An oocyte that has fertilized is called a zygote. Zygotes exist as a single cell but soon divide to become a two-cell embryo. Each of these cells will divide to become a four-cell embryo and then an eight-cell embryo. As the embryo continues to develop, the individual borders of the embryo’s cells come in contact with each other. This contact causes them to compact together into a stage called a morula. Following the morula stage, the cells in the embryo begin to expand and form a soccer ball shape. This stage is called the blastocyst stage. During this stage, cells in the embryo begin to differentiate into two separate structures. One group of cells is called the inner cell mass and will eventually develop into fetal tissue. Another group of cells is called the trophectoderm and will eventually develop into placental tissue

Many factors influence embryo development such as your diagnosis, maternal age, sperm quality, etc. It is not likely all oocytes retrieved will fertilize to become zygotes and not all zygotes will continue to develop into blastocysts. The embryos that develop to the blastocyst stage increase chances of creating pregnancies. Selecting an embryo for transfer on day 5, when zygotes have had sufficient time to develop to the blastocyst stage, helps your embryologist determine which embryo to transfer.