The developing human organism from the implantation of the fertilized ovum in the uterus, about two weeks after conception to the eighth week after conception.

The embryonic period of fetal development is one of rapid growth when the major organ systems begin forming and the main external features first appear. It is also a critical time for prenatal health and development—the most sensitive of all the stages of human development. During this period miscarriages or birth defects can be caused by a variety of factors, including maternal illness or use of alcohol and other drugs, and exposure to radiation. An enormous amount has been learned about embryos through ultrasonography and the study of miscarried embryos.

At the beginning of the embryonic period, a structure known as the embryonic disc is formed from the inner cells of the fertilized egg. The outer cells, collectively known as the trophoblast...