What term of pregnancy does cells organize to form body structure?

During the embryonic period, which spans from weeks 3 to 8 of pregnancy, cells organize to form the basic body structures and systems. This process includes gastrulation, where the three germ layers—ectoderm, mesoderm, and endoderm—are formed, leading to the development of organs and tissues. By the end of this period, the foundations for the major body systems are established.