According to Oxford Dictionaries, when speaking of human development, the term embryo refers to the first eight weeks of pregnancy, while fetus refers to the later stages of pregnancy after eight weeks. The first stages of pregnancy are referred to as the zygote, which is followed by the cells developing into an embryo.
According to MedlinePlus, cells within the embryonic stage begin to multiply and take on specific functions. These functions include creating blood and nerve cells. This process is important, as these different cells and their functions make up what will eventually become a human being with a heart, brain and organs. During the eighth week of pregnancy, the heart beats with a regular pattern, reports the Office on Women’s Health. Embryonic cell growth is fast during this stage of pregnancy resulting in external forms taking shape.
The term fetus refers to the stages of a pregnancy after eight weeks when the cells have formed enough to look more like a miniature human being. The fetus has arms and legs by the ninth week of pregnancy, and the brain and lungs begin to form at this point, according to MedlinePlus. By the 10th week of pregnancy, the fetus has eyelids, ears and other facial features. After the 10th week, the fetal period begins, marking the beginning of the gestational age of a human pregnancy.