Sometimes, the process of the embryo transfer is confused with implantation. During an embryo transfer, the embryo under ultrasound guidance and via a soft catheter is placed inside the uterus, generally 1 to 2 cm from the top of the uterine cavity. Implantation is the incident that hopefully will occur after the embryo transfer while the embryo is at the blastocyst stage and attaches to the uterus. This process will take place either 3-4 days after the performance of a Day 3 embryo transfer or 1-2 days after a Day 5 embryo transfer. It is a highly synchronized process in time and space that involves a molecular cross-talk between a receptive uterus and a competent blastocyst.
The structural and functional remodeling of the uterus that is mediated by estrogen and progesterone, is crucial in order to achieve a successful implantation. More specifically the endometrium becomes thicker and vascularized and the uterine glands begin to coil and boost their secretions. As for the blastocyst, there is the necessary condition of hatching that will set the blastocyst free of its protective coat i.e. the zona pellucida and thus permit direct physical contact between the embryo and the uterine endometrium.
Implantation consists of three stages: apposition, adhesion and invasion. At the stage of apposition the hatched blastocyst is located and makes contact with the implantation site of the endometrium. Through the adhesion stage there is massive communication and a much firmer attachment between the embryo and the endometrium which is driven by specific adhesion molecules. Invasion is an even further establishment of the blastocyst in the endometrium. Trophectoderm cells from the outer layer of the blastocyst migrate between the epithelial cells, displace them, and penetrate up to the basement membrane of the endometrium. The embryo sinks into the endometrial stroma and its original site of penetration into the endometrium becomes closed over by the epithelium, similar to a healing skin wound. The maternal spiral arteries are reconstructed in order to maintain a high blood flow between the fetus and the mother, so that nutrients and waste products can be transferred.
Successful implantation requires a high degree of preparation and coordination by the embryo and the endometrium. It is a very complex process that is only briefly summarized above and still challenging for researchers. It is also the beginning of a unique journey: the pregnancy.