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Prenatal Stages of Brain Development

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Instructor: Lisa Roundy

Lisa has taught at all levels from kindergarten to college and has a master's degree in human relations.

Prenatal brain development occurs during the first, second, and third trimester stages. Explore prenatal development, the growth that happens during each trimester, and learn about neural tubes and neurons. Updated: 08/30/2021

Prenatal Brain Development

Imagine that one day you decide to count nonstop until you get to one billion. Let's assume that you could count one number every second on average, so it would take you a billion seconds. After doing the math, this means that it would take approximately 32 years to count to one billion.

Now, let's consider the fact that one tiny cell in an embryo grows into a fully-developed brain with 100 billion cells by the time a baby is born. To put this into perspective, remember that it would take you roughly 32 years to count to one billion (averaging one number per second). This means it would take you about 3,200 years to count to 100 billion! Yet, in only nine months, a human brain - with 100 billion cells - is formed. This means that at least 250,000 brain cells are created every minute on average! Because of this rapid pace of development, proper prenatal care is essential to the development of an unborn child's brain.

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  • 0:06 Prenatal Brain Development
  • 1:22 First Trimester
  • 2:22 Second Trimester
  • 3:34 Third Trimester
  • 4:49 Lesson Summary
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The First Trimester

The first three months of pregnancy, or the first trimester, is when basic brain structure begins to form. In the first month, just 16 days after fertilization, an embryo forms the neural tube. The neural tube is the earliest nervous system tissue and eventually develops into the brain and the spinal cord.

During the second month of development, the neural tube begins to differentiate into brain cells and nerve cells. The brain cells transform into recognizable brain structures - most notably, the cerebral cortex is formed. The cerebral cortex is the outer layer of the brain. By the third month, this basic brain development allows the embryo to display reflexes and react to its environment. At the end of the first trimester, hormones are released that will determine whether development will continue to occur as male or female.

The Second Trimester

The second trimester is the fourth, fifth, and sixth months of a pregnancy. During the course of the second trimester, the brain becomes fully developed. Even though the brain has not reached its full size, most of the neurons within the central nervous system are present. A neuron is another name for a nerve cell, or the cells that conduct electrical impulses and make up the brain, spinal column, and nerves.

During the fourth month, the fetus begins to prepare itself for the outside world. The fetus can experience sensory sensations such as sound, taste, and smells. By the fifth month, the fetus is learning to control its reactions to these sensory sensations and to control its movement. The ability to consciously react to sensory sensations becomes even stronger during the sixth month. During the sixth month, another major mark of brain development occurs; the cerebral cortex splits into two separate hemispheres. Some researchers believe that the fetus develops the ability to remember at this stage.

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