The brain serves as the material foundation for intelligence, with fetal and infant stages representing the most critical periods for brain and central nervous system development. Strictly defined, the fetal period spans from the 9th week of pregnancy until birth, while the preceding gestational phase is technically termed the embryonic period.The embryonic stage marks the development from a single cell—the fertilized egg—into an organism with rudimentary human form;
the fetal stage primarily involves the continued growth and differentiation of organs and tissues that began forming during the embryonic stage. For the purposes of this book, the term "fetal stage" broadly encompasses both periods without distinction. The brain is an extraordinarily complex structure, with its most fundamental building blocks being brain cells, or neurons.
During embryonic development, brain growth parallels organismal growth, broadly divided into three phases: the first involves increasing cell numbers; the second sees growth in both cell numbers and volume; the third features continued volume expansion without further cell proliferation. These sequential processes overlap.
Among human developmental structures, the brain develops earliest. The neural plate emerges around the 18th embryonic day.Initially smooth, the brain's surface develops complex sulci and gyri by the 13th week. Within the cerebral cortex, specialized neurons for different functions primarily complete their differentiation and development between the 15th and 18th weeks of gestation. Human brain development accelerates rapidly from the mid-pregnancy period onward.
The first phase of rapid brain cell proliferation occurs between the 3rd and 6th months of gestation. The growth of brain cell volume and nerve fibers continuously increases brain weight.The second phase of rapid brain cell growth occurs between the 7th and 9th months of pregnancy. During this period, established neurons develop synaptic connections to facilitate the transmission of excitatory signals. The most active and vigorous period of human brain cell development spans the final three months of pregnancy through the first three months after birth. During this time, cell proliferation and cell enlargement overlap, driving rapid development. This overlapping growth pattern persists until around age two.Brain cell proliferation occurs as a one-time event; missed opportunities cannot be compensated for. Therefore, the fetal period and the first two years of life constitute a critically foundational stage for brain growth and development, determining both the quantity of brain cells and the quality of their maturation. After age two, development primarily involves increases in brain cell size and weight, alongside the cultivation of cognitive abilities.