This thesis explores the biological processes that are essential for pregnancy, focusing on how the trophoblast, a specific cell type from the embryo, interacts with the inner lining of the uterus. Implantation is the critical first step where the embryo attaches to the uterus, but it often fails. Even with advanced reproductive technologies, like IVF, success rates remain low. By examining the underlying physiological mechanisms of this interaction, gaps in our understanding may be discovered and therefore help us in better understanding this inefficiency of conception. Different key factors are examined, going from hormonal aspects to immunological aspects. Hormones like progesterone prepare the uterus to accept an embryo by creating a suitable environment. This environment needs to happen so that the embryo can nest in the uterus. The immune system balances between tolerating this newly introduced embryo and the protection against possible infections. When any of these systems fails, complications in pregnancy may occur.