How does an embryo attach to the uterus? It's information that may prove useful in studying and remedying infertility, as well as early pregnancy loss, and a life-threatening pregnancy complication called preeclampsia.
It all happens about six days after fertilization. The embryo at this point is about the same size as the egg, but it consists of approximately 100 dividing cells. As it was passing through the oviduct into the uterus it was enclosed by a membrane called the zona pellucida, which prevents the embryo from implanting itself. When the membrane does shed, however, it reveals a surface of cells coated with a protein that sticks to cells lining the uterine wall.
The embryo's sticking to the uterine wall happens gradually, something like a tennis ball rolling over a tabletop covered in syrup. The stickiness slows the embryo down until it eventually comes to a stop. Once the embryo stops moving, its surface cells, which will eventually become part of the placenta, send out finger-like extensions into the tissue of the uterine wall. They tap into the maternal blood supply there and construct a kind of pipeline through which the embryo takes nutrients and oxygen from the mother and in return gives her its carbon dioxide and waste material.
Sound like one too many horror or science fiction films you've seen? Well, for people who want a baby, this is an essential step to getting one.
"Researchers Discover How Embryo Attaches To The Uterus" (Science Daily)