During the prenatal period, human embryos go from a few cells to a fully formed baby in about 38 weeks. The human brain has its own timeline of growth during this period, and it tends to be consistent for healthy human fetuses. The major stages of prenatal brain development occur throughout 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 come up with the timeline for normal prenatal brain development thanks to several avenues of research. These include postmortem investigations on non-surviving embryos and fetuses in various fetal development stages. Doctors have also performed experiments and dissections on the fetuses of animals that have similar brain structures as humans, such as monkeys. Lastly, modern technologies such as imaging allow scientists to examine and monitor prenatal brain development in fetuses while they're still in their mothers' wombs. Through all of these methods, experts have charted the physical development of the brain as well as the emotional and mental development.
Prenatal brain development begins in the very first month of the prenatal period. During this time, all of the other major organs are taking shape. By the third week of life, the human heart has started its beating. It is about this time that the earliest form of a brain takes shape. Called the neural tube, this structure eventually evolves into the spinal cord and the brain.
In the second and third months of development, the embryo's facial features 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. Correspondingly, by the third month of prenatal development, the embryo begins to show reactions and reflexes. The fetus' reactions to outside experiences, such as bright lights and noises, continue to develop into the fourth month, as does the tiny being's ability to make facial expressions.
The fifth month of prenatal brain development begins to see the fetus actually control these expressions and reactions. At this point, physical development is such that the fetus can stretch and even flip in the womb. Experts believe that the ability to consciously react to sensations get even stronger during the sixth month, when the cerebral cortex splits into separate lobes. The fetus at this point might even have the ability to remember.
The final stages of prenatal brain development in the last trimester lead to a brain that becomes nearly similar to a fully formed human's, at least in structure. By month seven, the prenatal brain develops the grooved, curved look that's associated with the adult brain, and myelin develops on the outside of nerves from the brain to insulate them. Brain waves can be detected. Then in month eight, the major cortices form, such as the auditory and the visual, allowing the fetus to comprehend what he or she sees and hears. The fetal brain contains about as many neuron cells as an adult brain by the final month, though it will only be one-fourth the size of an adult's at birth.