Selective transfer of cryopreserved human embryos with further cleavage after thawing increases delivery and implantation rates

We investigated whether further in-vitro culture of human multicellularembryos that survive cryopreservation can select the viable embryos fortransfer. Embryos for cryopreservation were supernumerary multicellularembryos obtained after in-vitro fertilization (IVF) and intracytoplasmicsperm injection (ICSI) treatments, with <20% of their volume filled withanucleate fragments. These had been cryopreserved using a slow-freezing andslow-thawing protocol with 1.5 M dimethylsulphoxide as the cryoprotectant.From the start of our cryopreservation programme until September 12, 1994,the thawing strategy was to thaw frozen embryos up to the exact numberneeded for transfer. Embryos for transfer were selected on the basis oftheir morphological appearance and embryo transfer to the patient was doneon the day of thawing. From September 12, 1994 onwards we used a moreselective thawing strategy where a cohort of up to a maximum of 12 frozenembryos per patient is thawed from which embryos of the best morphologicalquality, and which are furthest advanced in terms of cleavage after a 24 hin-vitro culture period in Menezo B2 medium, are selected. We took deliveryrates, embryo implantation rates and birth rates into account to see ifthere is any difference between the following three types of transfersused: 187 transfers exclusively of embryos having continued to cleave afterthawing, 107 mixed transfers of embryos with and without further cleavageand 53 transfers exclusively of embryos with no further cleavage. Theoverall outcome in terms of delivery rate and embryo implantation and birthrates were not different between the new and the earlier thawing policies(6.6, 5.2 and 3.6% versus 6.0, 4.1 and 2.7% respectively). Only when adistinction was made between transfers on the basis of the presence ofembryos with further cleavage, did the advantage of selection on the basisof cleavage capacity become evident. Significantly higher delivery andembryo implantation and birth rates (11.2, 7.7 and 6.5% respectively) wererecorded with transfers exclusively of embryos with further cleavage versusmixed transfers of embryos with and without further cleavage (1.9, 2.9 and0.6% respectively). Fifty-three transfers exclusively of embryos with nofurther cleavage did not lead to any delivery. Our results demonstrate thatselection of human multicellular embryos which survive cryopreservation andcontinue to cleave in vitro can significantly improve the delivery rate pertransfer and the implantation rate per transferred embryo.