Neurons grow like wildfire before birth! The developing brain generates between 50 and 100 thousand new cells per second from the fifth through twentieth weeks of gestation. These brain cells migrate to different locations in the brain and begin to differentiate, although many more neurons than are needed are produced.
Eventually about 75 percent of these cells will die in the normal development of the brain and nervous system. This process – programmed cell death – is known as apoptosis. Of those cell connections present at birth, some 40 percent will be pruned, just like weeding a garden to dispose of what is not needed.
Birth to Age 3
It might surprise you to know that a three year old has about twice as many connections as an adult, but who has more “intelligence”? From birth through about age three there are vast numbers of connections and collections being recorded in the brain, but there is no understanding or organizing of this wiring. The brain does not yet have the ability to distinguish what is important.
Ages 3 to 12
From age three through twelve the brain begins to realize, as a result of collecting too much, that it has recorded events in more than one place, resulting in dense sets of synapses. At this point it begins to prune the excessive synapses in an attempt to get organized and eliminate what is not necessary.
Ages 12 to 19
The teenage years consist of more aggressive pruning as the brain begins to specialize and build an identity. While the brain does a ton of learning in the early years, the bulk of learning takes place after the frenzy of forming synapses has stabilized and the duplicated synapses have been pruned.
Adulthood ushers in a bit of a pruning plateau, where some connections are diminished and others are enhanced. These are the years of using the brain as life experiences fine-tune the neural connections!
For more on brain development, check out J. Madeleine Nash’s article, Fertile Minds, in TIME.