Brain Development Timeline

Brain development begins shortly after conception and continues throughout a person's life. At birth an infant has essentially all the neurons she will ever have. Processes such as myelination and synaptogenesis are well underway before an infant is born, but the brain is not complete at birth.

The Prenatal Period

Prenatal Photograph The formation of neurons begins very early in the human embryo.  By five weeks after conception, the cells in the developing brain begin dividing rapidly to form the 100 billion or so neurons that an infant's brain has at birth. Once formed, neurons begin to migrate to the correct location in the brain, and some synapses begin forming. 

 

Prenatal conditions — including temperature, pressure and fetal movements such as kicking— stimulate the development of synapses.  Myelination of neurons also begins prenatally, starting with the neurons of the spinal cord and brain stem.

The Newborn Period

Photograph of a NewbornAt birth, the infant brain weighs 2/3 to 3/4 pounds (300-350 grams) and contains 100 billion neurons.  Connections in the parts of the brain that control basic survival and reflexes are already well-developed, and myelination in those areas is nearly complete.

 

The newborn begins a rapid period of brain growth. Synaptogenesis continues very rapidly, with neurons in all parts of the brain making trillions of connections. Myelination continues in most parts of the brain. The neurons controlling hearing and vision rapidly become myelinated.

 Early and Middle Childhood

The early childhood years are a period of rapid change in the brain. During early and middle childhood, the brain forms and refines a complex network of connections in the brain through synaptogenesis, pruning, and myelination.

Photograph of  Young Child Playing

 

The process of forming connections is biologically driven, but experiences also promote synapse formation. The brain produces many more synapses than it will ultimately use. Researchers describe this process as synaptic overproduction. This rapid synapse formation continues throughout early childhood.

 

The process of myelination also continues during early childhood and is the major cause of the increase in a child's brain size.  In the first four years of life, the brain increases to 80% of its adult weight of 2.6 - 3.3 pounds (1200-1500 grams).

 

Pruning is a key process that shapes the brains of young children. Synaptic overproduction causes synapses to develop extremely rapidly. The pruning process refines these connections based on experience. Connections used regularly become stronger and more complex. Connections not used are considered non-essential, and the brain eventually prunes them away to increase efficiency.

 

As an example, an infant's brain has connections that allow her to hear sounds from all languages in the world. During the early years, the brain strengthens connections for sounds in the languages she hears regularly. Over time, the brain eliminates the connections for other sounds. This is why most adults have trouble distinguishing sounds that are not in our language.

Adolescence

Photograph of a TeenagerThe brain continues to change and mature throughout adolescence. An adolescent's brain reaches its adult weight by about age fourteen, partly because of increased myelination. 

 

As myelination and pruning continue during the teen years, adolescents become more capable of insight, judgment, inhibition, reasoning, and social conscience.  Increased activity in the frontal lobes enables the adolescent to begin comparing or interrelating several concepts at once.

 

Myelination of the frontal lobes is not complete until very late in adolescence. Some researchers estimate that frontal-lobe development continues until age 25 to 30.  The regions in the frontal lobe which are responsible for judgment, planning, assessing risks, and decision-making are the last areas to finish developing.

 

Repeated experiences create complex networks of synaptic connections.  Connections strtengthened through regular use become stronger and more complex. Selective pruning of non-essential connections also continues during adolescence.  Most pruning takes place between ages ten and sixteen. The pruning process enables the brain to operate more efficiently and provides room for networks of essential connections to expand. 

 

Synapse formation continues in adolescence, even as pruning is ongoing. The remaining dendrites continue to branch, grow, and form new synapses in response to new experiences.  Continued social, emotional, and cognitive development in adolescence is due, in part, to this ongoing growth in the brain.

Adulthood

Brain development continues into adulthoodEven in adulthood, the brain is continuously remodeling itself. The brain continues to develop connections throughout adulthood, but with two major differences:

 

The rate of synapse formation is much slower than in childhood.

 

Synapses are formed based only on specific experiences in the adult's life.

 

For example, adults who witnessed a catastrophic event — such as the collapse of the World Trade Center in New York City — formed connections in the brain that allow them to rememberand process what they saw. Adults who were not there, but watched news coverage of the attacks, formed different networks of connections that help them remember what they saw and heard on the news.

Aging and the Brain

Lifelong activity is important to maintain healthy brain growth. Areas of the brain that are not used regularly may eventually atrophy. Keeping the mind active is a key way to prevent brain atrophy. Activity can be as simple as reading, working crossword puzzles, or spending time talking to others and maintaining relationships. A person who isolates himself stops having experiences that keep the brain active.