How Does Implantation Take Place?

Implantation is the process by which the embryo attaches to the mother’s endometrium to receive oxygen and nutrients through the bloodstream and thus continues its development.

Implantation does not occur in every cycle, whether in natural conception or assisted reproduction cycles, as it requires optimal conditions for both the endometrium and the embryo.

The embryo must reach the uterus at the hatched blastocyst stage, meaning it must have 200 to 400 cells and have shed the outer layer surrounding it (zona pellucida). On the other hand, the endometrium must be receptive, with a thickness between 7 and 10 mm and a trilaminar appearance. This stage is known as the implantation window, and it lasts approximately 4 days.

During implantation, the trophectoderm of the blastocyst adheres to the endometrial epithelium through the action of adhesion molecules (integrins, selectins, fibronectins, etc.). It then moves toward the endometrial blood vessels to form new vessels, allowing it to connect with the mother’s blood supply. The tearing of small endometrial blood vessels causes implantation bleeding or spotting.

In an in vitro fertilization (IVF) treatment, the process is similar to natural conception, except that the embryo is placed in the uterus at the blastocyst stage (whether hatched or not).

Embryo implantation causes an increase in the human chorionic gonadotropin (hCG), estrogen and progesterone levels. These hormonal changes are the main cause of the first pregnancy symptoms, such as: nausea, fatigue, Breast tenderness, etc.