Step 1

In response to ovarian hormones, the stroma has undergone decidualization, and is edematous, vascularized, and rich in mucus and glycogen.

The blastocyst emerges from the zona pellucida (aka, it “hatches”*).

It can now interact directly with the maternal environment, and does so by releasing substances that prevent its rejection.
One of these substances, human chorionic growth hormone (hCG), preserves the corpus luteum within the ovary; recall that the corpus luteum produces progesterone, which is necessary for maintenance of the endometrial lining, and, therefore, the survival of the embryo.

Thus, many early pregnancy tests rely on the presence of hCG to determine pregnancy status.

Step 2

Apposition involves alignment of the embryonic pole to the uterine wall, to which it loosely attaches.

The blastocyst rolls over its surface (some texts specify that it is the inner cell mass that “rolls” within the trophoblast).
Proper alignment relies on signaling between the epithelia and embryo: pinopods, aka, uterodomes, on the uterine wall interact with microvilli on the blastocyst and establish a loose connection.

Implantation

Process that results in the encapsulation of the embryo within the uterine wall.
Typically occurs after the blastocyst reaches the uterine cavity.
Requires extensive cross-talk between the embryo and endometrium.