During which trimester does organogenesis primarily occur?

Organogenesis primarily occurs during the first trimester of pregnancy, which is a critical period for the development of the major organs and bodily systems in the embryo. This stage begins in the third week after fertilization and continues until approximately the end of the first trimester, around the 12th week. During organogenesis, the three germ layers formed during gastrulation—ectoderm, mesoderm, and endoderm—differentiate into specific tissues and organs. For example, the heart, lungs, kidneys, and liver start to take form during this time.

The second and third trimesters primarily involve further growth and maturation of these organs and systems rather than their initial formation. By the end of the first trimester, most major organ systems are established, although they will continue to develop and refine throughout the rest of the pregnancy. Hence, identifying the first trimester as the key period for organogenesis is essential to understanding human development.