Implantation, egg

Attachment of a fertilized ovum (egg) to the wall of the uterus. It occurs about 6 days after fertilization, when the blastocyst (early embryo) comes into contact with the wall of the uterus. As the cells of the developing embryo continue to divide, the outer cell layer penetrates the lining of the uterus to obtain oxygen and nutrients from the mother’s blood; later, this layer develops into the placenta. The embryo usually implants in the upper part of the uterus; if it implants low down near the cervix, placenta praevia may develop. Rarely, implantation occurs outside the uterus, possibly in a fallopian tube, resulting in an ectopic pregnancy.


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