The first step in embryonic development is the union of the egg and sperm to form the zygote. This zygote increases its number of cells on day 2, day 3, and so on until day 5.
It is at this moment that the embryo is called a blastocyst. The main characteristic of the blastocyst is the presence of a trophectoderm and an inner cell mass (ICM).
When the blastocyst implants in the woman's uterus, the trophectoderm becomes the placenta, while the ICM becomes the embryonic discs that will give rise to the fetus.