At which week does the fetal period begin?

• A. Week 9
• B. Week 6
• C. Week 12
• D. Week 3

Answer: (A)

The phase of development switches from organ formation to growth and maturation at this point. The embryonic period runs from fertilization through the end of the eighth week, and the fetal period begins at the start of nine weeks gestational age and continues to birth. So, starting at nine weeks, the body has established the basic organ systems, and the focus shifts to growth and refinement of those structures. That’s why nine weeks is the correct marker for the beginning of the fetal period. Earlier weeks are still part of the embryonic period (where organs are forming), while later weeks are part of the ongoing fetal period. If you’re thinking in terms of fertilization versus gestational age, nine weeks of gestational age corresponds to about seven weeks post-fertilization.