echoesof.science · Ages 15–16
Growth, puberty and the human lifespan
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Prenatal development has three stages: germinal (weeks 1–2), embryonic (weeks 3–8) and fetal (weeks 9–40). By 24 weeks a baby born prematurely has a chance to survive; by week 37 the organ systems are developed enough to survive outside the uterus without many premature-birth risks.
Infant growth is fast: average newborn length is 19.5 inches (about 49.5 cm), reaching 29.5 inches (about 74.9 cm) at 12 months and 34.4 inches (about 87.4 cm) at 2 years. Weight typically doubles in six months and triples in one year. A 2-year-old's brain is about 55% of adult size; by age 6 it is about 90%.
The growth velocity (growth per year) falls rapidly from birth to age four or five, is almost constant from five or six until the adolescent growth spurt, and then rises sharply. The spurt typically begins between 8 and 13 in girls and between 10 and 16 in boys.
Experiment: Growth velocity: analysing infant and childhood growth data
- Convert the textbook lengths to cm (1 inch = 2.54 cm): 19.5 in at birth, 29.5 in at 12 months, 34.4 in at 24 months.
- Plot length (cm) against age (months). Describe the shape of the curve.
- Calculate the growth velocity in cm per year for 0–12 months and for 12–24 months.
- Calculate the percentage decrease in velocity from the first year to the second year.
- Rounding: the textbook values are given to 0.1 inch. What is the largest possible error in each length in cm, and in each velocity?
- Extension: sketch the full growth-velocity curve from birth to adulthood using the description in the text (falling, then nearly constant, then the adolescent spurt).
| Age (months) | Length (inches) | Length (cm) | Velocity (cm/year) |
|---|---|---|---|
| 0 | |||
| 12 | |||
| 24 |
Questions
Calculate the growth velocity (cm/year) for 0–12 months and 12–24 months from the textbook data, and the percentage decrease.
Draw a flow diagram of the hormonal control of puberty, from the hypothalamus to the gonads, including one negative-feedback arrow.
🍎Explain the difference between life expectancy and healthy life expectancy, using the WHO figures for 2021.
The lengths are rounded to 0.1 inch. Estimate the uncertainty in the first-year velocity in cm/year.
Sources for every fact on this sheet: echoesof.science/school/growing-up/sources/