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Cryopreservation of Embryos

Cryopreservation of cleavage and blastocyst stage embryos has become a necessity with patients who have a greater number of high quality embryos than those transferred. Freezing of all embryos is recommended in cases of Ovarian Hyper-stimulation Sydrome (OHSS) as the risk of OHSS severity is significantly reduced if pregnancy does not occur. Additionally cryopreservation of all embryos is applied in cases where there are no optimal conditions for the implantation of embryos thus lowering the probability of a successful pregnancy.


“Vitrification” is the rapid and simple cryopreservation method where the solution is solidified at low temperature with no ice crystal formation. This is achieved by increasing the viscosity of the solution by using high cooling rates. This rapid cooling rate would minimize any potential chilling injury and osmotic shock to the embryos. Vitrification has proved to be of clinical value because of achieving better outcomes such as survival and pregnancy rates compared to the slow freezing method that was firstly established.


Culturing the embryos to the blastocyst stage allows a better optimization of embryonic selection thus maximizing pregnancy and implantation rates. This selection process has simultaneously reduced the number of embryos transferred per cycle and therefore blastocyst cryopreservation has become an important procedure in the IVF field.

It has been reported that expanded blastocysts exhibit poor survival rates after vitrification.
This is related to the presence of fluid in the blastocyst cavity (blastocoele) that causes intracellular ice crystal formation after vitrification. To prevent ice crystal formation and reduce the additional time of cryoprotectant exposure time for adequate dehydration of the embryo blastocyst undergo assisted shrinkage with the use of a laser shot prior to vitrification that is been reported to improve survival rates of blastocysts.

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