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Implantation is the earliest stage of embryo development in which the embryo invades the uterus (womb). After fertilization by sperm, the ovum (egg) divides into a small ball of cells, whose outer layer is specialized to invade the endometrium, which is itself prepared for implantation. The interface between embryo and endometrium develops into the placenta.

The term is also used to refer to the placing of drugs, prosthetics, or grafts in the body for treatment of disease.

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