School › Growing up › Ages 15–16
Growth, puberty and the human lifespan
Prenatal stages, infant growth rates, the shape of the human growth curve, the hormonal control of puberty with negative feedback, brain development, and what life expectancy and healthy life expectancy mean. With a growth-velocity data analysis.
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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.[1]
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%.[1]
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.[4][1]
Hormonal control of puberty: the hypothalamus releases GnRH, which makes the anterior pituitary release FSH and LH. In males LH stimulates the Leydig cells of the testes to make testosterone; in females the developing follicles of the ovaries release estrogens and progesterone. These hormones cause the secondary sexual characteristics, for example a deeper voice and facial, armpit and pubic hair.[5][2]
Negative feedback: rising testosterone acts on the hypothalamus and anterior pituitary to reduce GnRH release, keeping hormone levels in balance. Around the mid-40s to mid-50s the ovaries lose sensitivity to FSH and LH, and periods stop (menopause).[5]
The frontal lobes of the brain are still developing in adolescence, and brain growth continues into the early 20s. In 2021, global life expectancy at birth was 71.4 years and healthy life expectancy (years lived in good health) was 61.9 years, both back to the level of 2012 after the COVID-19 pandemic.[1][6]
Try it: Growth velocity: analysing infant and childhood growth data
You need
- Graph paper or a spreadsheet
- Calculator
- The data table on the worksheet
- Optional: anonymised, published growth-chart data provided by the teacher
Safety
This is a data analysis: no measurements of pupils are needed. If you use class data, collect it anonymously and voluntarily; never display individual pupils' heights or weights. Growth questions about a particular person belong with a doctor or the school nurse.
What to do
- 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).
What happens, and why?
Length rises from about 49.5 cm to 74.9 cm in the first year (velocity about 25.4 cm/year) and to 87.4 cm in the second year (about 12.5 cm/year), so growth velocity roughly halves (a decrease of about 51%). This fits the description of growth velocity falling rapidly from birth to age four or five. Values rounded to 0.1 inch can be out by up to 0.05 inch (about 0.13 cm), so each velocity is uncertain by up to about ±0.25 cm/year.[1][4]
Play: Where is it made?
Each statement is about the hormones that control puberty. Does it happen in the hypothalamus, the anterior pituitary, or the gonads (testes or ovaries)?
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Quiz
Other ages: Ages 6–7 · Ages 10–11