Final answer:
Fetal age begins at the ninth week from fertilization and progresses until birth, focusing on the growth and maturation of the organism. Embryonic age is from fertilization to the end of the eighth week, where fundamental organ systems and structures form. The transition marks a shift from basic development to more detailed growth and maturation.
Step-by-step explanation:
The difference between fetal age and embryonic age lies in the stages of prenatal development and the terminology used based on the time from fertilization. Embryonic age refers to the period from fertilization until the end of the eighth week, where rapid changes, such as the formation of the heart, nervous system, and primary organ structures occur. Fetal age, on the other hand, begins at the start of the ninth week post-fertilization and continues until birth, extending about 30 weeks, characterized by further development and maturation of the organs, the skeleton, and other physical features.
During the embryonic stage (weeks 3-8), the multi-cellular organism implants in the uterus, begins to form the placenta, and organ systems start to emerge, while the fetal stage (weeks 9-40) sees the differentiation of gonads, specialization of the circulatory system, and ossification of the skeleton. The embryo is highly susceptible to toxins as the major organs are forming, indicating its fragile state. By the end of the embryonic period, all organ systems are in rudimentary form, yet not fully functional.