Regarding oocyte cryopreservation, a recent systematic review found only five randomized controlled trials, three of them using the technique outlined above.[33–35] Survival rates were 97, 97 and 93%, while clinical pregnancy rates achieved after intracytoplasmic sperm injection were 39, 65 and 55% (the differences in pregnancy data may be explained by the different application: while the former study was performed in a routine infertility program, the latter two described application in oocyte donation). Essentially the same outcome was described in retrospective studies, where a similar technique was used.[36–41] No increase in embryonic aneuploidy or decrease of implantation potential was found after oocyte vitrification. Molecular profiles suggested that vitrified oocytes were biologically comparable to their fresh counterpart.
For preimplantation-stage embryos, the recent representative work of Cobo et al. based on 3150 warming cycles and 6019 embryos provides convincing data. The overall survival rate was 95% (with 100% blastomere survival in 93 and 95% of day 2 and 3 embryos, respectively). Implantation, clinical pregnancy, ongoing pregnancy and delivery rates were 36, 42, 33 and 38%, respectively. The authors did not find any difference related to the age of the embryo or the method for endometrial preparation.
These results are comparable to those published in 2005 by Kuwayama et al. about the outcomes of vitrification of close to 13,000 embryos in the initial paper, describing mass application of vitrification in human embryology, and clearly refer to the stability and reliability of the procedure outlined above. During the 7 years following 2005, more than 20 papers have provided detailed statistics by using vitrification techniques based on the same principle and reported roughly similar outcomes. Perinatal morbidity after transfer of cryopreserved embryos was found to be even lower than in fresh cycles.
Most studies agree that the length of storage does not negatively influence pregnancy rates after transfer.[46,47] Births were reported after transfer of frozen–thawed pronuclear-stage embryos and vitrified oocytes 20 and 5 years after cryopreservation, respectively.[48,49]
Expert Rev of Obstet Gynecol. 2013;8(2):181-190. © 2013 Expert Reviews Ltd.