If fertilization and subsequent implantation occur, decline of the endometrium is delayed until after parturition. As the blastocyst becomes embedded in the uterine mucosa in the early part of the second week, cells in the chorion of the developing placenta begin to secrete hCG and other luteotropins. These hormones maintain the corpus luteum and stimulate it to continue the production of progesterone and estrogens. Thus, the decline of the endometrium is prevented, and the endometrium undergoes further development during the first few weeks of pregnancy.
Implantation is the process by which the blastocyst settles into the endometrium.
The fertilized human ovum undergoes a series of changes as it passes through the uterine tube into the uterine cavity in preparation for becoming embedded in the uterine mucosa. The zygote undergoes cleavage, followed by a series of mitotic divisions without cell growth, resulting in a rapid increase in the number of cells in the embryo. Initially, the embryo is under the control of maternal informational macromolecules that have accumulated in the cytoplasm of the ovum during oogenesis. Later development depends on activation of the embryonic genome, which encodes various growth factors, cell junction components, and other macromolecules required for normal progression to the blastocyst stage.
The cell mass resulting from the series of mitotic divisions is known as a morula (L. morum, mulberry), and the individual cells are known as blastomeres. During the third day after fertilization, the morula, which has reached a 12to 16cell stage and is still surrounded by the zona pellucida, enters the uterine cavity. The morula remains free in the uterus for about a day while continued cell division and development occur. The early embryo gives rise to a blastocyst, a hollow sphere of cells with a centrally located clump of cells. This inner cell mass will give rise to the tissues of the embryo proper; the surrounding layer of cells, the outer cell mass, will form the trophoblast and then the placenta (Fig. 23.17).
Fluid passes inward through the zona pellucida during this process, forming a fluid-filled cavity, the blastocyst cavity. This event defines the beginning of the blastocyst. As the blastocyst remains free in the uterine lumen for 1 or 2 days and undergoes further mitotic divisions, the zona pellucida disappears. The outer cell mass is now called the trophoblast, and the inner cell mass is referred to as the embryoblast.
Implantation occurs during a short period known as the implantation window.
The attachment of the blastocyst to the endometrial epithelium occurs during the implantation window, the period when the uterus is receptive for implantation of the blastocyst. This short period results from a series of programmed actions of progesterone and estrogens on the endometrium.
Antiprogesterone drugs such as mifepristone (RU-486) and its derivatives compete for the receptors in the endometrial epithelium, thus blocking hormone binding. The failure of progesterone to gain access to its receptors prevents implantation, thus effectively closing the window. In the human, the implantation window begins on day 6 after the LH surge and is completed by day 10.
As contact is made with the uterine wall by the trophoblastic cells over the embryoblast pole, the trophoblast rapidly proliferates and begins to invade the endometrium. The invading trophoblast differentiates into the syncytiotrophoblast and the cytotrophoblast.
 The cytotrophoblast is a mitotically active inner cell layer producing cells that fuse with the syncytiotrophoblast, the outer erosive layer.  The syncytiotrophoblast is not mitotically active and consists of a multinucleate cytoplasmic mass; it actively invades the epithelium and underlying stroma of the endometrium.