An embryo is the stage of development between the zygote or fertilized egg and the newly formed offspring, formed by repeated mitotic division of the zygote. In humans, the term "embryo" is used to describe the stage of development from fertilization to 8 weeks of pregnancy, after which it is termed a fetus. The development of an embryo begins with fertilization, which results in the formation of a zygote. The zygote then divides repeatedly to give rise to a ball of cells. The stage when cells produced by the division of the zygote begin to form groups that develop into different tissues and organs of the body is termed as an embryo.