integrates all of this is about the irreversibility of timing effects. Development does not offer do-overs for critical windows. The brain does not get a second chance to wire itself, the heart to form its chambers, the palate to close. This makes prenatal development a paradigm case for the broader principle — relevant to critical periods in language, vision, and attachment — that *when* an experience occurs is often the decisive variable, not just what the experience is.