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 multicellular embryos that survive cryopreservation can select the viable embryos for transfer. Embryos for cryopreservation were supernumerary multicellular embryos obtained after in-vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) treatments, with <20% of their volume filled with anucleate fragments. These had been cryopreserved using a slow-freezing and slow-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 number needed for transfer. Embryos for transfer were selected on the basis of their morphological appearance and embryo transfer to the patient was done on the day of thawing. From September 12, 1994 onwards we used a more selective thawing strategy where a cohort of up to a maximum of 12 frozen embryos per patient is thawed from which embryos of the best morphological quality, and which are furthest advanced in terms of cleavage after a 24 h in-vitro culture period in Menezo B2 medium, are selected. We took delivery rates, embryo implantation rates and birth rates into account to see if there is any difference between the following three types of transfers used: 187 transfers exclusively of embryos having continued to cleave after thawing, 107 mixed transfers of embryos with and without further cleavage and 53 transfers exclusively of embryos with no further cleavage. The overall outcome in terms of delivery rate and embryo implantation and birth rates 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 a distinction was made between transfers on the basis of the presence of embryos with further cleavage, did the advantage of selection on the basis of cleavage capacity become evident. Significantly higher delivery and embryo implantation and birth rates (11.2, 7.7 and 6.5% respectively) were recorded with transfers exclusively of embryos with further cleavage versus mixed transfers of embryos with and without further cleavage (1.9, 2.9 and 0.6% respectively). Fifty-three transfers exclusively of embryos with no further cleavage did not lead to any delivery. Our results demonstrate that selection of human multicellular embryos which survive cryopreservation and continue to cleave in vitro can significantly improve the delivery rate per transfer and the implantation rate per transferred embryo.