Fertilized eggs, known as pre-embryos or embryos, are kept for a few days in special containers where they will develop in a culture medium. Embryo development requires a process of cell division by which the number of cells of the embryo will progressively increase.
The way an embryo develops will largely depend on their potential, i.e.  If there are chromosomal abnormalities or defects in other parts of the cell.
Embryos which develop normally have more chances to implant and produce a pregnancy.
The embryologist will ensure the best culture conditions for embryo development. He will monitor the development of the embryos during the culturing and select the best ones – those with more potential of implantation- to be subsequently transferred or frozen. Morphologically abnormal embryos will automatically be discarded.

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Microscopic observation of embryos.