The embedding of a fertilized mammalian egg into the wall of the uterus (womb) where it will continue its development. After fertilization in the fallopian tube the egg passes into the womb in the form of a ball of cells (blastocyst). Its outer cells (trophoblast) secrete adhesion molecules and enzymes, which destroy cells of the uterine wall and form a cavity into which the blastocyst sinks. In humans implantation occurs about six days after fertilization.