For the embryo to be able to adhere to the endometrium, it should have reached the blastocyst stage. It is a sphere-shaped structure that includes two layers: the external, which is made up of cells (trophoblast), which leads to the formation of the placenta, and the inner cell mass (ICM), which will give rise to all embryonic tissues.
In order for this structure to develop, the egg cell is fertilized in the Fallopian tubes, and starts developing whilst it travels from the tube or oviduct to the uterine lining.
Once on the surface of the uterus, the embryo will land and attach to the endometrial lining to start developing the placenta. To this end, the embryo has to break down the endometrial blood vessels located there and create new vessels that will allow it to nourish. After breaking down these small vessels, a light bleeding or spotting occurs, which is what we know as implantation bleeding.