Describe the stages of prenatal development.

There are three distinct phases of prenatal development. The first two weeks following conception are referred to as the germinal stage. During this period, after conception, the fertilized egg begins to divide. It eventually implants into the wall of the uterus. Weeks three through eight are called the embryonic stage. During this period, cells begin to specialize into specific types, and organ systems begin to develop. In week three, the beginning of the nervous system, the neural tube, is formed. By week four, a primitive heart is formed and begins beating. The embryo is supported by an amniotic sac that is filled with fluid. Materials are exchanged between the mother and embryo by a placenta. The last seven months of prenatal development are the fetal stage. By week twelve, most of the major organ systems have formed. Most of the rest of prenatal development is simply the further maturation of these systems and continued growth.