From a biological point of view, an embryo is considered to exist as soon as the union of the genetic materials provided by the two gametes (egg and sperm) takes place. This fact can be visualized on embryonic day one. At that moment it is observed that the oocyte has been fertilized, but it is no longer just an oocyte, since it contains the double genetic material that is visualized in the form of two pronuclei.
From there, millions of cell divisions take place until this two pronucleus embryo becomes a human being.
On embryonic day two and three, the cells are dividing and the embryo is made up of cells. On embryonic day 4, all the cells that divided begin to compact into a morula (they have a shape similar to a blackberry).
The next stage to appear is the blastocyst stage, which under optimal conditions appears on embryonic day 5. This stage is characterized because the cells are arranged in such a way that in one area of the embryo a cellular group accumulates, which is called the inner cell mass and will give rise to the embryo and fetus that we will later see in the ultrasound scans. While the rest of the embryo is surrounded by a row of cells that will give rise to the membranes and the placenta, which we call the trophoectoderm. And in the central area there is an area without cells with a liquid inside, which is called the blastocele and which will give rise to the amniotic fluid.
The difference between an early blastocyst and an expanded blastocyst is that, in the first case, the separation of these three parts begins to be intuited, initially appearing the blastocele which differentiates two cell groups, one that will give rise to the inner cell mass and the other to the trophoectoderm. At this stage the blastocyst is not clearly expanded and therefore the zones are not clearly differentiated. This embryonic stage usually begins to be seen at the end of the fourth day or at the beginning of the fifth day.
However, in the case of an expanded blastocyst, the cells already occupy a defined place in the embryo, and the inner cell mass, blastocele and trophectoderm are clearly seen. The expanded blastocyst is observed on the fifth or sixth embryonic day.