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Background and Objective: During the last few years, a trend has been noted towards embryos being transferred at the blastocyst stage, which has been associated with improved rates regarding implantation and clinical pregnancy in comparison to cleavage stage embryo transfers. There is a limited number of studies investigating this notion in oocyte donation cycles employing cryopreserved embryos. The aim of this study is to evaluate the implantation potential and clinical pregnancy rates between the day 3 cleavage stage and blastocyst stage embryo transfers in oocyte donation cycles employing vitrified embryos. Methods: This is a retrospective evaluation of oocyte donation frozen–thawed transfers completed in our clinic from January 2017 to December 2017. Intracytoplasmic sperm injection was conducted for all oocytes. Following fertilization, all embryos were cryopreserved either at the cleavage or blastocyst stage. Embryo transfer of two embryos was performed under direct sonographic guidance in all cases. Results: Our results confirmed a 55.6% clinical pregnancy (CP) resulting from day 3 embryo transfers, a 68.8% CP from day 5, and 71.4% CP from day 6. Significantly improved pregnancy rates were related to embryo transfers at the blastocyst stage when compared to cleavage stage transfers (68.9% and 55.6% respectively, p = 0.016). The risk with regards to multiple pregnancies was similar. Conclusion: Our findings indicate that in oocyte donation cycles employing vitrified embryos, embryo transfer at the blastocyst stage is accompanied with a significant improvement in pregnancy rates and merits further investigation.

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primary objective was met, with a cumulative spontaneous abortion rate of 7.4% with upper bound of the 95% confidence interval (CI; 4.4%, 11.5%) below the predefined threshold of 15.0% set based upon the observed 10.0% rate (CI 7.6%, 12.8%) in the pivotal trial (Stadtmauer 2013). Clinical pregnancy rates were 43.2% at 10 weeks post OR.CONCLUSIONS: This trial established a safety bridge between PVR produced via enhanced manufacturing processes and the legacy PVR based upon a rate of spontaneous abortion comparable to that observed in the pivotal phase 3 trial. Weekly administration of the PVR was well-tolerated with good pregnancy outcomes associated with its use in conjunction with HP-hMG stimulation. Based upon demonstrated safety and efficacy coupled with more convenient dosing than existing therapeutics, PVR offers an important option for luteal phase supplementation.