Britannica Sources used in this answer

What major changes occur during the embryonic period of human prenatal development?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

A series of medical illustrations showing the stages of childbirth: the baby's position at the onset of labor, flexion, internal rotation of the head, extension, external rotation of the head, and the uterus immediately after birth with the placenta and umbilical cord. — birth Sequential changes in the position of the child during labour.

The embryonic period, which lasts from the third to the eighth week of development, is a critical phase characterized by organogenesis, the process during which the body’s primary organ systems are formed.1

Key changes during this period include:

Gastrulation: Early in the third week, the embryo differentiates into three primary germ layers—the ectoderm, mesoderm, and endoderm—which serve as the precursors to all future tissues and organs.234
Neurulation: The formation of the central nervous system begins with the development of the neural tube, the precursor to the brain and spinal cord.24
Body Plan Establishment: The flat embryonic disk undergoes folding to become a cylindrical structure. By the fourth week, the embryo develops a recognizable C-shape, and limb buds appear.24
Organ Development: Internal organs, including the heart and liver, begin to form and function. By the end of the eighth week, most internal organs are well established, and the embryo begins to exhibit human-like external features, such as a distinct face, eyes, and jointed limbs.245

This period concludes when the basic structure of the organism is complete, marking the transition to the fetal stage.67