Four studies were included (n=8,824 cryopreserved embryos/blastocysts including 7,482 that underwent vitrification and 1,342 that underwent slow freezing). Three studies were randomised controlled trials (RCTs); methods of randomisation were unclear in all three studies.
Vitrification was associated with a statistically significant increase in the post-thawing survival rate of cleavage stage embryos and the survival rate of blastocysts compared to slow freezing; odds ratio for embryos 15.57 (95% CI 3.68 to 65.82, p<0.001; three studies; significant heterogeneity was found, p=0.001) and odds ratio for blastocysts 2.20 (95% CI 1.53 to 3.16, p<0.0001; two studies; no significant heterogeneity was found, p=0.06). All studies showed significantly higher survival rates in vitrification samples.
One study reported that vitrification was associated with a significant increase in the rate of embryo cleavage after thawing in vitrified samples (93% versus 90%, p<0.01) but found no significant difference between cryopreservation methods in blastocyst hatching rates (77.8% versus 100%).
Pregnancy rate: One study reported pregnancy rates per transfer of four-cell embryos of 27% for vitrification versus 32.1% for slow freezing and pregnancy rates per transfer of blastocysts of 53% for vitrification versus 51% for slow freezing. One study reported pregnancy rates per transfer of eight-cell embryos of 35% for vitrification versus 17.4% for slow freezing. Confidence intervals for all outcomes overlapped which suggested no significant difference.