Efficiency of human oocyte slow freezing: results from five assisted reproduction centres

It has been demonstrated previously that freezing oocytes within 2 h of retrieval increases the efficiency of cryopreservation via a slow-freezing/rapid-thawing protocol with 0.3 mol/l sucrose (SF/RT 0.3). The aim of this multicentre survey was to verify this observation on a larger scale. This was a retrospective study on the clinical outcome of 510 SF/RT 0.3 cycles divided into two groups: group A, freezing oocytes within 2 h of retrieval; group B, freezing oocytes more than 2 h after retrieval. The rate of best-quality embryos was significantly higher (33.24%) in group A than in group B (16.20%, P < 0.001). Pregnancy and implantation rates were 30.07% and 15.08% in group A versus 8.97% and 4.57% in group B (P < 0.001). Clinical pregnancy rates per thawed and per injected oocyte in group A were 5.53% and 10.41%, versus 1.46% and 2.77% in group B (P < 0.001). The overall yield from oocytes cryopreserved within 2 h of retrieval (group A) was 6.49 implantations per 100 oocytes thawed versus 1.74 for group B (P < 0.001). Embryo quality, pregnancy and implantation rates, and clinical efficiency of thawing cycles were all significantly improved when cryopreservation was carried out within 2 h of oocyte retrieval.