The process of growth and development within the womb, in which a single-cell zygote becomes an embryo, a fetus, and then a baby is known as prenatal development. The process of prenatal development occurs in three main stages. Germinal stage: The first two weeks after the implantation. Embryonic stage: The duration from the third week through the eighth week. Fetal period: The time from the ninth week until birth. During the first two stages of life, it is referred to as the embryo while as it reaches the third stage, the fetal period, it is referred to as the fetus. The fetal period of prenatal development marks more important changes in the brain. This period of development begins during the ninth week and lasts until birth. Implantation is a process in which a developing embryo, moving as a blastocyst through a uterus, makes contact with the uterine wall and remains attached to it until birth.