**5.3 Slow freeze versus vitrification for oocyte cryopreservation**

Slow freezing reports first began 13 years before literature on vitrification emerged. Both of these methods have demonstrated increasing efficiency over time, with continually improving live birth and ongoing pregnancy rates per transfer (Oktay et al., 2006). Although there is a lag in the data for vitrification outcomes, the number of babies born after vitrification is approaching that of slow freeze methods for oocyte cryopreservation (Noyes et al., 2009).

In a recent meta-analysis of randomized controlled trials (RCTs) comparing these two methods, vitrification was found to have better post-thawing survival rates for cleavage stage embryos (odds ratio [OR] 6.35, 95% confidence interval [CI] 1.14, 35.26) and for blastocysts (OR 4.09, 95% CI 2.45, 6.84) (Kolibianakis et al., 2009). A significantly higher number of embryos cryopreserved in the cleavage stage developed into blastocysts following vitrification. Clinical pregnancy rates, however, demonstrated no significant difference between slow freeze and vitrification protocols. This meta-analysis was undertaken to evaluate and summarize the available evidence for cryopreservation of human embryos, not oocytes. Additionally, the data amassed for this meta-analysis came from only 6 RCTs, only one of which commented on live birth rates. The authors, in light of this limited data, call for well-designed randomized controlled trials to further study differences between and advantages or disadvantages of these cryopreservation techniques.

Recently, a prospective randomized comparison of slow freeze versus vitrification for mature human oocyte cryopreservation was performed in Brazil (Smith et al., 2010). In this study, women with supernumerary oocytes retrieved (more than nine) were consented and randomized to either slow freeze or vitrification of these supernumerary oocytes. Demographic characteristics between the two groups of women were similar, including patient age, baseline laboratory values, and number of oocytes collected. Semen parameters were also similar between the groups and all oocytes were inseminated by intracytoplasmic sperm injection (ICSI). Oocyte survival after thawing was significantly higher in those having undergone vitrification. Additionally, a higher percentage of vitrified oocytes were fertilized (77% vs. 67% of slow freeze oocytes; p<0.03) and more of these zygotes underwent cleavage from day 1 to day 2 (84% vs. 71%, respectively; p<0.01). Perhaps the most important outcome for any assisted reproductive technology, however, is the rate of pregnancy. Biochemical and clinical pregnancy rates per thaw cycle were significantly higher in the vitrification group compared to the slow freeze group (46% vs. 17% and 38% vs. 13%, respectively; p<0.01 and p<0.02) (Table 1). Additionally, the two groups had similar rates of spontaneous abortion following embryo transfer. Perinatal outcomes were not evaluated by these authors. From case reports evaluating live births following oocyte cryopreservation, the average gestational age at delivery for slow freeze was 36.9 weeks, compared to 39 weeks' gestational age at delivery after vitrification (Noyes et al., 2009). This data suggests improved efficiency and a clinical advantage of oocyte vitrification for elective fertility preservation. Reproductive endocrinologists should be aware of this recent data when considering the implementation of oocyte cryopreservation into their clinical practice and when counseling patients seeking fertility preservation.


NA = not applicable; NS = not significant. SE = standard error. *Adapted and reproduced with permission from Smith et al., 2010.* 

Table 1. Oocyte survival and function following slow-freeze or vitrification for cryopreservation
