Embryo freezing can be undertaken at any given stage of in vitro development (fertilisation stage to blastocyst stage) with similar post thaw survival rates while the protocol of vitrification and warming is essentially the same other than a slightly shorter incubation time for the cleavage stage embryos.
Freezing and thawing using vitrification/warming at the zygote or pronuclear stage (fertilisation phase) enjoys excellent survival rates and this is presumably due to the membrane permeability changes that occur following fertilisation while it is also believed that the inner part of the cell (cytoplasm) becomes more stable rendering it less vulnerable to the osmotic fluctuations and overall stress which is exposed during the procedure.
Since all embryo transfers are undertaken at the blastocyst stage of development (day 5), any surplus embryos which resulted following a fresh replacement can be vitrified for subsequent use. The survival rate of blastocysts post thawing largely depends on the expertise of the embryologist and the quality of the embryos themselves. The poorer the quality is the lower the chances of survival. The method for both vitrification and warming is essentially the same as with freezing/thawing of oocytes or zygotes. With the immersion of the embryo into the equilibration solution the blastocyst will initiate dehydration hence it shrinks when seen under high magnification microscopy before it commences gradual expansion toward regaining its original size. The mechanisms of the warming procedure are the exact opposite to the vitrification one and these had been discussed in the oocytes vitrification/warming sector of this web page.
Following thawing the blastocysts should be allowed to culture under physiological conditions for approximately 2 hours prior to their potential replacement into the uterus of a patient.