Cryopreservation of day 2-3 embryos by vitrification yields better outcome than slow freezing

Objective: To compare the outcome of vitrification versus slow freezing cryopreservation for cleavage stage day 2-3 embryos. Design: A retrospective observational study. Setting: All thawed embryos assisted reproduction cycles between January 2010 and December 2012 at a single IVF laboratory of a Tertiary Medical Center. Patients: Five hundred and thirty-nine cycles of day 2-3 thawed embryos. Interventions: In 327 of the thawed cycles, the embryos were vitrified and in 212 of the cycles the embryos were derived from slow freezing embryos. Main outcomes measure: Embryo survival rate, blastomere surviving rate and pregnancy rate. Results: Embryo survival rate was significantly higher after vitrification compared with slow freezing (81.6%, 647/793 versus 70.0%, 393/562 embryos, p<0.0001). The clinical pregnancy rate per ET was significantly higher following vitrification compared to slow freezing, 20.0%, 63/314 versus 11.9%, 23/193, respectively (p=0.02). Conclusions: Vitrification of day 2-3 cleavage stage embryos yields better cycle outcome in all the parameters compared to slow freezing.