During which trimester does the majority of fetal organogenesis occur?

The majority of fetal organogenesis occurs during the first trimester of pregnancy. This period is critical as it encompasses the formation and initial development of vital organs and systems, including the heart, brain, liver, lungs, and kidneys. Organogenesis refers to the process through which the organs and structures of the fetus develop from the embryonic layers.

During the first trimester, particularly between weeks 3 and 8, the embryo undergoes significant changes. The cells differentiate into three primary germ layers: the ectoderm, mesoderm, and endoderm, which eventually give rise to all the organ systems. By the end of the first trimester, most of the major organs are formed, setting a foundation for further development during the subsequent trimesters.

In contrast, the second and third trimesters focus more on the growth and maturation of these already established organs rather than their initial formation. The postpartum period, while crucial for maternal recovery and bonding with the newborn, is outside the context of fetal development and organogenesis. Thus, identifying the first trimester as the correct answer highlights the importance of this stage in fetal development.