What is a fertilized egg called?

A fertilized egg goes through several stages of development, but is initially called a zygote. The next stages of development are blastocysts, embryos, and fetuses.

It takes three to five days for the zygote to pass through the fallopian tubes into the uterus, where it becomes a blastocyst. The blastocyst, little more than a congregation of cells, implants itself in the uterine wall, where the amniotic sac grows. The embryo stage follows where the body takes shape and the internal organs begin to develop. Eight weeks after fertilization, the embryo develops enough to be considered a fetus. During this final stage, the body components that formed as an embryo continue to mature until delivery.

–