Through the activity of the trophoblast, the blastocyst is  entirely embedded within the endometrium on about the 11th day of development. (Further development of the syncytiotrophoblast and cytotrophoblast is described in the section on the placenta.)
FIGURE 23.17 • Schematic diagrams of sectioned blastocysts. a. A human blastocyst at about 4.5 days of development showing formation of the inner cell mass. b. A monkey blastocyst at about 9 days of development. The trophoblastic cells of the monkey blastocyst have begun to invade the epithelial cells of the endometrium. In humans, the blastocyst begins to invade the endometrium at about the fifth or sixth day of development. c. A human blastocyst at 14 days after implantation. At this stage, the trophoblast cells have differentiated into syncytiotrophoblasts and cytotrophoblasts.
The syncytiotrophoblast has well-developed Golgi complexes, abundant sER and rER, numerous mitochondria, and relatively large numbers of lipid droplets. These features are consistent with the secretion of progesterone, estrogens, hCG, and lactogens by this layer. Recent evidence indicates that cytotrophoblast cells may also be a source of steroid hormones and hCG.
After implantation, the endometrium undergoes decidualization.
During pregnancy, the portion of the endometrium that undergoes morphologic changes is called the decidua or decidua graviditas. As its name implies, this layer is shed with the placenta at parturition. The decidua includes all but the deepest layer of the endometrium. The stromal cells differentiate into large, rounded decidual cells (see page 850). The uterine glands enlarge and become more coiled during the early part of pregnancy and then become thin and flattened as the growing fetus fills the uterine lumen.
Three different regions of the decidua are identified by their relationship to the site of implantation (Fig. 23.18):  The decidua basalis is the portion of the endometrium that underlies the implantation site. The decidua capsularis is a thin portion of en dometrium that lies between the implantation site and the uterine lumen.  The decidua parietalis includes the remaining en dometrium of the uterus.
By the end of the third month, the fetus grows to the point that the overlying decidua capsularis fuses with the decidua parietalis of the opposite wall, thereby obliterating the uterine cavity.
By the 13th day of development, an extraembryonic space, the chorionic cavity, has been established (see Fig. 23.17c).
The cell layers that form the outer boundary of this cavity (i.e., the syncytiotrophoblast, cytotrophoblast, and extraembryonic somatic mesoderm) are collectively referred to as the chorion. The innermost membranes enveloping the embryo are called the amnion (see Fig. 23.18).
The endometrium of the cervix differs from the rest of the uterus.