During which period of prenatal development are most major organs formed?

The embryonic period is crucial in prenatal development as it is during this time that most major organs and systems begin to form. This period typically spans from the second to the eighth week of gestation. During these weeks, the basic structures of the heart, brain, spinal cord, and other vital organs are established as the embryo undergoes significant changes, including the formation of the neural tube, which later develops into the central nervous system.

This phase is characterized by rapid cell division and differentiation, which is essential for the development of organs and systems. Any disruptions during this critical period can lead to congenital abnormalities or developmental challenges, emphasizing the importance of this stage for the overall health and viability of the fetus.

The other stages mentioned, such as the zygote stage and the trophoblast period, occur earlier and primarily focus on the initial cell division and implantation processes rather than organ development. The fetal period follows the embryonic period and predominantly involves growth and maturation of the already formed organs and systems, rather than their formation. Therefore, the embryonic period is the correct answer, as it is the defining time for the initial formation of major organ systems in the developing embryo.