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 ofone-cell human embryos, comparing a slow controlled-rate freezing procedurewith a rapid cooling procedure by direct plunging into liquid nitrogen. Weanalysed the numbers of embryos that were recovered immediately afterthawing (= recovery), the number of embryos morphologically intact afterthawing and subsequent dilution of the cryoprotectants (= survival), thenumbers of embryos undergoing further cleavage after 24 h of in-vitroculture (= cleavage) and the implantation of transferred embryos (=children born per frozen-thawed embryo transferred). We demonstrated thatthe recovery of embryos was greater after slow controlled-rate freezing.Survival was greater after rapid cooling and the number of embryosundergoing further cleavage was higher after slow controlled-rate freezing.Although the birth rate was twice as high after slow controlled-ratefreezing as after rapid cooling, this difference was not statisticallysignificant. In conclusion, our results show clearly that for the freezingof one-cell human embryos, slow controlled-rate freezing is more efficientthan rapid cooling. Before rapid cooling is used routinely in clinical in-vitro fertilization programmes, its safety and reproducibility must beconvincingly demonstrated.