Which stage primarily leads to the formation of all major organ systems?

The formation of all major organ systems occurs primarily during embryonic development, which is a critical period that takes place from the third week to the eighth week after conception. During this stage, the fertilized egg undergoes significant changes, and the basic structures of the body begin to form. This process includes the differentiation of the three germ layers—ectoderm, mesoderm, and endoderm—into various organs and systems, such as the heart, lungs, liver, and nervous system.

In contrast, during conception, fertilization occurs, but this is not yet a stage where organ systems are developed. The fetus does not undergo significant growth and development of organ systems until the fetal development stage, which follows embryonic development. While maturation involves the final stages of growth and development, it does not specifically pertain to the initial formation of organ systems. Hence, focusing on embryonic development highlights the critical timeframe during which major organ systems are established and structured.