Day 5 is also the first day that embryos will be considered for freezing. If the embryo transfer is performed on day 5, the embryos chosen for transfer are of the highest quality available. The remaining embryos that are growing vigorously will be frozen at this time for future use by the patient. If embryo transfer occurred on day 3, all of the high quality blastocysts will be frozen at this time. The remaining embryos of lesser quality or embryos that are growing well but have not made a sufficient number of cells are placed in the new Blastocyst Medium for one more day.
On day 6, embryos will be removed from culture to assess their growth for the final time. By day 6, the embryos will have had sufficient time to grow in the incubator to develop into quality blastocysts with a high number of cells. If they have reached this point in their development the embryologists will freeze them. It is not uncommon for embryos that were not ready for freezing on day 5 to go through a period of rapid development and to form quality blastocysts on day 6. These embryos often have more cell numbers than those which were transferred or frozen on day 5. There is no known difference in quality, freeze-thaw survival or developmental potential between day 5 blastocysts and day 6 blastocysts. Some embryos do not continue to grow and do not form viable blastocysts. This is true for every patient and is probably due to the fact that during the ovarian stimulation many follicles which begin to develop contain eggs that have no potential for development into a fetus. These embryos turn dark, begin to fragment and stop growing. They are not discarded until it is certain that they are no longer living. These same defective eggs would have given the same result had they developed in the body instead of in the lab.