Every fact in the three growing-up lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Where the facts are simplified or less certain: all ages for milestones, puberty and life stages are typical ranges or averages [1][2]; every child develops at their own pace, and a child who is earlier or later than the average is usually completely normal. The newborn length of about 50 cm is a rounded conversion of the textbook average of 19.5 inches [1]. The head-to-body proportions (about a quarter at birth, about a seventh in adults) are approximate [4]. Life expectancy figures (71.4 years, healthy life expectancy 61.9 years in 2021) are worldwide estimates from the WHO [6] and differ a lot between countries. The puberty hormones are described in a simplified way [5]. These lessons are general education, not medical advice: questions about a child's own growth or development belong with a parent or carer, a doctor or the school nurse.
All sources
[1] Psychology 2e, 9.3 Stages of Development
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
Prenatal development has three stages: germinal (weeks 1–2), embryonic (weeks 3–8) and fetal (weeks 9–40). By 24 weeks a baby born early has a chance to survive. Newborn reflexes (rooting, sucking, grasping, Moro) usually disappear around 4–5 months. Newborns weigh 5–10 pounds and are on average 19.5 inches long; weight typically doubles in six months and triples in one year; length is 29.5 inches at 12 months and 34.4 inches at 2 years. Babies hold their heads up around 6 weeks; most sit alone at 7 months; walking follows crawling. A 2-year-old's brain is 55% of adult size, about 90% by age 6. Adolescence begins at puberty (typically ages 12–18); growth spurt in girls begins between 8 and 13 (adult height 10–16), in boys between 10 and 16 (adult height 13–17). The frontal lobes are still developing in adolescence; brain growth continues into the early 20s. Adulthood begins around 20 with early, middle and late stages; physical abilities peak in young adulthood; menopause around 50; late adulthood begins in the 60s, when reaction time slows and muscle strength diminishes; crystallized intelligence holds steady or improves while fluid intelligence declines.
NHS (UK National Health Service) · Reliability: High
Type: National health service
Puberty is when a child's body begins to change as they become an adult. It is normal for puberty to start any time between the ages of 8 and 14. Signs include hair starting to grow around the armpits and genitals, girls developing breasts and starting periods, and boys developing a deeper voice.
Healthier Together, NHS North West London · Reliability: Medium-high
Type: NHS health information for young people
Puberty happens to everyone; it is part of growing up. The brain releases hormones that tell the body it is time to change. Not everyone goes through it at the same time. Changes include sweating more, greasier skin, hair growing in new places such as the armpits, and a rollercoaster of emotions, which is normal.
From birth until age four or five the rate of growth in height declines rapidly; the velocity is practically constant from five or six up to the beginning of the adolescent spurt; in boys from about 13 to 15 there is marked acceleration of growth. In the newborn infant the head represents about a quarter of the total length; in the adult about one-seventh.
[5] Biology 2e, 43.4 Hormonal Control of Human Reproduction
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
The hypothalamus sends gonadotropin-releasing hormone (GnRH) to the anterior pituitary, which releases follicle stimulating hormone (FSH) and luteinizing hormone (LH); at puberty these are released for the first time. LH stimulates the Leydig cells in the testes to make testosterone, responsible for male secondary sexual characteristics such as a deeper voice and facial, axillary and pubic hair. Estrogens and progesterone are released from developing follicles in the ovaries. Rising testosterone acts on the hypothalamus and anterior pituitary to inhibit GnRH release (negative feedback). In the mid-40s to mid-50s the ovaries lose sensitivity to FSH and LH and periods cease: menopause.
[6] COVID-19 eliminated a decade of progress in global level of life expectancy (news release, 24 May 2024)
World Health Organization (WHO) · Reliability: High
Type: International health agency
Global life expectancy at birth fell by 1.8 years between 2019 and 2021 to 71.4 years; global healthy life expectancy fell by 1.5 years to 61.9 years in 2021, both back to the 2012 level.