Biological Development: Prenatal, Neural, Pubertal, and Aging Processes
Biological development questions ask you to map physical change across the lifespan onto underlying mechanisms - genes, brain growth, hormones - and to know when the organism is most vulnerable to disruption.
Prenatal periods
- Germinal/zygotic (conception–week 2): cleavage, blastocyst, implantation.
- Embryonic (weeks 3–8): organogenesis. This is the window of greatest risk for structural malformation.
- Fetal (week 9–birth): growth and maturation; teratogens now mainly affect size, brain organization, and later behavior.
Teratogens act with dose, timing, genotype, and cumulative exposure. Examples: alcohol (no known safe threshold; Fetal Alcohol Spectrum Disorder brings facial dysmorphology, microcephaly, and attention/learning deficits), thalidomide (limb defects with exposure roughly days 20–36 post-conception), rubella in the first trimester (deafness, cataracts, cardiac defects), isotretinoin, lead, mercury, Zika, and maternal smoking (low birth weight).
Critical versus sensitive periods
A critical period has a hard boundary after which the capacity is lost; a sensitive period is a window of heightened plasticity with gradual decline. Sensory and language systems fit the sensitive-period model better than a strict critical-period one.
Brain and motor development
- Neurogenesis is largely prenatal; synaptogenesis, pruning, and myelination continue for decades.
- Sensory and motor cortex mature ahead of prefrontal cortex, which supports the usual cephalocaudal (head-to-tail) and proximodistal (center-to-edge) sequence.
- Infant reflexes (rooting, Moro, palmar grasp, stepping) appear and drop out on a schedule; persistence or absence is a screening flag.
- Motor milestones come in broad ranges: sitting without support ~6–9 months, walking independently ~12–15 months. Crawling is no longer treated as a required milestone in current CDC guidance.
Puberty and aging
- Adrenarche (adrenal androgens, ~6–9 years) precedes gonadarche. Girls: breast budding ~8–13, menarche ~12.5 in U.S. samples. Boys: testicular enlargement ~9–14. A secular trend toward earlier puberty is documented.
- Aging: menopause ~51, gradual andropause, presbyopia by the 40s, high-frequency hearing loss, bone-density loss, sarcopenia, and modest volume decline in prefrontal cortex and hippocampus.
Worked example
A woman takes a drug at 6 weeks post-conception; her infant is born with a heart defect. Reasoning: 6 weeks falls in the embryonic period, the peak window for structural organ malformation, so a structural defect is a plausible timing match. The same drug taken at 28 weeks would more likely produce growth restriction or behavioral effects. Dose and genotype still modulate risk, but timing is the first thing to test.
Traps that cost marks
- Swapping critical and sensitive periods.
- Reversing cephalocaudal and proximodistal.
- Claiming teratogens matter only in the first trimester - true for structural defects, not for brain and behavioral effects.
- Assuming every infant crawls on schedule.