Which landmark fetal development occurs during the first trimester?

The formation of organs is a crucial landmark of fetal development that occurs during the first trimester. This period is characterized by rapid cellular differentiation and organogenesis, where the foundational systems such as the heart, kidneys, liver, and other vital organs begin to develop from the embryonic tissue. By the end of the first trimester, most of the major organs are formed, setting the stage for further growth and development throughout the pregnancy.

While the growth of limbs, development of the nervous system, and formation of facial features also take place during the first trimester, they are more specific aspects of organ formation. For instance, limb development begins around the 4th week of gestation, while the rudiments of the nervous system start forming shortly after conception but are part of a larger process involving organ development. Facial features begin appearing later in the first trimester but are not as comprehensive in terms of the overall organ systems that form at this stage. Thus, focusing on organ formation encapsulates a broad and vital phase of early fetal development.