Indicate the age when organogenesis takes place.

• A. first 2 weeks in utero
• B. first 3 weeks in utero
• C. first month of life
• D. first week in utero

Answer: (B)

Organogenesis is the stage when the major organ systems begin to form from the three germ layers, and this starts during the embryonic period, roughly around week 3 after fertilization and continues through about week 8. That means the beginning of organ formation occurs within the first three weeks in utero. The first week is mainly fertilization and cleavage, and the early pre-embryonic period extends through the first two weeks, so those windows don’t capture the onset of organ formation. The first month of life is after birth, which is far too late. Hence, the timing that best fits the start of organogenesis is the first three weeks in utero.