After the penetration of the sperm into the egg, a zygote is formed. The zygote stage lasts the first 26-30 hours of development. The zygote completes meiosis and forms the second polar body and forms the female pronucleus. In parallel, the male pronucleus is formed. Both pronuclei move towards each other with the following localization of the chromosomes in the metaphase plate of the first division of mitosis. The process of mitotic division, which is called cleavage is characterized by the division of cells that do not increase in size. As a result, the total size of the embryo does not increase, while daughter cells become smaller. By the 4th day of development, when the embryo consists of approximately 12-16 cells, the blastomeres differentiate and form two cell layers. External blastomeres form trophoblast, while internal blastomeres produce embryoblast. By the 5th day of development, the embryo forms a blastocyst, which consists of 200 cells. During the period, the zygote and blastocyst continue moving through the fallopian tube towards the uterus. Here, cells of trophoblast eject finger-shaped processes for the penetration into the endometrium. At the same time, the endometrium continues to thicken under the influence of progesterone and, as a result, surrounds the blastocyst on all sides resulting in the implantation.