Brain Development

The brain grows at an amazing rate during development. At times during brain development, 250,000 neurons are added every minute!! At birth, almost all the neurons that the brain will ever have are present. However, the brain continues to grow for a few years after birth. By the age of 2 years old, the brain is about 80% of the adult size.
You may wonder, "How does the brain continue to grow, if the brain has most of the neurons it will get when you are born?". The answer is in glial cells. Glia continues to divide and multiply. Glia carries out many important functions for normal brain function including insulating nerve cells with myelin. The neurons in the brain also make many new connections after birth.
The Brain During Development
The nervous system develops from embryonic tissue called the ectoderm. The first sign of the developing nervous system is the neural plate that can be seen at about the 16th day of development. Over the next few days, a "trench" is formed in the neural plate - this creates a neural groove. By the 21st day of development, a neural tube is formed when the edges of the neural groove meet. The rostral (front) part of the neural tubes goes on to develop into the brain and the rest of the neural tube develops into the spinal cord. Neural crest cells become the peripheral nervous system.

At the front end of the neural tube, three major brain areas are formed: the prosencephalon (forebrain), mesencepalon (midbrain) and rhombencephalon (hindbrain). By the 7th week of development, these three areas divide again. This process is called encephalization.

Average brain weights at different times of development:
AGE BRAIN WEIGHT (grams)
20 weeks of gestation 100
Birth 400
18 months old 800
3 years old 1100
Adult 1300-1400
Brain Weight

(Data from Dekaban, A.S. and Sadowsky, D., Changes in brain weights during the span of human life: relation of brain weights to body heights and body weights, Ann. Neurology, 4:345-356, 1978) The top graph on the left shows the brain weights of males and females at different ages. The bottom graph shows the brain weight to total body weight ratio (expressed as a percentage). The adult brain makes up about 2% of the total body weight.
More on Brain Development


Time Magazine (February 3, 1997) had an excellent article about brain development called Fertile Minds. Dr. Arnold Scheibel has written a detailed, yet easy to understand description of the Embryological Development of the Human Brain. Read about brain reorganization, neuron migration, axon guidance and visual development in pages developed by the Society for Neuroscience.

Other excellent resources:
• Secret Life of the Brain - a PBS special exploring the brain from birth to old age
• Baby Talk - Learning Language - from US News and World Report, June 15, 1998
• Development and Neurobiology Column
• Stages of Development
• Understanding TV's effects on the developing brain
• The TEEN Brain - articles from US News and World Report (August 9, 1999). Some of these articles are available on-line.
• Inside the Teenage Brain - PBS
• Zero to Three - BrainWonders
• The brain during old age

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