What marks the beginning of the fetal period?

The beginning of the fetal period is characterized by the significant development and maturation of the fetus, which starts at approximately the ninth week of pregnancy. During this time, the developing organism transitions from the embryonic stage, where basic structures and organ systems begin to form, into the fetal stage, where growth and refinement of these structures occur.

In the fetal period, the focus shifts from the formation of organs to their growth and functional development. This stage continues until birth, during which the fetus undergoes notable changes in size, complexity, and development, preparing it for life outside the womb.

In essence, the ninth week is crucial because it signifies the switch to a phase where the developing human is called a fetus and is poised for the various physiological changes required for survival after birth. This understanding is foundational for grasping human development in prenatal stages.