Following fertilization, the major events of the first week of development are cleavage, formation of the blastocyst, and the beginning of the implantation process. Fertilization can only occur after ovulation, when a secondary oocyte reaches the ampulla of the uterine tube. Here it meets several hundred sperm, only one of which will penetrate the outer protective layers, due to built-in mechanisms to prevent polyspermy. Following fertilization along the uterine tube, implantation occurs in the posterior wall of the uterus, except in the event of an ectopic pregnancy.
Extending over 24 hours, fertilization is the sequence of events from the initial contact between the sperm and oocyte, to the joining of the male and female chromosomes on the metaphase plate at the first mitotic division to form a zygote. As a result, a diploid cell is formed with a new genotype and its genetic sex determined, stimulated to undergo mitosis.
The male and female pronuclei then complete mitosis and commence a series of cell divisions known as cleavage, beginning about 30 hours after fertilization. As it continues on its path to the uterus, the zygote divides into two cells, now called blastomeres, then four, then eight. During this time, it is still surrounded by the zona pellucida, which caps its growth. Therefore, as the cells divide, they become smaller. When nine or more cells are present, the process of "compaction" occurs. This establishes the embryo as a multicellular organism by the formation of cell-surface adhesion glycoproteins which facilitate cell-to-cell communication. Previously, each blastomere was totipotent, meaning it could be removed and survive on its own to form a new organism. The cells are now interdependent.
Around the third day after fertilization, a ball of 12-15 cells enters the uterus. This stage is called the morula (meaning "mulberry"). As the circular ball of cells with an inner cell mass and an outer cell layer arrives in the uterine cavity, the zona pellucida begins to disintegrate. A blastocoele, or fluid-filled cavity, begins to form, dividing the blastomeres into two cell groups. The outer cell layer forms the trophoblast, a single layer of cells around the periphery that will form the future embryonic part of the placenta. At one pole of its interior surface, a clump of cells forms, the inner cell mass, which will become the future embryo. This fluid-filled embryo is called the blastocyst.
After floating in the uterine cavity for about two days, the blastocyst completely loses its zona pellucida, and begins to grow rapidly. At day 6, it latches onto the endometrial epithelium, with the side of the inner cell mass (embryonic pole), attached to the uterine wall. Eventually the embryo will be attached by its dorsal side.
This early implantation induces the trophoblast to differentiate into two layers, the cytotrophoblast and syncytiotrophoblast. The cellular cytrotrophoblast surrounds the embryo as a single layer and the syncytiotrophoblast, a multinucleated layer with no cell boundaries, invades the endometrial epithelium to begin the implantation process.