How Humans Grow: From One Cell to Adulthood

One Cell, One Body

Every human being begins as a single fertilised cell — a zygote about 0.1 mm across. From that starting point, a series of extraordinary biological processes unfolds over roughly two decades, producing a fully grown adult of some 37 trillion cells arranged into specialised tissues, organs, and systems.

Growth is not simply about getting bigger. It involves differentiation — the process by which identical early cells become specialised as neurons, muscle fibres, bone cells, or skin — as well as maturation, as organs and systems gradually become fully functional. Understanding these stages gives a clearer picture of why children develop the way they do, and what drives the milestones we observe.

Before Birth: The Fastest Growth of All

The nine months of prenatal development represent the most dramatic growth rate of any period in human life. In just 38 to 40 weeks, a single cell divides and transforms into a newborn with a beating heart, functioning kidneys, a brain, and a complete skeletal frame.

The first two months after conception are called the embryonic period. During this time, all the major body structures are laid down. By eight weeks, the embryo — now called a foetus — has a recognisable human form, though it is only about 3 centimetres long. The remaining seven months are largely a period of growth and refinement, as organs grow larger and more specialised.

The brain develops throughout the entire prenatal period and continues developing actively for years after birth — more on that shortly.

Infancy and Early Childhood: Rapid Early Growth

Newborns grow faster in the first year of life than at any other time after birth. A typical baby roughly doubles its birth weight by about five months and triples it by around twelve months. Height increases by approximately 25 centimetres in the first year alone.

This rapid growth is driven primarily by growth hormone, produced by the pituitary gland — a small structure at the base of the brain. Growth hormone stimulates the liver to release insulin-like growth factor 1 (IGF-1), which acts on growth plates in bones and on many other tissues to promote growth and cell division.

Brain development during infancy is particularly intense. Neurons form trillions of new connections (synapses), and a process called myelination — coating nerve fibres in a fatty insulating layer that speeds up nerve signals — begins and continues for years. The rapid cognitive and motor milestones of the first two years (sitting, crawling, walking, talking) reflect this neurological growth.

Between ages 2 and 5, growth slows relative to infancy but remains steady. Children gain roughly 5 to 8 centimetres and about 2 kilograms per year. Body proportions shift: the head, which is disproportionately large at birth, becomes more in proportion as the trunk and limbs grow faster.

Middle Childhood: Steady and Coordinated

From around age 6 to the beginning of puberty, children grow at a relatively steady, moderate pace — roughly 5 to 6 centimetres and 2 to 3 kilograms per year. This phase is sometimes called the latency period for growth, though that term is slightly misleading: significant brain, muscle, and immune system development continues throughout this stage.

Motor skills become increasingly refined and coordinated during this period. Children develop better balance, hand-eye coordination, and fine motor control — skills that allow increasingly complex physical and cognitive tasks. The brain continues to prune excess synaptic connections (a process called synaptic pruning) while strengthening the connections that are used most, gradually increasing processing efficiency.

You can see how hormones drive development throughout this and every other stage in our hormones guide.

Puberty: The Hormonal Turning Point

Puberty is triggered by the hypothalamus signalling the pituitary gland to release luteinising hormone (LH) and follicle-stimulating hormone (FSH). These hormones stimulate the gonads — the ovaries in females and testes in males — to produce sex hormones: primarily oestrogen and progesterone in females, and testosterone in males.

These sex hormones drive the physical changes of puberty and are also responsible for the adolescent growth spurt — the most dramatic phase of height increase after infancy. Peak height velocity during the growth spurt is typically 8 to 12 centimetres per year. Girls tend to enter puberty and the growth spurt about 1 to 2 years earlier than boys, on average, though there is wide individual variation.

Beyond height, puberty involves maturation of reproductive organs, development of secondary sex characteristics, increases in bone density and muscle mass, and changes in body fat distribution. Brain development during adolescence is also significant — the prefrontal cortex, responsible for reasoning, impulse control, and planning, continues developing well into the mid-twenties.

Our growth and development guide covers the biological mechanisms of puberty in greater depth.

How Bones Know When to Stop Growing

Long bones — the ones in the arms, legs, and fingers — grow at special regions near their ends called epiphyseal plates, or growth plates. These plates are made of cartilage, which is softer than bone and can be added to as the bone elongates. As new cartilage forms on one side of the plate, older cartilage on the other side is converted to bone through a process called endochondral ossification.

Sex hormones, particularly oestrogen (present in both males and females), eventually cause the growth plates to close — converting entirely to bone. Once fused, bones cannot lengthen further. Growth plate closure typically happens in girls between ages 14 and 16, and in boys between ages 16 and 18, though both timing and pace vary significantly between individuals.

This is why the growth spurt associated with puberty ultimately ends: rising sex hormones first drive rapid growth, then close the very plates that allowed the growth to happen.

Brain Development: The Last Organ to Fully Mature

Of all the body's organs, the brain takes longest to reach full maturity. While the brain is close to its adult volume by around age 10, its internal organisation continues changing well into the mid-twenties. The prefrontal cortex — the region behind the forehead that handles planning, judgment, impulse control, and social reasoning — is the last area to fully mature.

This extended development period is not a design flaw; it reflects the brain's extraordinary adaptability, shaping itself in response to experience and learning throughout adolescence. It also helps explain some characteristic adolescent behaviours, including increased risk-taking and emotional reactivity, which have a genuine neurological basis.

You can explore the brain's structure and function further in our human body flashcards tool, or read about development in the context of ageing in our aging explained post.

What Influences How Tall You Grow?

Height and overall growth are influenced by a combination of genetic and environmental factors. Genetics accounts for roughly 60 to 80% of height variation in populations. The remaining influence comes from nutrition, health during childhood, sleep, hormonal function, and socioeconomic factors that affect access to food and healthcare.

Adequate nutrition is particularly important. Protein provides the amino acids needed to build new tissue. Calcium and vitamin D support bone mineralisation. Zinc is required for growth hormone function. Prolonged deficiency in any of these can impair growth, which is why access to a balanced diet during childhood has measurable effects on adult height at the population level.

Sleep also matters more than many people realise — growth hormone secretion peaks during deep sleep. Chronic sleep deprivation during childhood can interfere with normal growth hormone release, one of many reasons adequate sleep is considered essential during development.

This is general educational information. If you have concerns about a child's growth — whether it seems too slow, too fast, or otherwise unusual — a paediatrician is the right person to assess and advise.