In fetal development, what gestational age defines the embryo versus the fetus?

• A. 0-8 weeks and 8 weeks to birth
• B. 0-2 weeks and 2-8 weeks
• C. 1-6 weeks and 6-40 weeks
• D. 4-8 weeks and 8-36 weeks

Answer: (A)

The boundary between the embryonic and fetal periods is based on what’s happening in development. The embryonic period covers organ formation, called organogenesis, and lasts through about the first eight weeks after fertilization. By the end of week eight, the major organ systems are established, and the focus shifts to growth and maturation in the fetal period, which runs from roughly week nine to birth. Some resources describe this cutoff as eight weeks for the end of the embryonic stage and from eight weeks onward for the fetal stage, which is a common exam convention. That’s why the answer pairing embryonic development up to eight weeks with fetal development from eight weeks to birth is used, even though many texts explicitly state the fetal period begins at week nine.