Final answer:
The term referred to from conception to 12 weeks is the embryonic period. In this stage, the embryo undergoes significant development including the formation of organ systems.
Step-by-step explanation:
The term that is referred to the time from conception to 12 weeks is a. Embryonic period. During this time frame, a developing human is called an embryo. This period, which spans from weeks 3 to 8 after fertilization, involves significant growth and changes that include cell division, migration, and differentiation. By the end of the embryonic period, all of the organ systems are formed in their rudimentary state. In contrast, the fetal period begins at the start of the ninth week after fertilization and lasts until birth, while the neonatal period spans the first to the thirtieth day of life outside of the uterus.