Vitrification of cleavage stage day 3 embryos results in higher live birth rates than conventional slow freezing: a RCT

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Abstract
Is the live birth rate (LBR) per embryo thawed/warmed higher when Day 3 cleavage stage embryos are cryopreserved by vitrification compared with slow freezing? The LBR per embryo thawed/warmed was higher after vitrification than after slow freezing on Day 3, based on better embryo survival, quality and availability of embryos in the vitrification group. Post-thawing survival rate of cleavage-stage embryos has been reported to be higher after vitrification than after slow freezing. This RCT was performed in an academic tertiary center between September 2011 and March 2013. If supernumerary embryos were available on Day 3, patients were randomized at the time of cryopreservation using a computerized system to determine a simple allocation to the vitrification group or the slow freezing group and all embryos were frozen with the same technique. The primary outcome of this study was the LBR per embryo thawed/warmed. Power calculation revealed that 184 thawed embryos were needed in each group (β = 0.8, α < 0.05) to test the hypothesis that the LBR per embryo thawed/warmed was significantly higher (16%) after vitrification than after slow freezing (6%). Patients P < 0.0022; relative risk (RR) 3.23; 95% confidence interval (CI) 1.64–6.35). Similarly, the implantation rate per embryo thawed/warmed was higher after vitrification (20.7% (45/217) than after slow freezing (7.5% (15/200); P = 0.0012; RR 2.76; CI 1.59–4.81). The survival rate was significantly higher after vitrification (84.3% (183/217) than after slow freezing (52.5% (105/200); P < 0.0001). Significantly more embryos were fully intact after vitrification (75.4% (138/183) than after slow freezing (28.6% (30/105); P < 0.0001). The number of transfers was significantly higher after vitrification (90.1% (109/121)) than after slow freezing (73.7% (70/95); P = 0.0024). Survival rates in the slow freezing group were low in this study. Additional RCTs are needed to compare reproductive outcome after vitrification and after slow freezing with 1,2-propanediol and 0.2 M sucrose, since this method has been reported to have better survival than the method used in our study. Our findings are only applicable to the specific slow freezing cryopreservation medium used in our study, and not to any other commercially available media. When compared with slow freezing using 1,2-propanediol and 0.1 M sucrose as cryoprotectant, vitrification of Day 3 cleavage stage embryos resulted in a higher LBR per embryo warmed, and may therefore result into a higher cumulative delivery rate after one oocyte retrieval. None. NCT02013024.

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