Human embryogenesis is the process of cell division and cellular differentiation of the  embryo that occurs during the early stages of development. In biological terms, human development entails growth from a one celled zygote to an adult human being.  Fertilisation occurs when the sperm cell successfully enters and fuses with an egg cell (ovum). The genetic material of the sperm and egg then combine to form a single cell called a zygote and the germinal stage of prenatal development commences. Embryogenesis covers the first eight weeks of development and at the beginning of the ninth week the embryo is termed a fetus. Human embryology is the study of this development during the first eight weeks after fertilisation. The normal period of gestation (pregnancy) is nine months or 38 weeks. The germinal stage, refers to the time from fertilization, through the development of the early embryo until implantation is completed in the uterus. The germinal stage takes around 10 days.