How Your Baby’s Brain Grows
During fetal development, your baby’s brain produces at least twice as many brain cells as he needs. The excess cells are only loosely wired in with a few dendrites and synaptic connections to other brain cells. If they do not make a certain minimum number of connections with other brain cells, and do not receive enough nourishing nerve growth factor as a result of the synapses that have been made, these dendrites and synaptic connections are programmed to eventually wither and die.
At least 40 per cent -and sometimes as many as 75 per cent – of these early brain neurons are lost during prenatal development. This pruning process starts around the seventh month and reaches its peak during the eighth month of pregnancy. By stimulating your baby in the womb, more neural pathways are hard-wired in to expand the range of possibilities for future development. After he is born, the number of synapses in his brain will again explode exponentially to form quadrillions of new synaptic connections. This process will continue up until puberty. Selective pruning will then once again dominate as your child’s brain undergoes its last stage of shaping as he matures into adulthood.
Your baby is born with the maximum number of brain cells he will ever own. Yet at birth, his brain only weighs around 350g. During the first year of life, his brain will almost treble in weight to up to 1,000 g. By the time he reaches adulthood, his brain will contain hundreds of millions of cells fewer than he has at birth, yet will still weigh an average of 1.5 kg. The number of nerve cells present have halved since birth, yet the brain has quadrupled in weight. How can this be?
As each neuron develops a new connection with another nerve cell, it triggers the appearance of tiny bumps on the cell’s surface from which a fine, dendritic extension develops rather Ike a TV aerial. Dendrites act to increase the volume and surface area of each cell so that it has a larger ‘landing site’ for connecting up with axonal endings (telodendria) from other nerve cells. The more connections a neuron receives, the more dendrites it grows in response. In other words, the more information the neuron senses is out there, the more antennae it puts “put to receive it.
The neuron is literally making itself more receptive to information from other parts of the brain. By putting out ‘feelers’ in this way, each neuron cell body becomes increasingly connected to the neural net, which therefore expands at a rapid – almost exponential – rate. At the same time, the dendrites are putting out more thorny spines which, pith increased stimulation, expand from the thin popsickle shape to the thick, mushroom shape as described previously. It is this increasing number of branches and spines that fill most of the grey matter and contribute to the increase in volume and thickness of the brain.
In fact, as the cerebral cortex expands in size with continued stimulation and learning experiences, the cell bodies in the cortex are pushed further and further apart by their proliferating dendrites. This widening continues until around age 1 year, then selective pruning starts to remove unwanted and under-used synaptic connections as those in active circuits continue to grow and become stronger. This dismantling of unused circuits doesn’t seem to occur to any great extent until puberty, however, but increases in rate from the age of around 10 years.
Another major contributor to brain weight are the glial cells and myelin sheaths. As the brain becomes more active, the glial cells get larger and wrap themselves around the growing neurons to supply them with nourishment. The fatty myelin sheaths surrounding the axons of some brain cells form when glial cells multiply in number and wrap themselves around an axon like concentric bands of insulating tape. By the time of birth, only the nerve cells controlling vital functions such as breathing heart rate, temperature, reflexes, seeing hearing and touch have started to myelinate.
Nerve cells in the cerebellum are not fully myelinated until the age of 2. As nerve tracts gain their fatty sheaths and electrical transmission becomes faster, so your baby acquires new skills such as reaching for objects, standing crawling walking running and hand skills. The parts of the brain involved in learning (hippocampus, cerebral hemispheres) and the part of the brain involved in controlling the sleep-wake cycle (reticular activating system) will not have matured and myelinated until your child has reached puberty. As a result, the human ability to learn, remember, think, interpret and plan all continue to mature until adulthood.
The tripling in brain weight during the first year of life is a growth rate unique to humans. This is essential to the survival of our species. Your baby has to be born at a stage of development when he is not so helpless he cannot survive, but his brain is still small enough to be delivered through your pelvis. By the time of birth, your baby’s head is already around 2 per cent larger than your birth canal, and will only fit through because his skull bones are designed to overlap each other, allowing the shape of the head to mould.
If the brain were much larger, enough to support intelligent behaviour, normal delivery would become impossible. It is thought that Neanderthal man became extinct due to his over-large head interfering with childbirth. Cro-Magnon man, on the other hand, with his smaller head, continued to multiply and gave rise to Homo sapiens – modem man.