According to the Encyclopedia for Children's Health, prenatal development refers to the time and process it takes for a single fertilized cell to become an embryo then a fetus. The Encyclopedia for Children's Health states that his time is generally around 38 weeks and occurs in three stages: the germinal stage, the embryonic stage and the fetal stage.
The Free Dictionary defines the germinal stage of embryonic development is the time between fertilization and implantation of the ovum. It undergoes several divisions of the cells, makes its way to the uterus and implants into the endometrium, or uterine lining, according to the Encyclopedia for Children's Health.
The Encyclopedia for Children's Health states that the embryonic stage begins after implantation and lasts for eight weeks. During that time, cell division continues and clusters of cells begin to merge in order to take on different functions. One process leads to the formation of germ layers, which eventually differentiate into separate tissues and structures; during this stage, all essential and internal and external structures have been formed, according to the Encyclopedia for Children's Health.
According to the Cleveland Clinic, the fetal stage begins after week eight; during this stage the fetus grows bigger, stronger and more fully formed. The nervous system starts to grow, the heartbeat is audible through a Doppler and the reproductive organs are developed; the Cleveland Clinic explains that this stage lasts until delivery.