The human brain has a consistent growth timeline during the prenatal period, with major stages occurring throughout the nine months of pregnancy. Research methods include post-mortem investigations, experiments on animals with human-like brain structures, and modern imaging technologies. The neural tube forms in the first month, the cerebral cortex in the second, and the senses and facial expressions in the fourth. The fetus begins to control these expressions and reactions in the fifth month, and the cerebral cortex divides into separate lobes in the sixth. By the last trimester, the brain becomes almost fully formed in structure.
During the prenatal period, human embryos grow from a few cells to a fully formed baby in about 38 weeks. The human brain has its own growth timeline during this period and tends to be consistent for healthy human fetuses. The major stages of prenatal brain development occur during the nine months of pregnancy, including the creation of the neural tube in the first month, the growth of the cerebral cortex in the second month, the development of the senses in the fourth month, and the separation of the cortex into lobes in the sixth month.
Scientists have been able to work out the timeline for normal prenatal brain development through several avenues of research. These include post-mortem investigations of non-surviving embryos and fetuses at various stages of fetal development. Doctors have also performed experiments and dissections on the fetuses of animals that have human-like brain structures, such as monkeys. Finally, modern technologies such as imaging allow scientists to examine and monitor prenatal brain development in fetuses while they are still in the womb. Through all of these methods, experts have traced the physical development of the brain as well as emotional and mental development.
Prenatal brain development begins in the first month of the prenatal period. During this time, all other major bodies are taking shape. By the third week of life, the human heart has started beating. It is in this period that the first form of the brain takes shape. Called the neural tube, this structure eventually evolves into the spinal cord and brain.
In the second and third months of development, the facial features of the embryo begin to develop into a more human form, including the mouth, nose, eyes, eyelids, and ears. The neural tube at this time begins to transform into what will become the main parts of the brain, including the cerebral cortex. As a result, by the third month of prenatal development, the embryo begins to show reactions and reflexes. The fetus’s reactions to external experiences, such as bright lights and noises, continue to develop in the fourth month, as does the baby’s ability to make facial expressions.
The fifth month of prenatal brain development begins to see the fetus actually controlling these expressions and reactions. At this point, physical development is such that the fetus can stretch and even turn over in the womb. Experts believe that the ability to consciously react to sensations becomes even stronger during the sixth month, when the cerebral cortex divides into separate lobes. The fetus at this point may also have the ability to remember.
The final stages of prenatal brain development in the last trimester lead to a brain that becomes almost like a fully formed human’s, at least in structure. By the seventh month, the prenatal brain develops the grooved and curved appearance associated with the adult brain, and myelin develops on the outside of the brain’s nerves to insulate them. Brain waves can be detected. Then, in the eighth month, the main cortices, such as the auditory and visual cortices, form, allowing the fetus to understand what it sees and hears. The fetal brain contains about as many neuronal cells as an adult brain by the last month, even though it will be only one-quarter the size of an adult at birth.
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