Best Practices on Oocyte Cryopreservation: Slow Freezing or Vitrification?

By Setareh Tais, ND

Oocyte cryopreservation is an important aspect of the in vitro fertilization protocol, whereby harvested oocytes are cooled to sub-zero temperatures to stop all biologic activity and preserved for future fertilization. The first human birth from a frozen oocyte was reported in 1986 – and since then there has been significant improvements in the methods of cryopreservation to improve IVF success rates. The long-established technique is slow-freezing (SF) of oocytes in the presence of low concentrations of cryoprotectant agents. This process is said to minimize the risk of intracellular ice formation and structural damage. In the last decade more IVF centers are using vitrification (VT) to cryopreserve oocytes. In this method, a high initial concentration of cryoprotectants are used along with an ultra-rapid cooling process to transform the cells into a glass-like structure to avoid ice crystal formation.  Recently a 5-year retrospective study of the national Italian Assisted Reproduction Technologies (ART) data registry was conducted to compare oocyte survival, fertilization, and clinical pregnancy rates with SF and VT methods. 14,328 cycles with 11,599 embryo transfers, 1850 pregnancies and 1342 live births were studied. The results showed a higher oocyte survival rate with VT (63.1%) compared to SF (51.1%), lower fertilization rate with VT (70.1%) compared to SF (71.6%),  and high pregnancy rate with VT (14.4% per cycle and 18% per transfer) compared to SF (12% per cycle and 14.8% per transfer). This study demonstrates better clinical outcomes using vitrification for oocyte cryopreservation.

Reference:
Levi Setti PE, Porcu E, Patrizio A, Vigiliano V, de Luca R, d’Aloja P, Spoletini R, Scaravelli G. Human oocyte cryopreservation with slow freezing versus vitrification. Results from the National Italian Registry data, 2007-2011. Fertility and Sterility 2014. 102(1): 90 – 95.

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