Six RCTs were included in the review (n=977 cycles). There were four RCTs of vitrification compared with slow freezing, and two RCTs of ultra-rapid freezing compared with slow-freezing. The quality of the included trials was generally poor, with three trials scoring 0 out of a possible 6 points, two trials scoring 2 points, and one trial scoring 4 points.
Vitrification versus slow-programmed freezing: There was a statistically significantly higher incidence of clinical pregnancies with embryo vitrification compared with slow freezing (OR 1.55, 95% CI 1.03 to 2.32; I2=0%; four RCTs). Compared with slow freezing, vitrification had a statistically significantly higher incidence of ongoing pregnancies (OR 1.82, 95% CI 1.04 to 3.20; I2=27%; three RCTs) and embryo implantation rate (OR 1.49, 95% CI 1.03 to 2.15). There was no significant difference between vitrification and slow freezing in terms of multiple pregnancies, miscarriage rates or live-birth rates.
Ultra-rapid freezing versus slow-programmed freezing: There was a statistically significantly lower incidence of clinical pregnancies with embryo ultra-rapid freezing compared with slow freezing (OR 0.35, 95% CI 0.16 to 0.76; I2=0%; two RCTs). Compared with slow freezing, ultra-rapid freezing had a statistically significantly lower incidence of ongoing pregnancies (OR 0.37, 95% CI 0.17 to 0.81; I2=0%; two RCTs). There was no significant difference between ultra-rapid freezing and slow freezing in terms of multiple pregnancies, miscarriage rates or live-birth rates.
Vitrification versus ultra-rapid freezing: Indirect analysis indicated that compared with ultra-rapid freezing, vitrification rates resulted in a statistically significantly higher incidence of clinical pregnancies (OR 4.43, 95% CI 1.84 to 10.66) and ongoing pregnancies (OR 4.92, 95% CI 1.88 to 12.87). There was no significant difference between vitrification and ultra-rapid freezing in terms of multiple pregnancies, miscarriage rates or live-birth rates.
Subgroup analysis: For the direct analysis, the odds of a clinical pregnancy for embryos cryopreserved and/or transferred at the cleavage stage were significantly higher in the slow-freezing group compared with ultra-rapid freezing group at the time of cryopreservation (OR 0.32, 95% CI 0.12 to 0.84) and at the time of embryo transfer (OR 0.35, 95% CI 0.16 to 0.76). For the indirect analysis, the odds of a clinical pregnancy for embryos cryopreserved and/or transferred at the cleavage stage were significantly higher in the vitrification group compared with ultra-rapid freezing at the time of cryopreservation (OR 0.14, 95% CI 0.04 to 0.54) and at the time of embryo transfer (OR 0.16, 95% CI 0.05 to 0.52).