The stage of the conceptus when organogenesis is occurring is referred to as the embryo stage. During this stage, which typically spans from around the 3rd to the 8th week of development in humans, the major organs and body systems begin to form. This includes the development of the neural tube, which will give rise to the central nervous system, as well as the development of organs such as the heart, lungs, liver, and kidneys. After the completion of organogenesis, the conceptus is referred to as a fetus. Therefore, the correct answer is "Embryo."