LAUSR

RESEARCH QUESTION Is there a difference in blastocyst formation between fresh and vitrified-warmed sibling oocytes and can this difference be attributed to changes in embryo morphokinetics? DESIGN Between February 2016 and December 2017, 472 metaphase II (MII) oocytes in 67 donor-recipient cycles from 27 different healthy anonymous oocyte donors were allocated for fresh transfer (FSHO) (n = 220) to a synchronous recipient (n = 36) or vitrified (VITO) (n = 252) to be warmed and transferred to another recipient (n = 31). Embryos derived from the FSHO and their sibling VITO were analysed for morphokinetic development using time-lapse imaging, blastocyst formation and clinical outcome. RESULTS Time-lapse analysis showed an overall delay in cleavage rate from the time of pronuclei disappearance up to the time of blastulation in the VITO compared with their sibling FSHO. Twelve morphokinetic variables were significantly different between the groups. On Day 5 significantly more FSHO embryos developed to blastocyst (expansion 1-6) and reached the full blastocyst stage (expansion 3-6) compared with the VITO embryos [53.2% (84/158) versus 40.0% (64/160); P = 0.0244 and 48.1% (76/158) versus 31.3% (50/160); P = 0.0028, respectively]. The embryo utilization rate was similar in both groups at the time of cryopreservation; 51.3% (FSHO) versus 45.0% (VITO) (P = 0.3124). The pregnancy rate per cycle was 47.2% (17/36) in FSHO patients and 48.4% (15/31) in VITO patients (P = 1). Limitations in this study: non-randomized, small study size and not powered to detect differences in clinical outcomes. CONCLUSIONS Timing of development is altered and blastocyst formation is delayed in embryos derived from vitrified-warmed donor oocytes compared with their fresh sibling counterparts. Although preliminary results suggest that the clinical impact of this delay may be limited, this needs further investigation in larger randomized studies.