Fertilization occurs when the sperm successfully enters the ovum's membrane. (Ovum is the egg) The genetical material of the sperm and egg that combine to form a single cell, called a zygote, and the germinal stage of prenatal development commences.[1] The germinal stage refers to the time from fertilization, through the development of the early embryo, up until implantation. The germinal stage is over at about 10 days of gestation.[2]

The zygote contains a full complement of genetic material and develops into the embryo. Briefly, embryonic developments have four stages: the morula stage, the bastula stage, the gastrula stage, and the neurula stage. Prior to implantation, the embryo remains in a protein shell, the zona pellucida, and undergoes a series of cell divisions, called mitosis. A week after fertilization the embryo still has not grown in size, but hatches from the zona pellucida and adheres to the lining of the mother's uterus. This induces a decidual reaction, wherein the uterine cells proliferate and surround the embryo thus causing it to become embedded within the uterine tissue (this is when you can get a positive test) The embryo, meanwhile, proliferates and develops both into embryonic and extra-embryonic tissue, the latter forming the fetal membranes and the placenta. In humans, the embryo is referred to as a fetus in the later stages of prenatal development. The transition from embryo to fetus is arbitrarily defined as occurring 8 weeks after fertilization. In comparison to the embryo, the fetus has more recognizable external features and a set of progressively developing internal organs. A nearly identical process occurs in other species. 

• Prenatal (sperm fertilizes egg - birth)

• Embryo (fertilization - 8 weeks after fertilization)

• Zygote, the single cell stage which occurs after fertilization

• Blastocyst, the stage prior to implantation, when the embryo is a hollow sphere

• Post-implantation embryo, the period 1 – 8 weeks after fertilization (3 to 10 weeks gestation)

• Fetus, (10th week of pregnancy - birth)

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