Embryo quality
On the 3rd day, 2 days after oocyte retrieval, the eggs (now called embryos) are once more examined for cleavage. First at this stage, the embryologist is able to say something about the quality of the embryos, and their chance to result in a living pregnancy.
The optimal embryo has now cleaved into 4 cells of equal size, and there is a visible nucleus in each cell. In addition, the embryologist looks at the number of small fragments inside the zona pellucida, which represent sequestered cellular material. It there are many fragments, the embryo is not suitable to be frozen.
The embryologist also looks at the thickness of the zona pellucida. In case the zona pellucida is very thick, it may be advantageous to place a small incision in the membrane. This is called assited hatching (see video link). Before the embryo implants in the uterus, it has to get out of the zona pellucida. This is called hatching. A thick zona pellucida can prevent embryos from hatching. It thus becomes trapped inside the zona and therefore can not implant into the uterus.
We normally transfer the embryos back 2 days after oocyte retrieval. If there are many embryos which are equally good, it may be advantageous to grow them for longer time, to see if all embryos develop further. Usually there are some embryos in which development stops. By culturing for 5 days to the blastocyst stage, it can be easier to choose the right embryo which provides the greatest opportunity for pregnancy. On the other hand, there may be fewer embryos for freezing.