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A fetus is a developing baby, or any type of mammal I suppose as long as it's a vertebrate, after the embryonic stage and before birth.
A baby is known as an embryo until about 8 weeks, then it is known as a fetus until birth.
The sperm fertilizes the egg and it begins to grow as an embryo. During the eight week of development it becomes recognizable as being human and is called a fetus.