A fertilized egg goes through several stages of development but initially is called a zygote. The next stages of development are blastocyst, embryo and fetus.
It takes between three and five days for the zygote to traverse the fallopian tubes into the uterus, where it develops into a blastocyst. The blastocyst, little more than a congregation of cells, implants in the uterine wall, where the amniotic sac grows. The embryo stage follows where the body takes form and internal organs begin to develop. Eight weeks after fertilization, the embryo is developed enough to be considered a fetus. During this final stage, the bodily components that formed as an embryo continue to mature until delivery.