Growth

Soon after fertilisation, the zygote begins to divide. The single cell divides to form two cells, then four and so on.
As the embryo grows into a foctus, and the foetus grows into a baby, this cell division continues. Each cell grows, then divides, grows, then divides - over and over again. This carries on all through childhood, until a person has reached adulthood and stops growing.

Growth takes place as cells grow and divide.

Development

The change from a zygote to an adult does not involve only growth. It also involves development. As the tiny embryo develops, its organs gradually form. For example, it develops a heart, lungs and a brain.
When a baby is born, it has all of its organs. But development continues. Its muscles become stronger as it learns to crawl, walk and run. Its brain develops, as it learns to talk and play with toys.
Each person is an individual. Each of us develops in slightly different ways, and at a slightly different pace. The chart shows the main stages in development that everyone passes through. Notice that there are no sharp changes from one stage to another.

The main stages in a person's development.

Adolescence

At around the age of 12 or 13 in boys, and 10 or 11 in girls, a big step in development takes place. The reproductive organs and the brain undergo quite large changes. There is often a growth spurt (a sudden, rapid period of growth) at this time.
This time of change from childhood to adulthood is called adolescence.
Changes in the reproductive organs In girls, menstruation begins. Hormones procluced by the reproductive organs cause changes in body shape, as breasts develop and hips widen. Hair begins to grow in the armpits and other parts of the body.
In boys, sperm production begins. Hormones produced by the reproductive organs cause body shape changes, as shoulders broaden. The voice becomes deeper. Hair begins to grow on the face, armpits and other parts of the body.
Changes in the brain
The brain does not grow any larger during adolescence. But there is quite a lot of reorganisation in the brain, which makes a person think and feel differently from when they were a child.
• The person becomes better at making decisions and planning ahead. The ability to think logically improves. This is a time when many people find they can learn more quickly .
• Emotions may become stronger. People may worry more. They may begin to have romantic feelings. They become more self-aware.
• During adolescence, there is often a strong need for approval by friends and others. Young people may want to be like a role model. This can be stressful if they set themselves impossible standards, perhaps trying to be like someone they see on television.