The most difficult hurdle for the floating blastocyst is to become attached to the uterine lining (endometrium). Like trying to dock a tanker coming into port, the blastocyst is first slowed down by long molecules that extend from the endometrium (mucins), followed by a cascade of molecules that bring the trophoblasts into closer and closer contact with the endometrium. Once intimate contact is made the trophoblasts begin to invade into the endometrium, beginning the process of placentation.
Excerpted from: From Trophoblast to Human Placenta