A key step in all IVF cycles is to evaluate embryonic development after fertilization.
The embryologist examines whether fertilization has been successful within 16-20 hours after egg insemination. To do this, he will look for the presence of two polar bodies, and two pronuclei. All of them fuse together to give rise to the zygote (one-cell embryo).
As one shall see in this picture, by zygote we refer to the first stage of embryonic development. When it reaches the blastocyst stage (day 5-6), it is expected to attach into the maternal uterus, thereby marking the start of a new pregnancy.