When do the brain, spinal cord, and internal organs begin to develop in a human embryo?

In human prenatal development, the brain, spinal cord, and internal organs begin to develop early in the embryonic period, with these foundational changes typically completed by about the fourth week after conception.1

The process begins with the formation of the neural plate approximately 18 days after fertilization. This structure subsequently folds to form the neural tube, which serves as the precursor to the central nervous system, with the cephalic portion developing into the brain and the caudal portion becoming the spinal cord. Concurrently, through the process of gastrulation, the embryo differentiates into three tissue layers—the ectoderm, mesoderm, and endoderm—which give rise to the various internal organs and systems of the body.21