A randomized comparison of the cryopreservation of one-cell human embryos with a slow controlled-rate cooling procedure or a rapid cooling procedure by direct plunging into liquid nitrogen

We conducted a randomized prospective study of the cryopreservation of one-cell human embryos, comparing a slow controlled-rate freezing procedure with a rapid cooling procedure by direct plunging into liquid nitrogen. We analysed the numbers of embryos that were recovered immediately after thawing (= recovery), the number of embryos morphologically intact after thawing and subsequent dilution of the cryoprotectants (= survival), the numbers of embryos undergoing further cleavage after 24 h of in-vitro culture (= cleavage) and the implantation of transferred embryos (= children born per frozen-thawed embryo transferred). We demonstrated that the recovery of embryos was greater after slow controlled-rate freezing. Survival was greater after rapid cooling and the number of embryos undergoing further cleavage was higher after slow controlled-rate freezing. Although the birth rate was twice as high after slow controlled-rate freezing as after rapid cooling, this difference was not statistically significant. In conclusion, our results show clearly that for the freezing of one-cell human embryos, slow controlled-rate freezing is more efficient than rapid cooling. Before rapid cooling is used routinely in clinical in-vitro fertilization programmes, its safety and reproducibility must be convincingly demonstrated.