Final answer:
From implantation until the end of the second month of pregnancy, the unborn offspring is known as an embryo.
Step-by-step explanation:
An unborn offspring from implantation to the end of the second month of pregnancy is called an embryo. The embryonic stage includes weeks 3-8 after fertilization. During this period, the embryo undergoes significant development, beginning as a blastocyst that implants in the uterus and growing to possess rudimentary organ systems.
By the end of the eighth week, the embryo exhibits foundational structures for all major organ systems, which will continue to develop and become functional as the embryo transitions into the fetal stage starting from the ninth week of gestation.